Formation of Images
Light, Sound and Waves

Investigations with ray streaks

for 14-16

In these experiments, students look at the behaviour of rays of light. These 'rays' are not ideal rays, but narrow streaks or blades of light that emerge from the slits of a comb illuminated by a small lamp. They see for themselves some important properties of lenses, rays and images - effectively drawing ray diagrams in light. As well as showing the action of lenses and mirrors, the experiments model a variety of optical instruments. Some of these experiments could be set up as a circus with groups of students moving from one to the other. 
 
If laboratory benches are not long enough, or flat enough, board can be placed on a bench (or two stools), painted white or covered in white paper to show up the ray streaks.

Up next

Experiments with a fan of rays

Ray Diagrams
Light, Sound and Waves

Experiments with a fan of rays

Practical Activity for 14-16

Class practical

A group of short, introductory experiments to investigate the behaviour of concave and convex lenses using rays of light.

Apparatus and Materials

  • Metal plate with five parallel slits or multiple slits (e.g. a wood-graining comb)
  • Barriers
  • Holders for slits and barriers
  • Cylindrical lenses (see technical notes)
  • Plane mirror
  • Either:
  • Ray box, with vertical filament
  • Or:
  • Lamp stands
  • Lamps with vertical filaments
  • Housing shields

Health & Safety and Technical Notes

Many ray boxes of traditional design become very hot after a lesson of use. The class should be warned, and provided with heat-resistant gloves or cloths if they need to handle the ray box when still hot.

Read our standard health & safety guidance


Ideally you will provide plano-cylindrical lenses, with powers of about + 7 D ( f =14.3 cm), + 10 D ( f =10 cm), + 17 D ( f =6 cm) and — 17 D ( f =-6 cm). The most commonly available at the time of writing (Jan 2007) are + 7 D, + 13 D, — 7 D and — 13 D, generally biconvex and biconcave. These will perform the experiments as written, although plano-convex or plano-concave lenses will enable the aberrations to be reduced. The lenses should be 5 cm wide in order to give extensive fans of rays, and 5 cm high so that the rays extend a great enough distance.

Plastic lenses have steeper curves for the same powers compared with glass lenses and the aberrations are therefore much greater (glass has a higher refractive index than plastic).

Do not give students all the lenses, slits, etc., straight away. At the beginning they should have the lamp and power supply, a multiple slit, barriers, white paper, and just one lens, the weakest convex lens (probably + 7 D). Later give samples of all the lenses for a short period of general play, with a warning that lenses are rather fragile and are easily scratched. After that, it is best to provide only those lenses, etc., that are needed for each experiment.

A screen with 5 or more slits is preferable to the 3-slit screen supplied with most ray boxes. If it is possible to provide home made screens like this, the effect will be increased. Such a screen can be supported in the path of the beam coming from the box. In these experiments a multiple slit refers ideally to one of these. The spherical aberration of experiment 5 will not show with a 3-slit screen.

If the ray looks fuzzy, it may be because the lamp filament is not vertical, parallel to the slits in the screen. Or it may be because the lamp has a crooked filament. In the latter case, the cure is to change lamps. The crooked filament can, in some cases, even give an impression of crooked rays. Home-made slits which are not really straight and vertical are also apt to produce crooked rays.

The shields for the lamps come in two slightly different designs; some are placed around the lamp but others are big enough to put the supporting arm of the lamp into the half-length slot, allowing the light to come out of the long slot. It is worth experimenting to see which is the best way for your equipment.

For all the experiments, three-quarters blackout is strongly advised.

