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?

Answer 1

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.