In black and white photography, a photon energy of about 4.00 x 10 to the negative 19th power I is needed to bring about the changes in the silver compounds used in the film. Explain why a red light used in a darkroom does not affect the film during developing.

Answer 1

for red light λ=0.62 – 0.76 μm
max f=c/ λ=3•10⁸/0.62•10⁻⁶=4.84•10¹⁴ Hz
ε=hf=6.63•10⁻³⁴•4.84•10¹⁴ =3.2•10⁻¹⁹ J
Since 3.2•10⁻¹⁹ J < 4•10⁻¹⁹ J red light does not affect the film .