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Name: Bobby G. P.
Status: N/A
Age: 60s
Location: N/A
Country: N/A
Date: Thursday, August 22, 2002

How close must two observers be so both can see a single photon?

A single photon can only be seen by one observer. The process of detecting a photon destroys it, i.e., converts the energy to another form. In the case of the eye, it converts the photon's energy to chemical energy.

Actually, the eye is not sensitive enough to "see" a single photon.

Greg Bradburn


For two observers to "see the same photon", they must be the same person. To see a photon, one of the nerves on your retina absorbs the photon. This is also how a radio antenna "sees" a radio wave. One beam of light has millions or billions of photons in it. Exactly how many depends on the intensity of the light (i.e. brightness) and how wide the beam is. Most of the visible light photons that enter your eye get absorbed, never to exist again. The little bundle of energy carried by the photon is transferred to a nerve. The nerve transports the energy to your brain.

Many photons you cannot see such as radio and television photons will pass right through you. You never see them. Infrared photons do not get absorbed by the retina of your eye, but they can be absorbed by your flesh. This is one of the mechanisms your body uses to get warmed by a fire. High energy photons such as ultraviolet light and x-rays cannot be seen either. Still, anybody who has had sunburn has definitely felt his skin absorb ultraviolet light.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College

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