Experiments with a single ray

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)

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.
2. Optical centre: undeviated rays, using the + 7 D lens

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.
3. Using only three rays instead of many (only if a multiple slit screen has previously been used)

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