The human eye can respond to as little as 10^−18 J of light energy.
For a wavelength at the peak of visual sensitivity, 550 nm, how many photons lead to an observable flash?

Question

The human eye can respond to as little as 10^−18 J of light energy.
For a wavelength at the peak of visual sensitivity, 550 nm, how many photons lead to an observable flash?

anonymous · Answer

The key equation is going to come from Mr Planck:$$E=h \nu $$where h is Planck's constant; and ν is the frequency.  This equation gives you the energy per photon at a given frequency.  Alas, you're given wavelength, but that's easy enough to convert to frequency given the  following equation:$$c=\lambda \nu $$where c is the speed of light; λ is the wavelength; and ν is again frequency.  As soon as you know the energy of a photon with a wavelength of 550nm, you should know how many photons you would require to accumulate 10^-18J.  Be careful with your units.