# Young's slits with light

##### Class practical

Two overlapping patches of light make bright and dark bands, giving direct evidence for the wave nature of light.

#### Apparatus and materials

Graticule, 1/2 mm or Ronchi ruling

Lens (+14D)

Greaseproof paper screens, small

Power supply for lamp

Metre rule

Lamp, holder and stands

Double slit, e.g. made from glass microscope slide

Double slits holder

#### Health & Safety and Technical notes

The lamp should be 48 W / 36 W, otherwise the fringes will be dim and hardly visible. Screen the lamp so that light is directed only towards the slits, and does not flood the room.

The whole point of this experiment is to see the pattern of fringes clearly, something that needs skill to arrange. See apparatus section on 'Setting up Young’s experiment with light' for details of how to make the interference pattern visible, and how to make the double slits.

#### Procedure

a Setting up. Allow at least a metre between the lamp filament and the slits, and several metres from the slits to the screen. A darkened room is essential. The slits must be parallel to the filament of the lamp.

b Viewing. Get students to look at the fringes with the naked eye, from behind the screen. It may be necessary to remind them to pull their heads back to a reasonable distance from the screen. Seeing the fringes – ‘light + light’ making ‘more light’ in some places but making 'no light' in other places – is most important.

c Measurements. First measure the fringe spacing, x, by averaging over many. It may be best to mark the screen in pencil when the pattern is visible and later, in daylight, measure the spacing between the marks. Measure the distance from the slits to the screen, D, with a metre rule. Finally, measure the separation of the slits, s, (e.g. by comparing the slits with a calibrated graticule, using a magnifying glass to help).

d Estimating the wavelength. Lead the students through the geometry – see note 2 below – and then ask them to work out a very rough estimate of the wavelength.

#### Teaching notes

1 Students see for themselves that two overlapping patches of light can make bright and dark bands, giving evidence for the wave nature of light. They can then go on to estimate the wavelength of light. Be careful that the second aim does not override the first.

2 Geometry
If the central band is at P and the next bright band at Q, the path difference, S1Q – S2Q must be one wavelength. Draw S2M perpendicular to TQ. Then S1M is the path difference, single wavelength. With the big distances and small angles involved, the triangle S1S2M will be similar to PQT. Then, by similar triangles:

wavelength / S1S2 = PQ / TQ

wavelength = distance between slits x fringe separation / distance from slits to screen

3 It’s worth explaining to students that making a rough estimate is often very good science. Progress in physics does not always consist of measuring one more decimal place with great precision. Here, a rough guess at the tiny wavelength of light is worth a great deal because it tells us why light seems to cast sharp shadows; yet it warns us that wave effects will become important when we go into fine detail.

4 Using white light, students will be estimating an average wavelength for the visible spectrum. The human eye is most sensitive in the green region, and the rough estimate is likely to be near to a value for green light, say about 5 x 10-7m. You may want to interpose red and green colour filters. Switching quickly between them will make the fringes seem to grow and shrink, demonstrating the difference in spacing.

5 Lasers are very easy to use as the light source, but it would be a pity of students went away thinking that a laser is essential to see this effect.

This template with two sets of semi-circular lines can be used with an OHT to simulate the interference of coherent waves from two point sources.

This experiment was safety-checked in January 2007