Ask A Scientist© Zoology Archive

name       Bill
status     other
age        60s

Question - Many people hit deer while driving at night.
Why can't you see deer eyes glow at night when you drive past them?

The commonly observed phenomena of animals' eyes "glowing in the dark" is a
result of some exterior light, usually a flashlight, spotlight or headlight,
reflecting back from the animal's eyes. You will not see it unless you, the
light, and the animal are at the right angles - you will only see a deer's
eyes glow in headlights if the deer is more or less in front of the car and
its eyes turned just right toward you.
J. Elliott
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This is a complex area of optics. It involves the same "red eye" effect in
flash photography. Different animals reflect different colors. The
keywords to look this up include: retroreflectivity and dew
heiligenschein. Robert Greenler refers to this in a chapter on diffraction
in his book _Rainbows, Halos, and Glories_ , Cambridge University Press 1980.

This effect is used by 3M in their reflective tapes and in highway
paint. Small spheres reflect the light back. Two other technologies --
corner cube reflection (Stimsonite) and parabolic reflectors (illumiNITE)
-- are also retroreflective, but you can skip those in your search.

---Nathan A. Unterman---
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Bill,
Many animals with 'night vision' have reflective pigment behind the retinal
photoreceptors at the back of their eyes. Light coming into the eye
therefore stimulates the photoreceptors on the way in and once again as it
reflects back through them. That doubles their perception of the light's
brightness. If you shine a light directly into their eyes at night, you see
the reflected light from that pigment on the back of their eyes. Unless the
animal looks directly at the source of the light, you won't get the full
effect.
CED

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