Viewing sharp shadows

Demonstration

Interesting diffraction effects seen in shadows.

Apparatus and Materials

• Greaseproof paper to make a translucent screen
• Dressmaking pins (with ball heads), one or more
• Wax, hard, to attach objects to plate
• Wax
• Metal plate with 1.5 mm hole
• Light source, compact
• Power supply, low voltage, variable, to supply 8A at 12V
• Retort stands, bosses and clamps, 3
• Plate glass, one piece to hold objects for casting shadows

Health & Safety and Technical Notes

Take care when using a razor blade to rule a narrow slit. Single-edged blades are safer. If double-edged ones are used, cover one edge with several layers of PVC tape.

Read our standard health & safety guidance

Avoid inserting lenses, or the magic will be lost.

Place the compact light source at one end of the lab, with the 1.5-mm pinhole just in front. Place the translucent screen near the other end of the room. In the middle, between them, place the objects to cast shadows. The objects should be far from the lamp, at least 3 metres, preferably 5 metres. The screen should be at least 1.5 metres beyond the objects. It is essential to avoid glare from the bench tops (cover with black cloth if necessary}.

A sewing needle, a pin, a sheet of metal or card with small holes drilled in it, and a small disk or steel ball (maximum diameter 5 mm) all make good shadow-casting objects. Stick them with wax on a piece of plate glass. Perhaps because it is natural, a human hair seems to do best of all.

A dressmaking pin, with spherical glass head (diameter about 4 mm) is best of all for showing the white spot. It can be held separately in mid-air. Or several can be used with one source, to cast shadows on several screens.

The room should be fairly dark. However, if you try to show diffraction in a completely blacked-out room then students cannot see what is happening, and discipline problems could ensue. Students will need time to let their eyes become dark-accommodated.

Procedure

1. Let students stand near the collection of objects and look at them. If they hold a piece of paper just beyond the objects, they may catch sharp shadows.
The diffraction effects...
2. Ask students to move to the translucent screen, going round behind it to look at the shadows there. Remind students to hold their heads 1/4 metre or more behind the screen (as in reading a book.)
The shadow of a disk...
3. To a physicist, the strangest shadow of all is that of a small ball or disk: there is a white spot in the centre of the shadow. One can just see this in a long, very dark room, if one expects it. Our source is too large, and students will probably miss it, unless the source is made smaller. Place a metal plate with a hole 1 mm diameter just in front of the lamp. Then ask students what they see.
Diffraction by a slit...
4. If time permits, change to a set of three prepared slits: wide, medium, and very narrow. Suitable slit widths are 1 or 2 mm, 1/4 to 1/2 mm, 1 to 3 hundredths of a mm. The narrowest slit needs no microscope to check its width; judge it by its diffraction spread, which should be 2 or 3 cm on a screen at 1 metre. These could be ruled on a coated glass slide, the narrowest with a razor blade.

Teaching Notes

• As part of a higher level course, you may want to show what happens with a V-shaped slit or a variable slit. If students are familiar with pinhole cameras, you might ask able students what this suggests about the diameter of a suitable pinhole.
• Whether or not you have done this earlier, you may want to bring out a ripple tank and show diffraction through single and multiple slits, and round obstacles.

This experiment was safety-tested in February 2006