Photon Calculation

Question

The human eye is able to detect as little as 2.35×10−18 J2.35×10−18 J of green light of wavelength 510 nm. Calculate the minimum number of photons of green light that can be detected by the human eye. Consult the table of physical constants as needed.

Answer 1

E = hc/λ

E = energy of 1 photon

h = Planck's constant = 6.626x10^-34 Js

c = speed of light = 3x10⁸ m/s

λ = wavelength in meters = 510x10^-9 m

E = (6.626x10^-34 Js)(3x10⁸ m/s) / 510x10^-9 m

E = 3.90x10^-19 J per photon

2.35x10^-18 J x 1 photon / 3.90x10^-19 J = 6.0 photons that can be detected by the human eye under these conditions.