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Re: Cat's eyes

At 05:07 PM 9/3/00 -0400, Ludwik Kowalski wrote:

Sorry I missed this part, a lens with a flat mirror in the focal
plane (whose normal is parallel to the lens axis) does indeed
send the beam of parallel light backward.

OK.

But only when the beam is strictly parallel to the lens axis.

Really? That's news to me. Strictly?? Zero degrees????

Perhaps your ray-tracing muscle needs exercise:

1) Derive a general expression for how the efficiency of retroreflection
falls off as a function of angle (and of numerical aperture). Do the
one-dimensional case, to keep things simple.

Hint: the mirror is in the FOCAL PLANE.

2) Estimate the numerical aperture of a dark-adapted kitty.

3) Combine (1) and (2) to get a value for the angle, for comparison with
experiment.