Procedure

  1. Fan of rays of light
  2. Shine light from the lamp through the multiple slit and make 'rays' on the paper, which is laid on the bench in front of the lamp. Raise or lower the lamp until the rays continue right across the paper.
  3. Rays and lenses (first + 7 D; later various lenses)
  4. Place the weak positive cylindrical lens (+ 7 D), in various positions in the fan of rays produced ina, and look at what it does. For a short time, try out other lenses available in place of the + 7 D lens. Return to the + 7 D lens and try using barriers to cut down the aperture of the lens.
  5. Lens forming a real image
  6. Carefully adjust the + 7 D lens to ensure that its face is perpendicular to the central ray. Shut down the aperture if necessary until all the rays that come through seem to go on through one point. This is a 'real image'. Again, try other lenses in this way.
  7. Fan of rays and two lenses
  8. Use two lenses of the same power (e.g. + 7D, in the fan of rays. Try the lenses different ways round and observe any effect.
  9. Fan of rays and a stronger lens
  10. Replace the lenses with the single strongest convex lens available (probably + 13 D or + 17 D). Use barriers to restrict the aperture and lessen the spherical aberration.
  11. Fan of rays and a negative lens
  12. Replace the positive (convex) lens with a cylindrical negative lens (concave, probably – 13 D or – 17 D). Look along the rays from the outgoing end and see 'where they seem to come from'. Also look from above and again 'see where the rays seem to come from'. Move the lamp towards and away from the lens and note the changing position of the virtual image.
  13. Effect on image of changing object distance
  14. Changing object distance
  15. (using lens + 7 D, later with + 13 D or – 17 D) Go back to the weak positive lens and try moving the lamp nearer and farther away. You can also try this with the strong positive lens. Observe that a clear image is always formed and that the distance of the image from the lens depends on how far away the lamp is from the lens.
  16. Virtual images
  17. Once more with the weak positive lens, move the lamp so close to the lens that a virtual image is formed. Look along the rays from the outgoing end and see where the rays seem to come from. Then look at the rays from above and see where they seem to come from.

Teaching Notes

  • Students should begin to realize that the light travels in straight lines and that an object is seen when light enters the eye. A lens bends light rays so that the rays pass through an image point. We think we see the object at the image point.
  • It is worth spending time organizing the students at the beginning so that they can get the equipment set up quickly and safely. Power supplies, in the darkened laboratory, can have dangerously trailing cables. Lenses have a nasty habit of getting themselves underneath heavy power supplies too. If the initial organization is done well then this will help the students to work at their own pace. Diagrams can be drawn by putting paper on the ray streaks and sketching over them.
  • Carry out the experiments as a continuous programme, with students moving at their own speed. In this way, extension work for faster students can be encouraged. The division into separate parts 1, 2, 3 . . . in these notes is an artificial one, intended only to help when you are making preparations.
  • The rays of light passing through the comb are just like a sunbeam travelling through patchy clouds. The length of them can be adjusted by raising and lowering the lamp.
  • With step 2 , visit the students to make sure that they have tried placing the lens so that it is not twisted, but receives the rays more or less normally. Offer them small barriers to shut off some edge rays if they wish.
  • This is a new amusing game for young people and they will do many things with the lenses which do not seem sensible or profitable to a physicist. They do not know what properties of lenses and rays they are looking for, and we suggest that you should respect that ignorance and let them play quite freely for a while. Suggested lenses are + 17 D; - 17 D; an extra + 7 D, and perhaps a plane mirror.
  • The rays from a single point object (the filament) do, after passing through the lens, pass through an image point and go straight on through it. The lens can be twisted with respect to the rays, and turned so the plane or convex side is facing the incident beam of light.
  • Again, in step 3, encourage students to ask for more lenses. This may seem like a repeat experiment. But it is a time for students to show that they can produce a good image by placing the plane side of the lens towards the lamp (if they have plano-convex lenses) and the lens perpendicular to the central ray. (The rays will otherwise fall on a caustic curve after they pass through the lens.) This is exciting in its own right, and shows the effects of astigmatism. Closing down the aperture will show the effect of only using the paraxial rays on the sharpness of the image. The lamp can be moved sideways, and backwards and forwards, to show the effect on the image position.
  • Whichever way round the stronger lens is placed in step 5 with respect to the incoming rays, the lens seems to suffer from a problem; all the rays do not go through a single image point, and some of the rays may be coloured at the edges. To get a good image the lens should be stopped down (aperture reduced) and turned (if it is plano-convex), so that equal amounts of refraction take place at each surface; the screen receiving the image can then be placed at the circle of least confusion. In optical instruments the problem is solved by adding together a number of lenses to produce the same refraction and the misbehaviour of one lens is compensated for by the opposite behaviour of another.
  • Spherical aberration is natural, optical behaviour. It results from rays of light being bent, according to the law of refraction, at the surfaces of a spherical lens. Fortunately, a spherical lens does produce an almost perfect image for rays from a point object, when the object is near the axis of the lens and the aperture is small, so that none of the rays hit the lens surfaces very obliquely.
  • Step 6 will raise questions of virtual images, but there is no need to labour the idea at this stage (see teaching note ??? ). We can show rays which spread out after passing through a concave lens as though they have come from a point near to the lamp. The rays don't actually pass through a virtual image. By looking along the rays, from the opposite end to the lamp, the rays will appear to be straight.
  • It is also interesting to show what happens when the - 13 D or - 17 D lens is teamed up with a + 13 D or - 17 D lens; the joint effect of the lenses is to behave like a parallel block of glass.
  • Students should not make any record of the changes of image distance in step 7. The purpose of this experiment is to let students see that a clear image is always formed and that the distance of the image from the lens depends on how far away the lamp is (using the word image in the sense of a place through which rays cross accurately). As the lamp distance becomes less, the image distance becomes greater. If the distance between the lamp and lens is less than one focal length, the lens will produce a virtual image. The special position of the lamp at the focus of the lens producing parallel rays after passing through the lens could be noted.
  • Moving the lamp sideways (if practicable), perpendicular to the rays, will produce an image which is off the principal axis of the lens.
  • Step 8 is the time to discuss virtual images with students while they are looking at their lens and rays together. (This is not a good time for a demonstration. This would interrupt and spoil this period of students' own explorations.) A + 7 D lens placed near to the lamp shows the rays spreading out after passing through a convex lens, producing a virtual image. Looking along the emergent rays towards and through the lens, the rays appear straight, not even bent by the lens. Looking above the rays, the emergent rays will appear to meet on the same side of the lens as the lamp.
  • When students are doing experiments with lenses receiving a fan of rays, it may help if you move round amongst them, to straighten a lens or offer an extra lens to try. You can slide a small piece of cardboard in across the fan, to cut off ray after ray. The demonstration of successive rays being cut off or admitted seems to help students to understand what is going on. You can do this at any point in this sequence of experiments.

