Photoelectric Effect Applet
Einstein in 1905 explained
this phenomenon by looking at light as packets of energy
(photons).
For example:
The energy of Blue Light
with wavelength about 400nm
E = hf = hc/λ = 6.626x10^-34 J.S x 3 x
10^8m/s / 400 x 10^-9m
Energy of Blue Light =
4.97 x 10^-19 Joules
Energy of Red Light at 700nm = 2.84 x
10^-19 Joules
We can see there is a difference of energy between the red and blue light.
Now if we had a material that can only eject electrons when 3.00 x 10^-19
Joules or more
was absorbed, then no matter how many strong the red light beam, nothing will ever emit.