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Re: [Phys-l] refraction question



On Fri, Apr 23, 2010 at 7:49 AM, Anthony Lapinski
<Anthony_Lapinski@pds.org> wrote:
...suppose you shine red light (say, 680 nm) from
air into water. Since n = 1.33, both the velocity and wavelength will
decrease by this factor. Thus, (680 nm)/1.33 = 511 nm. This is the
wavelength of green light! We've probably all done this demo with a red
laser, and the beam remains red.

The beam remains red, because the frequency doesn't change as you
change medium, and ultimately all light has to go through your
vitreous humour before it hits your retina. It doesn't matter what
colour you think it should be in the water, you can't experience that
colour/wavelength, just as you can't experience the colour/wavelength
of light in air. (Unless someone does something *really* nasty to your
eyes...)

So color depends on frequency. In class I
usually say that color depends on wavelength. Lasers are rated by their
wavelength. Instead, should lasers -- like tuning forks -- be rated by
their frequency since this quantity never changes?

For the interference experiments I do in high school, knowing the
wavelength is more useful since that is what we use in our equations
(e.g. double slit expt.). The interference pattern is being formed in
air (well, on the screen, but you know what I mean, I hope). It would
be interesting to do this underwater -- you would still see a red
pattern (vitreous humour and all that), but the spacing would be
different. In that case, the frequency would be the more useful
quantity.

Mike

--
Michael Porter
Colonel By Secondary School
Ottawa, Ontario, Canada