This experiment was safety-tested in January 2007

Up next

Experiments with a single ray

Ray Diagrams
Light, Sound and Waves

Experiments with a single ray

Practical Activity for 14-16

Class practical

More introductory experiments investigating the behaviour of concave and convex lenses with rays of light.

Apparatus and Materials

  • Metal plate with five parallel slits or multiple slits (e.g. a wood-graining comb)
  • Barriers
  • Holders for slits and barriers
  • Cylindrical lenses (see technical notes)
  • Power supply, 12 V
  • Plane mirror
  • Either:
  • Ray box, with vertical filament
  • Or:
  • Lamp stands
  • Lamps with vertical filaments
  • Housing shields

Health & Safety and Technical Notes

Many ray boxes of traditional design become very hot after a lesson of use. The class should be warned, and provided with heat-resistant gloves or cloths if they need to handle the ray box when still hot.

Read our standard health & safety guidance


Ideally you will provide plano-cylindrical lenses, with powers of about + 7 D ( f =14.3 cm), + 17 D ( f =6 cm) and – 7 D ( f =-6 cm). The most commonly available at the time of writing (Jan 2007) are + 7 D and – 7 D, + 13 D and – 13 D, generally biconvex and biconcave.) These will perform the experiments as written, although plano-convex or plano-concave lenses will enable the aberrations to be reduced. The lenses should be 5 cm wide in order to give the extensive fans of rays and 5 cm high so that the rays extend a great enough distance.

Plastic lenses have steeper curves for the same powers compared with glass lenses, and the aberrations are therefore much greater (glass has a higher refractive index than plastic).

Warn students that the lenses are rather fragile and are easily scratched. After this instruction it is best to provide only those lenses, etc., that are needed for each experiment.

If the ray looks fuzzy, it may be because the lamp filament is not vertical, parallel to the slits in the screen. Or it may be because the lamp has a crooked filament. In the latter case, the cure is to change lamps. The crooked filament can, in some cases, even give an impression of crooked rays. Home-made slits which are not really straight and vertical are also apt to produce crooked rays.

The shield for the lamps come in two slightly different designs; some are placed round the lamp as shown in the diagram but others are big enough to put the supporting arm of the lamp into the half-length slot, allowing the light to come out of the long slot. It is worth experimenting to see which is the best way for your equipment.

For these experiments, three-quarters blackout is strongly advised.

Procedure

  1. A single ray hitting a lens (+ 7 D and - 7 D)
  2. Set up the lamp with a single slit, so that a single ray emerges. You can make the ray thinner and brighter by placing a + 7D lens just behind the slit. Watch what happens to the ray when it hits various places on a positive lens. Repeat the experiment using a negative lens. If stronger lenses are available, try these as well.
  3. Optical centre: undeviated rays, using the + 7 D lens
  4. Return to the + 7 D lens, and using the single ray find out whether there is any place on it such that a ray hitting it emerges with its direction unchanged. Try twisting the lens or moving the ray so that the ray strikes the same part of the lens at a different angle.
  5. Using only three rays instead of many (only if a multiple slit screen has previously been used)
  6. Using the multiple slit, place a lamp a considerable distance away from the slit, and use a strong positive (+ 15 D or + 17 D) lens to form an image. Is the image a good one? If barriers are added to mask the aperture down to a small central region, it can be. Try the same experiment using a three slit screen and arrange for the ray through the centre slit to hit the centre of the lens.

