refraction and diffraction

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From: Paul Doherty (pauld@exploratorium.edu)
Date: Fri Oct 06 2000 - 10:09:46 PDT 

Message-Id: <l0311071eb603b74e8784@[192.174.2.173]>
Date: Fri, 6 Oct 2000 09:09:46 -0800
From: Paul Doherty <pauld@exploratorium.edu>
Subject: refraction and diffraction

Paul

So, Paul, without using QED, or any three syllable words, why do different
wave lengths of light bend at different angles when passing from one index
of refraction material to another?

Also, what is your best simple model for how a diffraction grating works?

Curt G

Curt

So about dispersion.
Every atom has an electron cloud that oscillates with a resonant frequency
in the ultraviolet.
The closer the frequency of the light is to the ultraviolet the more the
electron cloud of those atoms is driven into resonant oscillation and the
more the light interacts with the atoms.
Therefor blue light interacts more, slows down more, and bends more than
red light.

To understand a diffraction grating start with two slits.
Light is a wave.
It goes through both slits (1800 model).
When the light goes through a slit it spreads out.
The light from the two slits adds together (interference) and makes a pattern.
In the pattern the angle to the first maximum to each side of the central
maximum is a function of the wavelength of the light so red light bends
more than blue. Opposite prisms.

When you add more slits to make a diffraction grating the first maximum to
the side remains at the same angle but the angular spread of the resulting
interference pattern gets smaller.

Hope this helps.

Paul D

Paul "But it is more complicated than that!" Doherty,
Senior Staff Scientist, The Exploratorium.
pauld@exploratorium.edu, www.exo.net/~pauld

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