How images form

Demonstration

Investigating the formation of real images.

Apparatus and Materials

• Light source, compact (100 W 12 V)
• L.T. variable voltage supply (12 V 8 A)
• Aluminium screen with 50 mm circle of holes
• Plano-convex lens, large
• Retort stands, bosses and clamps, 3
• Screen, white
• Greaseproof paper

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

The plano-convex lens should be of condenser quality, about 100 mm in diameter, 150 mm focal length. A suitable lens is obtainable from Knight Optical, part number LPC15050.

The aluminium screen has a series of small holes drilled in a circle of about 50 mm diameter, so that a cone of rays is produced when a light is shone at it.

Procedure

1. Position the lens about half a metre from the lamp, with its convex side towards the lamp. This reduces aberration. Hold the aluminium screen with holes between the two, and about 100 mm from the lens. Position the lamp, aluminium screen and lens so that the lens forms a real image about half a metre beyond the lens on the screen.
2. This is the same arrangement as for

   Real and virtual images in a smoke box

   but without the box or smoke.
3. Adjust the positions so that the pencils of light from the outer holes in the screen meet the lens at places near its edge.
4. Show that there are rays proceeding from the aluminium screen to the lens. Do this by moving a piece of greaseproof paper to catch those rays in the space between the screen and the lens.
5. Ask what happens to the rays beyond the lens. Bring students close enough to see what does happen. Do this by waving a sheet of greaseproof paper in the space between the lens and the image, and on beyond.

Teaching Notes

• Although the lens bends pencils of light to pass through the image point, neither rays nor image are visible to an audience in a dark room without smoke to scatter light.
• The lens forms a real image some distance beyond the lens.
• If the plate is removed, an image of the lamp will be seen clearly on the paper screen.
• The lens is stronger for blue light than red light, and so coloured images of the lamp will be seen at different distances from the lens.

This experiment was safety-tested in August 2006