Teaching Notes

  • It is worth spending time organizing the students at the beginning so that they can get the equipment set up quickly and safely. Power supplies, in the darkened laboratory, can have dangerously trailing cables. Lenses have a nasty habit of getting themselves underneath heavy power supplies too. If the initial organization is done well then this will help the students to work at their own pace. Diagrams can be drawn by putting paper on the ray streaks and sketching over them.
  • The rays of light passing through the comb are just like a sunbeam travelling through patchy clouds. The length of the rays can be adjusted by raising and lowering the lamp.
  • In step 1 you can show students how to make the ray brighter and thinner by placing a lens between the lamp and single slit, just behind the slits. If students seem to be bothered by the addition of this lens, do not supply it. (A +10 D lens works well).
  • In step 2 the ray which passes through the lens undeviated passes through the optical centre of the lens. Undeviated rays can be found in rays fired at the lens from all directions just by twisting the lens. This is a helpful ray to draw when constructing optical diagrams. Two undeviated rays, from the top and bottom of the object, will enable the magnification to be calculated.
  • With stronger lenses it is not possible to find undeviated rays that go straight through the optical centre. There are rays that enter the lens, take a slanting path through the thick glass of the lens and come out parallel to the original direction. These are called undeviated rays because their final direction is the same as before they entered; they only stagger a little sideways.
  • With the intermediate rays missing, the three rays in step --c-- will appear to form a perfect image. This is, of course, cheating, but it is useful for showing simple diagrams of optical instruments with rays. In the case of more complicated models, this dishonest trick provides a simplification that is almost essential for a first look. This untruthful simplification is used all the time in traditional optical diagrams, usually without any warning. This is, of course, cheating, merely concealing the spherical aberration which is there from the rays passing through the outer edges of the lens.

This experiment was safety-tested in March 2007

Up next

Model of a camera

Ray Diagrams
Light, Sound and Waves

Model of a camera

Practical Activity for 14-16

Class practical

A simple model of a camera, demonstrating image formation, focus and the effect of aperture on depth of field.

Apparatus and Materials

For each student or group of students

  • Lamp, stand, housing (one left- and one right-handed), 2
  • Multiple slit
  • Holder for slits
  • Barriers, 2
  • Plano-cylindrical lens, approximately + 7 D
  • Power supply for lamp
  • A4 paper, plain white, one sheet

Health & Safety and Technical Notes

Warn students that their lamps may become hot if they are left on for a series of experiments of this type.

Read our standard health & safety guidance


Procedure

  1. Show students how to stand the cylindrical lens on its base, with the multiple slit about 10 cm in front of it. You might then give them these instructions:
  2. Put the sheet of paper at one end of your table. Your model camera will be there. Place your lamp about 1 metre from the paper. Raise or lower it until light from it shines right across the paper. Place your lens, upright, about 10 centimetres from the end of the paper nearest the lamp. Mount the multiple slit at the end of the paper so that rays of light come shining through it to the lens. Can you see the image of the lamp where all the rays meet? Draw an outline of a camera box on the paper with a front opening where the lens is, and a place for the film at the back where the image is, as in the lower diagram above.
  3. Place a second lamp beside the first, with its beam also shining through the multiple slit. You can see how two object-points make two image-points where the film should be. You might imagine these are the head and toes of a person being photographed.
  4. Now move one of the two lamps nearer the camera. See how you get a patch on the film instead of a point. A patch like that for each point of the original object would make an 'out-of-focus' picture. Keep the two lamps where they are, at unequal distances. Make the front opening of your lens narrower by pushing in some small barriers to stop the outer rays from getting through. See what that does to the out of focus patch on the film.

Teaching Notes

  • The lamp needs to be carefully positioned to enable the beam to show up across 1 m of the bench top.
  • When the second lamp is moved towards the lens, the focus of its beam of rays will be further from the lens, and thus form a patch where students have drawn the film.
  • Forming a smaller aperture with the barriers will increase the camera's depth of field, so that the out of focus effect will be reduced.

This experiment was safety-tested in January 2007

Up next

Depth of field for a camera

Ray Diagrams
Light, Sound and Waves

Depth of field for a camera

Practical Activity for 14-16

Class practical

A model camera using lamps, to investigate depth of field and magnification.

