Two-Slit Interference
Light Sound and Waves

Setting up Young's experiment with light

Practical Activity for 14-16 PRACTICAL PHYISCS

Apparatus and materials

You will need:

  • Colloidal graphite (Aquadag)
  • Soft brush
  • Glass microscope slide
  • Metal straight-edge
  • Steel needle
  • Translucent screen, small
  • Lamp, holder and stands
  • Power supply for lamp
  • Light source, holder for double slits

If you wish to determine the wavelength of light, you also need:

  • Metre rule
  • Ronchi rulings or calibrated graticule

Safety

The use of two razor blades, as suggested in some texts, is deliberately omitted here. The suggested methods are much safer than razor blades.

Technical notes

Preparing the slides

Paint microscope slides quickly with Aquadag, using a soft brush. Allow the slides to dry. Hold each slide up to a bright light and make sure it is thoroughly opaque. Once you are satisfied, scratch the slits in a soft black film on glass.

Ruling the slits

There are several good ways of ruling slits on the coated slides ranging from simple hand scribing to the use of a special device.

  1. Rough ruling by hand. Drag a blunt pin or needle along a metal ruler. To make the second slit, hold the ruler there but tilt the scriber a little and drag it again. The edge of a small screwdriver or the tip of a fine biro also makes a good scriber.
  2. Special ruling device. Insert the glass slide in the ruling device and rule one slit in the Aquadag with a blunt needle, or etc, held up against the cross-piece. Then displace the slide slightly by turning the thumbscrew on the end of the device, then rule the second slit.
  3. The eye of a darning needle, broken across, can be used as a miniature pitchfork to rule double slits.
  4. The old fashioned ruling pen used by draughtsmen rules parallel double slits very well. The separation of the slits is easily adjusted by the knurled knob that moves the two blades closer or farther apart. Make that adjustment by trial, then run the pen along a ruler. If the width of each slit should be greater, grind the tips of the blades a little. This is probably the best method.

Size and separation of slits

In a large lab, slits 0.5 mm apart, centre to centre, will give fringes that are wide enough for students to see easily, provided the screen is several metres from the double slits.

If each of the two slits is itself about 0.25 mm wide, only two or three bright fringes will be visible — because such a wide slit does not spread the light over a broad diffraction pattern. However, those fringes will be brightly illuminated.

If each slit is only about 1/8 mm wide (and the slits are still 1/2 mm apart) the pattern of fringes will be just as coarse; but more fringes will be visible; and the pattern will look much less bright.

In a small room, it may be necessary to rule the two slits closer together to make the fringes far enough apart to be seen easily.

Testing the slits

Hold the slits just in front of one eye and look at a distant line-filament lamp. Then the eye acts as a 'tele-scope' focused more or less for infinity. You will see Young's fringes and can judge them for brightness and separation. This arrangement should be a teacher's test: it should not be used by students, because the use of the observer's eye in this way is very confusing and likely to spoil the essential message of the experiment.

Seeing the pattern of fringes

At a distance of several metres, there will be little light on the viewing screen, so the experiment must be done in a laboratory with good blackout. If you do not have such a laboratory, you could set it up inside plastic down-pipe. For details see the website given below.

Use lamps with very straight, vertical filaments. Bulbs of 36-watt would be better than 24-watt ones. Orient the filament of the lamp parallel to the slits. The lamp should be screened so that light is directed only towards the slits, and does not flood the room.

Use a translucent screen, one that scatters light through a small angle. Each student in turn stands immediately behind the screen so the light that comes almost straight through it to the eye. Suitable materials for the screen are kitchen greaseproof paper, waxed paper, and oiled tissue. (Architects' tracing linen is unsuitable because it scatters light through a large angle.)

To reduce glare from polished table tops, cover them with black cloth. Where necessary, place screens to stop reflected light from reaching the viewing screen.

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