Diffraction

• When a wave is disturbed by the edge of a barrier circular wavefronts are produced.
• If the wave passes through a gap in a barrier circular wavelets are produced at each edge and the rest of the enegy just goes straight through
• The frequency, wavelength and speed of the waves is not altered.... but the amplitude of the part of the wave that spreads out is lower - as the energy is distributed over a bigger area!

Diffraction can be demonstrated by using a ripple tank in the laboratory

Large gap in the barrier

If the gap is large, compared to the wavelength of the waves passing through ,the circular disturbances are tiny compared to the undistrubed wavefronts.

Therefore most of the energy just continues through without change.

But, at the edge, the straight wavefront breaks into circular fronts and so just the energy at the edges spreads out (diffracts).

The percentage of the total energy that gets through the gap that breaks up and spreads out (diffracts) is tiny compared to the total amount that gets through - therefore the majority of the energy just travels straight and you are hardly aware of the bit that 'spreads'

This means that:

large straight wavefronts get through the big gap - travelling undisturbed

a tiny disturbance at the edge of the straight wavefront leads to slight curving of the wavefronts at the edge and a slight spread

this leads to a 'shadow area' where very little of the energy travels to.

Small gap in the barrier

If the gap is small (about the same size as the wavelength of the wave passing through) the circular disturbances that get through are massive compared to the undistrubed wavefronts. Therefore hardly any of the energy just continues through without change. Rather, at the edges the straight wavefront breaks into circular fronts spreading the energy out behind the barrier (considerable diffraction occurs).

As this is the way that virtually all of the energy gets through:

only circular wavefronts are observed passing through the tiny gap in the barrier (instead of straight ones)

the energy spreads out behind the barrier

there is no shadow area behind the barrier.

• This 'getting of proper circular waves' is only really the case in practice if the gap is about the same size as the wavelength of the energy passing through it (or smaller!).
• Therefore the wavelength of the energy wave must be considered before deciding whether diffraction occurs to any great degree or not.
• But you have to remember that some of the signal is diffrated at the edge of a barrier whatever the wavelength - and the bigger the wavelength the bigger the effect!

Why can you hear around corners but not see round them?

A doorway is a tiny gap for a soundwave of wavelength 1.3m (a note of middle C) but is enormous for a light wave of wavelength 600 nm!

Therefore we can hear round corners, because the sound waves diffract around the edges of the doorway- making the bulk of the energy spread out in all directions through the 'gap' of the door.... but we can't see round doorways, beacuse a miniscule quantity of the light energy will diffract and the bulk of it will travel straight through.

Why do radiowave signals 'bend' around mountains into the valleys below whereas microwave (Cell phone) signals get blocked by them?

Radiowave signals have a much longer wavelength than the microwave (or very shortwave radiowaves - as cell phone companies prefer to call them!). Therefore the diffraction they suffer is much greater and the signal 'spreads' out into the valley area.... it is weaker than if it had been able to go through without the mountain there.... but it does get through!

Why does light split into its colours when it is diffracted?

The wave's appreciation of the 'size of the gap' depends on its wavelength. Red light has a bigger wavelength than blue light. therefore a gap looks smaller to a red ray of light than it does to a blue one! Therefore the red end of the spectrum of light diffracts more than blue end when white light is passed through a tiny gap (such as that found in a diffraction grating). 'Laser specs' have diffraction gratings as lenses and let you see lots of pretty coloured effects when you look at anything... they are letting you see diffraction patterns.

Diffraction Spikes

In the image on the right, the brightest stars can be seen to exhibit 'diffraction spikes'. These most commonly appear as four relatively faint projections radiating outward from the star arranged 90 degrees from each other, but can also appear as two projections on opposite sides of the star. Diffraction spikes are caused by light diffracting around the support vanes of the telescope's secondary mirror.

(taken from http://www.chapman.edu/oca/gallery2/artifact.htm)

• This is a great interactive page from the University of Salford!