Apparatus and Materials

  • For each student or group of students
  • Lamp, stand, housing (one left- and one right-handed), 2
  • Multiple slit
  • Holder for slit
  • Plano-cylindrical lens, approximately + 7 D
  • Piece of pale coloured filter to fit lamp
  • Power supply for lamp
  • Barriers for creating aperture

Health & Safety and Technical Notes

Warn students that their lamps may become hot if they are left on for a series of experiments of this type.

Read our standard health & safety guidance


The switch-on current for two 12 V 24 W lamps may trip the overload cut-out on some supplies. Try connecting one lamp first and then adding the other.

Procedure

  1. Place the lamps side by side, about 1 metre from the lens, and look at the images A' and B'. Place a piece of pale colour filter in front of one lamp. This will make it easier to tell which rays come from which lamp. Hold a sheet of plain paper vertically across the two image points to represent a photographic film.
  2. Move one lamp much farther or nearer, to produce an out-of-focus patch on the film made by light from that object-point. Bring in barriers to stop down (reduce) the lens aperture and observe the effect on the focus of the blurred lamp.

Teaching Notes

  • Arranging the lamps in this experiment is not easy, and students will need help.
  • Two lamps can be used to represent two objects, or the top and bottom of a single object. If the light from the two lamps is of different colours, the path through the camera from different objects will be seen clearly. In the first diagram below, both objects (parts) are at the same distance from the lens.
  • In the second diagram, above, one lamp is moved forward, so each object's distance from the lens is different and their images are not in focus in the same plane.
  • However, if the aperture is reduced, less blurring of the image of the moved object occurs in this image plane. This is shown in the third diagram, above.
  • The overall clearest images of both objects are seen in a plane situated between the two images.
  • For a spherical lens, this would be called the circle of least confusion. Cheap, fixed focus cameras have tiny apertures. For average scenes they produce a reasonable photograph if the film is placed at the circle of least confusion.
  • With the lamps side by side, students can measure the separation of the lamps (size of the object) and the separation of their images (size of the image), and so calculate the magnification (or in this case, diminution).
  • Explain that this object lies across the lens axis; it is not an object lying along the axis of the lens.

This experiment was safety-tested in January 2007

Up next

Model of a telescope with ray streaks

Ray Diagrams
Light, Sound and Waves

Model of a telescope with ray streaks

Practical Activity for 14-16

Class practical

Using lamps to model a telescope and measure its magnification.

Apparatus and Materials

For each student or group of students

  • Lamp, stand, housing (one left- and one right-handed), 2
  • Triple slit
  • Holder for slit or blocks to hold slit
  • Plano-cylindrical lens, approximately + 7 D
  • Plano-cylindrical lens, approximately + 14 D
  • Pale coloured filter for one lamp (optional)
  • Power supply for lamps

Health & Safety and Technical Notes

Warn students that their lamps may become hot if they are left on for a series of experiments of this type.

Read our standard health & safety guidance


The height of the lamp will need to be adjusted to give a balance between brightness and the length of the rays.

The three slits should be placed close to the +7D lens so that the central ray goes, undeviated, through the centre of the lens, and the outer rays go through the edge of the lens. This will reduce the effect of spherical aberration.

The lenses must be carefully placed so that the rays from both lamps only make small angles with the axis of the objective. The eyepiece must be moved so that it treats the two sheaths of rays equally.

Procedure

  1. Place the two lamps at one end of the bench to act as the top and bottom of the object. Hold the triple slit about 80 cm or more from the ray boxes. Stand the weak positive lens, +7D, as the objective, just beyond but close to the slits with its curved face towards the object. The eyepiece is the strong positive lens, +14D, which should also have its curved face towards the object.
  2. With only one lamp switched on, move the eyepiece so that the rays from the lamp emerge to form a parallel beam. Now add the second lamp very close beside the first one. (To make the rays from the second lamp more easily distinguishable, place a sheet of pale coloured gelatine or Cellophane in front of that lamp.)
  3. The magnification of the object by the telescope can be measured like this: trace the final image rays in a straight line back to the plane of the lamps; then compare the distance between the two lamps with the distance between these two points.
  4. The ratio of the focal lengths: focal length of eyepiece/ focal length of objective
  5. ...will also give the magnification, and the agreement with the ray result can be checked.

