# Beams of light and wave effects .

1. Nov 10, 2004

### Cheman

Beams of light and wave effects.....

Let us first think of a ripple tank and how this is used to demonstrate diffraction nad interference- this is one wave front so when passes through small gap curves the wavefront, spreads the wave out and thus causes diffraction. We often use this analogy to describe light.

But surely a beam of light is very different to a water wave for the simple reason that it is not just one wave but lots of wave chains travelling together in a beam. So how do we explain why diffraction of a beam of light is the same as you would expect from 1 single wave like a water wave?

2. Nov 10, 2004

### Integral

Staff Emeritus
Suppose you had multiple sources of water waves, would you not still observe the diffraction through a single slot? The presence of waves of differing frequency's do not effect diffraction effects, only make it harder to see clearly. That means that the wave patterns in your wave tank will be very complex.

Both cases of waves, water and light are modeled by the same differential equation, in a single spatial dimension the wave equation is:

$$U_{tt} = \alpha U_{xx}$$

Since the waves obey the same mathematics, why would we expect them to behave differently?

3. Nov 11, 2004

### Laser Jock

The key in seeing these interference and diffraction effects with light is coherence. If all the "wave chains" are in phase with each other (ie. coherent), then these effects are easily visible and the behavior is identical to waves in a ripple tank. As the light becomes less coherent, these effects can become washed out.