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MCAT Question A Day - 9/9/13 - Answer!

The human eye can respond to light with a total energy of as little as 10-18 J. If red light has a wavelength of 600 nm, what is the minimum number of red light photons that the eye can perceive?

A.  1
B.  2
C.  3
D.  5

The correct answer is (C). E = hc/l = (6.63 * 10-34 J s)(3.00 * 108 m/s)/(600 * 10-9 m) = 3.31 * 10-19 J. This is the energy of each red photon. The number of such photons needed to produce a total of 10-18 J of energy is:

(10-18 J)/(3.31 * 10-19 J/photon) ~ 3 photons

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