Teaching Notes

  • With two lamps, students can see that the first (real) image formed by the objective lens is much smaller than the original object (represented by the space between the two lamps). But if students look along the two parallel beams emerging from the eyepiece, they will see that these seem to come from a distant virtual image which is much bigger than the object.
  • A frequent error is to use just one lamp with the three slits, because it is easier, and to assume that the two outside rays come from the top or the bottom of the object. But all the rays come from the same point on the object (the lamp). So a second lamp is needed to represent another position on the object.
  • Colouring the light from each lamp differently makes it easier to discuss what is happening.
  • Removing the slits so that the full cone of light is seen is a wonderful sight, especially if the beams from the two lamps are differently coloured.
  • Moving the lamp sideways shows how the images move. The two real images formed by the objective lens will be seen clearly. But the rays, emerging from the eyepiece now form a rather complicated picture, except that the 'eye-ring' will be clearly visible as a place where the two sheaths of rays cross, making a narrow region where an observing eye receives everything. This works especially well if the three slits are being used.
  • Instead of using two lamps, you may use a single lamp and move it to and fro, perpendicular to the axis of the telescope. You will see the emergent beam of parallel rays tilting to and fro much more strongly than the rays arriving at the objective lens. This also illustrates the magnification, but it is not so convincing to beginners as the two-lamp method.

This experiment was safety-tested in January 2007

Up next

Ray model of telescope with field lens

Ray Diagrams
Light, Sound and Waves

Ray model of telescope with field lens

Practical Activity for 14-16

Class practical

Using an intermediate lens to widen the field of view and to invert the image, producing a terrestrial telescope.

Apparatus and Materials

For each student or group of students

  • Lamp and stand, 2
  • Left and right handed housing
  • Triple slit
  • Holder for slit
  • Barriers, 2
  • Plano-cylindrical lens, approximately + 7 D
  • Pale coloured filter for one lamp (optional)
  • Power supply for lamps
  • Additional plano-cylindrical lens +7 D for field lens

Health & Safety and Technical Notes

Read our standard health & safety guidance


Warn the students that their lamps may trip the overload cutout on some supplies. Try connecting one lamp first and then adding the other one.

Procedure

  1. Set up the telescope model with two lamps as in:

    Model of a telescope with ray streaks


  2. Place an extra lens +7D at the real image formed by the objective lens. Observe the effect of this on the light meeting the eyepiece and on the eye-ring.

Teaching Notes

  • If a converging lens is placed exactly at the images produced by the objective lens, it does not alter the convergence or divergence of the fan of rays that passes through the image. You might say to students:
  • "If you stick your thumb on the glass of a magnifying glass and look at your thumb through the glass does it look anywhere else except just behind the lens?"
  • If the distance from object to lens is zero, the distance of image from lens is also zero, and the magnification is 1.
  • However, this extra lens tilts the whole fan of rays so that they emerge pointing in a different direction (except for a ray which happens to hit the extra lens just in the middle). When we make a ray model of a telescope with two lamps, the fans of rays through their two real images cannot both pass through the centre of the eyepiece. So although the extra lens does not alter the angle between rays within a fan, it does tilt the two fans to pass through a more central region of the eyepiece.
  • The advantages are: a larger field of view, less aberration, and an eye-ring that is not so far outside the eyepiece. The enlarged field of view is the most important.
  • A terrestrial telescope can be made so that the final image is the same way up as the object. Place a convex lens so that it is twice its focal length beyond the first image produced by the objective, and it will produce a real image at twice its focal length beyond itself. There is no magnification, but the new image becomes the object for the eyepiece and it is the same way up as the object. The telescope is, of course, now longer by four times the focal length of the additional lens.

This experiment was safety-tested in January 2007

Up next

Model of a microscope with ray streaks

Ray Diagrams
Light, Sound and Waves

Model of a microscope with ray streaks

Practical Activity for 14-16

Class practical

Using a lamp to model the passage of rays through a microscope.

Apparatus and Materials

For each student or group of students

  • Lamp and stand
  • Lamp housing
  • Triple slit
  • Holder for slit or blocks to hold slit
  • Plano-cylindrical lens, approximately + 7 D
  • Plano-cylindrical lens, approximately + 13 D
  • Power supply for lamps

Health & Safety and Technical Notes

Warn students that their lamps may trip the overload cutout on some supplies. Try connecting one lamp first and then adding the other one.

Read our standard health & safety guidance


Procedure

  1. The lamp serves as an object. Position the lamp so that it is approximately 30 cm from the triple slit. Place the + 7D lens close to the triple slit, on the opposite side to the lamp, with the curved face of the lens facing away from the lamp.
  2. Move the lamp up towards the lens until the image distance is two or three times the object distance.
  3. Place the + 13D eyepiece lens beyond the focal plane of the objective lens, with its curved face towards the objective. Move the lamp quickly to and fro perpendicular to the axis, and watch the changing tilt of rays emerging from the eyepiece.

