Class practical

Identifying that the image in a plane mirror is virtual.

Apparatus and Materials

For each student or group of students

• Large sheet of white paper (A3)
• Plane mirror
• Ray box or lamp (12 V 24 - 48 W)
• Low voltage power supply for ray box
• Multi-slit screen (comb)
• Barriers
• Holder for mirror

Health & Safety and Technical Notes

Read our standard health & safety guidance

Many ray boxes of traditional design become very hot after a lesson of use. Warn the class and provide them with heat-proof gloves or cloths if they need to handle the ray box when still hot.

Procedure

1. Place the plane mirror upright on the paper and arrange the lamp, multiple slit screen and barriers so that a fairly narrow fan of rays from the lamp hits the mirror. Move the lamp quite close to the multiple slits, so that the virtual image of the lamp will be somewhere on the actual paper.
2. Ask students to 'look along the reflected rays to see where they come from' and to look down on the pattern from above, and see where the reflected rays seem to come from.

Teaching Notes

• Encourage faster students to try to put some marking device at the place the rays seem to come from, such as a large pin stuck on the paper or a small block of wood.
• The crossover of the rays in the reflection can be quite confusing to sort out at some angles.
• Use the name virtual image and ask students where it is. Able students will be able to trace the rays back carefully towards the virtual image, showing that it is perpendicularly as far behind the mirror as the object is in front.
• It could be useful to bring out the ripple tank to show circular ripples being reflected from a barrier as in:

  Reflection of a circular pulse by a barrier

This experiment was safety-tested in January 2007

Demonstration

Comparing the paths of a reflected ray of light and a rubber ball.

Apparatus and Materials

• Light source, compact (100 W 12 V)
• L.T. variable voltage supply (12 V 8 A)
• White screen (500 mm x 300 mm approx)
• Card with slit (5 mm wide approx)
• Retort stand and boss
• Plane mirror
• Rubber ball

Health & Safety and Technical Notes

Be aware that compact light sources using tungsten-halogen lamps without filters are significant sources of UV. Ensure that no-one can look directly at the lamp.

Read our standard health & safety guidance

Procedure

1. Set up the compact light source with the card and slit in front of it, so that a thick ray of light splashes across the vertical white screen.
2. Catch the ray with a piece of plane mirror held in the hand and reflect it across the screen.
3. Bounce the rubber ball against a hard wall or floor to show its reflection.

Teaching Notes

• This demonstration introduces the idea of reflection. It is not intended for careful measurements, but students should see some connection between the angles.
• Make sure that students observe the similarity between the path of the ray of light, and the path of the ball.
• You could ask the question 'What is light made of... bullets?'. If students have seen waves being reflected in a ripple tank, they might also give waves as an answer.

This experiment was safety-tested in August 2006

Class practical

Showing angle of incidence equal to angle of reflection.

Apparatus and Materials

• For each group of students
• Ray box or lamp (12 V 24 - 48 W)
• Low voltage power supply for lamp
• Single slit
• Plane mirror
• Holder for mirror
• Paper protractor (see below)
• White paper

Health & Safety and Technical Notes

Many ray boxes of traditional design become very hot after a lesson of use. Warn the class and provide them with heat-proof gloves or cloths if they need to handle the ray box when still hot.

Read our standard health & safety guidance

A suitable protractor template is provided (see below).

A cylindrical lens can be fitted within the ray box to give a clear, long ray streak.

Procedure

Set up the apparatus to produce a single ray streak on the paper. Stand the mirror on the paper protractor with the two bases aligned. Students will quickly see the equal angles.

Teaching Notes

• Students should see rays of light being reflected at a plane mirror. They should extract some kind of rule about equal angles. It is possible to sketch in a series of rays in order to keep a record of the experiment. But it is just as easy to read off the angles of incidence and reflection from the protractor, if it is aligned with the mirror.
• An alternative is to use a Hartl disc or some other arrangement, which has a protractor and a scheme for showing the behaviour of a single ray.
• The template (see below) with two sets of parallel lines can be used with an Over Head Transparency (OHT} to simulate reflection and interference of plane waves, at a straight barrier.
• Photocopy or print the lines onto an OHT. Cut into two sets. Use the lines as wave fronts towards the mirror at an angle.
• This is not real interference but rather a Moire Pattern analogue.

This experiment was safety-tested in January 2007.

Class practical

The caustic curve and how the aberration can be reduced by reducing the aperture.

