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?

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