Teaching Notes

  • This is a difficult model to arrange. It should be a class experiment, but many students will need help from the teacher.
  • With a high-power objective it is difficult to get the lamp close enough, and to see the rays clearly enough, even when using the three slit comb. Using a + 7D objective and the + 13D eyepiece lens will make an instrument over 80 cm long.
  • It may be possible to crowd two lamps close enough together to serve as top and bottom of the 'object'.
  • If the rays are not clear, the comb can be removed and the full cone traced along the bench.

This experiment was safety-checked in March 2007

Up next

Model of a microscope using a car lamp

Ray Diagrams
Light, Sound and Waves

Model of a microscope using a car lamp

Practical Activity for 14-16

Class practical

Using a lamp to model the passage of rays through a microscope.

Apparatus and Materials

  • For each student or group of students
  • Plano-cylindrical lens, approximately + 13 D
  • Plano-cylindrical lens, approximately + 7 D
  • Triple slit
  • Power supplies for lamp filaments

Health & Safety and Technical Notes

Read our standard health & safety guidance


The lamp is made from a car tail-light lamp. The lamp is designed to fit into a special base, but this is probably not worth the expense. A small wood block can be drilled with a hole to provide a push fit for the lamp. Two wires should be soldered to the contacts on the lamp and a third wire to the cap. The lamp should be pulled into the block with its axis horizontal so that the filaments are vertical. The two filaments act as two very close object points.

For the rays to be of approximately equal brightness, the 21 W filament should be run at 6 V and the 5 W filament at 12 V.

Procedure

  1. Place the + 13D objective lens, with its plane side towards the lamp, about 12 cm from the filaments. This makes the images of the two filaments about 20 cm from the lens. A narrow aperture for the objective lens is needed for a clear image.
  2. Place the + 7D eyepiece beyond this image, so that the final image appears to come from the object plane. It may help to set up the model if one of the filaments is switched off.
  3. If the ray streaks are not clear, then the slits can be removed and the full cone rays traced along the bench.
  4. The magnification can be measured by measuring the size of the object and the final image.

Teaching Notes

This experiment was safety-tested in January 2007

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About telescope lenses

Ray Diagrams
Light Sound and Waves

About telescope lenses

Teaching Guidance for 14-16

If a bi-convex eyepiece is not used and a plano-convex or meniscus lens is used instead, it should be placed with its plane or concave side towards the eye. That arrangement, which looks like the opposite of the best arrangement for minimizing spherical aberration, is the correct one for an eyepiece. This is because that lens is dealing with small pencils of rays coming from the real image, which the observer is looking at with the eyepiece.

Those narrow pencils are like thick rays, and they are almost parallel to the axis. The eyepiece must deal with these thick rays as well as possible, and for that the eyepiece should have its convex face turned to receive those thick rays. On the other side of the eyepiece the narrow pencils, or thick rays emerging from the plane side of the eyepiece, will pass through the eye ring, so they form a strongly converging group. The eyepiece, arranged this way round, treats those thick rays with less spherical aberration than it would the other way round. Those rays which hit the outer region of the eyepiece lens are bent a little more than the ideal amount, because there is very little spherical aberration. But if the eyepiece were the other way round that extra bending would be much greater, and would have two effects.

  • It would give extra magnification to the outer parts of the final virtual image, making distortion.
  • It would give curvature of field to the image, so that the outer portions are farther away than the central part.

To see how that curvature of field arises, one must look at the differential ‘extra bending’ between the outer and inner rays of each small pencil. This story of the eyepiece is not something to discuss with students at an introductory level.

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Ray streaks and ray diagrams: some cautions

Ray Diagrams
Light Sound and Waves

Ray streaks and ray diagrams: some cautions

Teaching Guidance for 14-16

The ray box is a useful and convenient piece of apparatus for demonstrating optical phenomena, but be aware of its limitations. When using ray boxes, you need to make sure that students do not make invalid assumptions. Take care to bring out the correct physics. Here are some suggestions.

Ray streaks

The pattern of light emerging from the comb (or slits) in front of a ray box (or simple lamp) is often treated as if it consists of discrete rays, but it does not. What you see on the paper fixed to the bench are streaks of light, each emanating from a range of points along the extended object of the lamp filament. A succession of beams constitutes a line across the surface on the bench. This is more correctly described as a shadow of the comb (or slits) cast onto the paper. (A point source too would produce ray streaks, though the intensity of light is likely to be reduced.) Note that the height of the lamp filament affects the length of these streaks.

