Laser diffraction

24 June 2010

Ingredients

Laser Pointer Cling film
Silica gel Interestingly shaped or patterned plastic

Instructions

Do not shine the laser in your own or any one else's eyes

Shine the laser through each of the objects and look at the patterns produced.

Try shining the laser through a slit in a piece of paper.

Result

The slit produces a series of spots on the wall at right angles to the slit. The other objects will produce a variety of different patterns.

Diffraction from a slit, silica gel, a bottle covered in condensation and a cd

Explanation

Light is an electro-magnetic wave, basically a wave of magnetic and electrical and magnetic fields. This means that it has a lot of the properties of a wave. The wavelength corresponds to its colour, and the wave height (amplitude) corresponds to its brightness.

If you imagine waves in the sea hitting a small gap in a wall, the waves will spread outwards, this effect is called diffraction.

If you have two gaps in the wall you will get two circular wave patterns spreading out. In some areas a peak from one gap will always meet a wave peak from the other hole, the two waves add together producing particularly high waves 9bright light)

in other places the peaks from one wave will meet the troughs from the other wave, so the two waves cancel out and you are left with flat water (darkness).

In this setup the light and dark patches occur at particular angles, producing light and dark spots on a wall, in what is known as an interference pattern.

Diffraction from a point Diffraction from 2 slits
If waves meet a thin gap in a wall (or laser light meets a small gap) the waves will spread out If there are two gaps the waves will interfere producing larger waves (bright light) in some directions and smaller waves in others (darkness)

 In fact if the gap is wide compared to the wavelength of light (bigger than a 2000th of a mm) the waves going through the hole interfere with one another to form a pattern. the bigger the hole the closer the fringes are together. This is why the distance between the spots changed as you changed the angle of the laser.

Diffraction from a single slit Diffraction through a wide slit
If the hole is larger the waves going through the hole interfere with one another forming a pattern. In this case a bright spot straight ahead. The wider the hole the closer the bright spots or fringes are together in the pattern.

These are all very simple cases, but if you shine waves at any pattern then they will project a different one on the wall. In many cases this pattern is what you would expect to see (a shadow or something similar) but if you have many very small objects (such as silica gel which is made up of minute balls of silica, or a scrumpled up piece of cling film the patterns can be surprising and beautiful.

Why doesn't it work with normal light?

You do get diffraction effects with normal light, but they are much less pronounced, this is because normal light is much less coherent than laser light. This means that if you look at light at two different times or places, the waves will sometimes be in time constructively interfereing and others be out of time destructively interfering. This means that any interference pattern that is produced will change from microsecond to micosecond, and your eyes will only see the average of all possible patterns which we consider normal behaviour.

Normal single colour light A single wavelength of normal light
2 bits of normal light If you interfere it with another you find that they move in and out of phase at random. So any interference patterns will change with time.
Laser light Laser light waves on the other hand move in time so the interference patterns are stable.

Normal light is still slightly coherent over a few wavelengths, so if you are dealing with very small structures only a few wavelengths across such as a bubble or oil film
you can still see interference effects.

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