Re: questions about light

laura wrote:

2. How can I change the wave-length of a given light ray?

i) In vacuo, you can do this simply by changing your velocity relative
to the source, or vice versa.
ii) I am pretty sure that you also do this when the light ray is
passing through a refractive medium. Since the energy of a given photon
(E=hf) remains constant, this means that the frequency f remains
constant. Therefore, since the speed of light c is reduced in a
refractive medium, this means that its wavelength (lambda = c/f) must
also be reduced.
The reason why you don't see fish change colour when you stick your
head in a fish pond or aquarium is that the cones in the back of your
eye respond to the wavelength of light in the viscous medium of the
eye, not to the wavelength in the water.

What is the
greatest precision achieved for this operation?

In the case of i) above, such Doppler shifts can be measured much more
accurately using diffraction gratings to compare emission or absorbtion
spectra of known molecules, than by passing the light through coloured
media (as suggested by Hontas Farmer)

John Bell
2nd discussion moderator at http://global.accelerators.co.uk
(Change John to Liberty to respond by email)

.

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