If you make home-made slits which, deliberately, are not perfectly parallel or straight, they can produce wavy rays. The waviness of those rays might help students think about what is said in the previous paragraph: it is all a matter of shadows.

Be ready, therefore, to clarify any misunderstanding that might take place.

Ray diagrams

Misconceptions can also arise from the accepted practice of using only two or three ray streaks in experiments, or rays in corresponding ray diagrams.

The image of an extended object (typically a light source) will be formed from a flux of light passing through the optical system, not just two or three rays. The lens focuses cones of light. Limiting the explanation to two or three rays makes drawing easy, and in many cases correctly predicts the outcome. But without cautions from the teacher, this can mislead and confuse students. For example, many students think that a small opaque disc placed in front of a lens will produce a hole in the image, whereas in practice it simply reduces its brightness. The Newtonian reflecting telescope is a good illustration of an obstruction doing this.

For this reason it is often better to use a multiple slit (or often 5-slit) shutter in front of the ray box rather than the 3-slit shutter normally provided.

This Guidance note was inspired by an article in the journal Physics Education by Prof Laurence Viennot, University Denis Diderot, Paris. [ Physics Education Vol 41 (2006}, 400-408]

Physics Education


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Ray box or lamp?

Ray Diagrams
Light Sound and Waves

Ray box or lamp?

Teaching Guidance for 14-16

In many optics experiments, the apparatus we show is a free-standing lamp on a stand. This may be shielded by housing, which comes in both left and right configurations, to aid in bringing the lamps close. This has several advantages.

  • It is easy for students to understand that the object is the glowing filament itself.
  • Used with a multi-slit comb, this apparatus produces a broad fan of ray streaks, needed in some experiments.
  • Two lamps can be brought side by side to create the top and bottom of an imaginary object. This enables a comparison of the positions of the two lamps in the image produced by a lens. With ray boxes it is difficult to get two lamps close enough together.
  • The height of the lamp above the bench is adjustable. When used with slits, this enables the user to alter the length and brightness of the ray streaks it produces.

Ray boxes will produce good rays, but they tend to emphasize parallel incident rays. Some designs have a built-in lens, but this is not always wanted.

A colour filter, such as yellow or magenta, placed in front of the lamp will produce more exciting ray streaks than white rays. A colour filter also helps to identify what happens to light from the top and bottom of an object when the light then passes through a lens. You may be amazed at the difference in response from students.

NOTE: Whether you use a ray box or a simple lamp to produce ray streaks, it is essential that the filament is in the same plane as slits in the comb (i.e. is vertical).

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Teaching ray optics

Reflection
Light Sound and Waves

Teaching ray optics

Teaching Guidance for 14-16

At introductory level, simple experiments can help students to realize that light travels in straight lines and that an object is seen when light from the object enters the eye. A lens bends light rays so that the rays pass through an image point and we think we see the object at that point.

Treated as open-ended experiments they show students the way in which light behaves with real lenses in optical instruments.

Photograph courtesty of Jim Jardine

Most of the experiments described on this website are suitable for intermediate level courses. After completing them, students should be able to draw a diagram of light rays (not formal ray construction diagrams) showing the following.

  • Rays travel out from an object point in all directions, going fainter as they go farther.
  • All rays from a remote object point pass through an image point.
  • Rays from a remote object point which pass through a lens and proceed to a real image point after the lens, continue straight on through that point.
  • Rays from an object point which pass through a lens forming a virtual image emerge along lines that appear to come straight from the image point.
  • Every ray aimed at a central point in a lens (called the optical centre) passes through undeviated.

The real behaviour of rays falls short of the ideal of passing through images exactly. Students will see this and learn a little about correcting for that aberration.

The ray optics equipment suggested in these experiments looks simple, but some practical skill is needed to get the best out of it. Teaching notes provided with each experiment will help you ask the right questions of students struggling to get results.

You will be better prepared for student questions if you try out the experiments carefully beforehand. It is also advisable to read traditional textbooks that go beyond what students need to know for examination purposes. For example, knowing that the minimum distance between object and image is four times the focal length of a converging lens will enable a teacher to choose a lens that suits the length of a demonstration bench.

A well-organized cafeteria of equipment, under teacher control, will encourage students to do their own experimenting. In this way, extension work for faster students can be encouraged.

At intermediate and advanced level, ripple tanks can be brought in when needed, to show reflection or refraction for example. Wave theory predicts that all parts of a wavefront starting from a small light source arrive in phase at the image. This requires all paths from the object to take the same time.

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