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Tap 506-2: Diffraction of light Apparatus

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TAP 506-2: Diffraction of light


  • torch bulb in holder with power supply

  • diffraction gratings, 2 each of various spacing, e.g. 100 lines mm-1, 300 lines mm-1

  • red, blue and green filters (one of each colour)

  • 2 glass slides (e.g. microscope slides)

  • lycopodium dust


Lycopodium is dried pollen. Some people are allergic to pollen so, before use, it is wise to check for hay-fever sufferer. (See Safeguards in the School Lab 4.4). Lycopodium powder in air will explode so extinguish all flames before using it.


Observe what happens when light is diffracted by:

Explore the effects of changing the separation of the lines and the wavelength of the light.

Set up a small (6 V) lamp. You are going to observe how light from the lamp is diffracted in different situations.

Write down or sketch what you observe.

Hold a diffraction grating and a colour filter close to your eye.

Look through it at the lamp. Rotate the grating.

(A diffraction grating consists of many parallel, equally spaced lines, as much as several thousands in each centimetre.)

Repeat with a finer or a coarser grating (using the same colour filter as before).

Now use two diffraction gratings. Place one on top of the other, so that their lines are at

90 degrees to each other. You have made a grid of lines through which the light is diffracted. Look through the grid at the lamp.

Try holding the gratings so that their lines cross at a different angle – say, 60° or 45°. Try combining gratings with different line spacing.

Finally, sprinkle fine dust (e.g. lycopodium powder) on to a glass slide. Blow off any excess dust. Look through the slide at the lamp.

(Now you are looking at light diffracted by a random array of tiny particles.)

Repeat the above, using a different colour filter.

Sum up your observations by saying how the diffraction patterns depend on:

Practical advice

This activity is intended to remind students of diffraction effects they may have previously observed. Points to bring out are:

  • the size of the pattern increases with increasing wavelength (red light is diffracted more than blue)

  • the size of the pattern depends inversely on the spacing of the diffracting objects (the finer the grating, the more widely spaced the diffraction images)

  • the geometry of the diffraction pattern depends on the geometry of the diffracting objects

  • a random arrangement of small obstacles gives rise to a diffraction pattern that is a set of concentric rings.

External reference

This activity is taken from Salters Horners Advanced Physics, section PRO, activity 17

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