## 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 * 10^{8}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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