Question

The human eye is a complex sensing device for visible light. The optic nerve needs a minimum of 2.0 × 10−17 J of energy to trigger a series of impulses that eventually reach the brain. How many photons of blue light (475 nm) are needed?

Answer 1

number of photons can be calculated by dividing the needed energy by the energy per photon.

The minimum energy needed is given as 2 x 10^-17 joules
Energy per photon = hc / lambda where h is planck's constant, c is the speed of light and lambda is the wavelength
Energy per photon = (6.626 x 10^-34 x 3 x 10^8) / (475 x 10^-9)
                              = 4.18 x 10^-19 J

number of photons = (2 x 10^-17) / (4.18 x 10^-19)
                               = 47.79 photons which is approximately 48 photons