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Physics Notes: Behaviour of visible light rays 3. Diffraction e.g. by a CD

Visible light  3. Investigating the DIFFRACTION of visible light rays - CD acts as a diffraction grating

Doc Brown's Physics exam study revision notes

3. Investigating the DIFFRACTION of visible light

A ray box system is no good for investigating the diffraction of light.

Instead, you can use water waves in a ripple tank.

Remember both light and 'ripples' are transverse waves.

So, in effect, you are using water waves to model light waves.

(You can of course investigate reflection and refraction with the ripple tank too.)

Diffraction is the effect of waves spreading out when passing through a gap or passing by a barrier. In effect, waves go round corners! and it doesn't matter if its sound, light or water waves - they all diffract and bend round corners! The effect is so small with light (tiny wavelength), you don't notice it, but you see water waves bending around walls of a harbour and you can hear sounds from round a corner.

You should appreciate that significant diffraction only occurs when the wavelength of the wave is of the same order of magnitude as the size of the gap or obstacle.

A: There is a relatively small diffraction effect when waves pass through a wide gap that is much bigger than the wavelength of the wave.

B: You get the maximum spreading or diffraction when the light waves pass through a gap of similar size to the wavelength of the incident waves.

You can see these effects with transverse water waves at the seaside as waves hit the protective walls of a harbour BUT you need a very tiny slit to observe diffraction with light waves because of their tiny wavelength.

Can you observe the diffraction of light?

When you hold up a fine needle towards a bright light, the edges aren't quite sharp because the light rays are diffracting (bending) around the pin's surface.

A diffraction grating consists of a large number of equally spaced parallel fine slits or fine grooves. When light falls on a grating, the light from each slit/groove diffracts and will interfere with the light from the other slits. When you get constructive interference for a given wavelength, that colour is enhanced and hence observed

A DVD or CD (compact disc) has a spiral, grooved track (usually composed of very fine pits) or grooves in the metal. On the surface of a CD, the grooves are approximately parallel. Consequently, the light reflected from a small portion of the surface of the CD diffracts as if it was from an approximately parallel source. This results in the coloured 'rainbow' patterns you see when viewing white light reflected from the surface of a CD which is referred to as a
reflection grating.

Keywords, phrases and learning objectives for the behaviour of visible light rays

Know how to investigate the diffraction of visible light rays by a CD which acts as diffraction grating to produce the visible spectrum of colours.

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