Number of Photons?

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.

(a) How many photons of red light (660 nm) are needed?

(b) How many photons of blue light (475 nm) are needed?

Answer 1

Please remember the formula between the energy and frequency or wavelength:
E = hν = hc/λ
where E the energy in J, h the Plank constant 6.626x10^-34 J•s, ν the frequency in 1/s or Hz, c the speed of light 2.998x10^8 m/s, and λ the wavelength in m (1 nm = 10^-9 m). Therefore one-photon energy of 660nm light is:
E = hc/λ = (6.626x10^-34 J•s)*(2.998x10^8 m/s)/(660x10^-9 m)
= 3.01x10^-19 J
Please complete the solution to (a) and continue with the same approach on (b).