Apparatus and Materials

For each student or group of students

• Ray box or lamp (12 V 24 - 48 W)
• Low voltage power supply for lamp
• Multi-slit screen (comb)
• Cylindrical concave mirror
• Large sheet of white paper (A3)
• Plain card
• Card with slot (see technical notes)
• Barriers

Health & Safety and Technical Notes

Many ray boxes of traditional design become very hot after a lesson of use. Warn the class and provide heat-proof gloves or cloths if they need to handle the ray box when still hot.

Read our standard health & safety guidance

Suitable mirrors are available from a number of suppliers eg Griffin Education or Technology Supplies.

Some manufacturers provide a five slit screen with the ray box. However, more slits than this will make the experiment more impressive. With home made slits it is important to ensure that the slits are parallel and perpendicular, or the rays produced may appear crooked.

The card with slot should be large enough to block out all light from the ray box, but with a central slot that will allow three rays to pass through. Less effective would be to replace the multi-slit screen with the conventional three slit screen provided with the ray box.

Procedure

1. Place the lamp and multi-slit on the sheet of paper to make a wide fan of rays. Place a (concave) cylindrical mirror in this fan. The mirror must have a large aperture (at least 120°, and preferably 180°) so that it forms a caustic curve on the paper. Slide the plain card in front of the comb as an obstruction to cut off ray after ray.

2. Now use the card with the wide slit in it, in front of the comb, so that a narrow fan of rays hits the mirror and forms an image. If the card is moved across the comb, the image will slide around the caustic curve.

Teaching Notes

• This is FA Meier's famous mirror and comb experiment. Students will see a caustic curve, which they will ever afterwards recognize as the curve on the tea or coffee in a cup. It's a delight; the diagrams do not do it justice.
• The image using the multi-slit screen will show spherical aberration. Reducing the aperture with a wide slit reduces this. Using only three rays makes it appear that the aberration has been removed.
• If you are using a shiny metal mirror it can be turned round and the convex mirror investigated.
• Another look at the concave mirror in the smoke box would be a good idea now. See the experiment:

  Real an virtual images in a smoke box

This experiment was safety-tested in January 2007

Demonstration

This experiment is well worth the effort of setting it up.

Apparatus and Materials

• Concave spherical mirror, large, with holder
• Red marble
• Blue marble
• Small cardboard box (5 cm or larger) e.g. matchbox
• Stand for box
• Small reading lamp
• Plasticine or Blu-tack

Health & Safety and Technical Notes

Read our standard health & safety guidance

The concave mirror should be as large as possible and preferably have an aluminized front surface. Suitably sized mirrors are obtainable from the supplier: Ocean Direct.

For the additional effect with a lamp, the aperture diameter across the mirror's face should be at least as big as the mirror's radius of curvature.

Photo courtesy of Mike Vetterlein

Procedure

1. Fix the red marble, R, on the under side of the top of the small box using Plasticine or Blue-Tack. Place the box with its open side facing the mirror at such a height that the red marble is near the centre of curvature of the mirror, a very small distance below the mirror's axis. The red marble is thus hidden from the observer.
2. Place the blue marble, B, as a decoy, to catch the observer's eye. This is also fixed with Plasticine or Blue-Tack to the box, but on the upper side of the top. B is the same distance from the mirror as R, but a little above the axis and a little to one side. (As B is above the box it is clearly visible to an observer.)
3. Face the mirror and look at the visible marble, B. You can see an equally real red marble beside it.

Teaching Notes

• It is essential to place the blue marble carefully so that the two marbles do not seem to overlap but appear side by side. A little empty bed of Plasticine, or Blue-Tack stuck beside the blue marble, will look like a support for the red marble's image.
• With the object placed at the centre of curvature of the mirror, there is no spherical aberration.
• You can look at the image with a magnifying glass. This then becomes a Newtonian telescope.
• For an additional effect, place a small lamp (spotlight) beside the observer's head so that it lights up the visible marble B. Any rays from the lamp which hit the image beside the visible marble will pass right through that image, continue to the mirror, and be reflected to the concealed marble R. This means that the imaginary marble is illuminated as well.
• Both marbles must be located correctly. Shields of black paper or card can be arranged to limit the observer's view. The demonstration works best as a set piece with a frame constructed especially for it.
• With a mirror of smaller aperture, the concealed marble needs to be illuminated directly by a lamp concealed somewhere between the marble and the mirror.
• Scientific toy shops have a version of this consisting of two concave mirrors facing each other, one above the other. The top mirror has a hole cut in it, and an object placed just below the hole on the bottom mirror will produce an image which appears to float in space.

This experiment was safety-tested in January 2007