Photoelectric effect

Question

A 100 W sodium lamp gives off light with a wavelength of 589 nm uniformly in all directions. A totally absorbing screen is placed at a distance of 10 km from the lamp. If the screen has an area of 1.0 m² , what is the maximum rate of photons absorbed by the screen?

Please explain the step clearly. Thank you.

Answer 1

Hi there.

Start with how many photons are emitted from the point source per unit time.

Power = 100W = Energy/time = number of photons/time x energy per photon

Energy per photon = hc/l, where l is wavelength, h and c constants, l you know. This will be a very big number, h is very small (1e-34)

Then multiply the number of photons/time by 1m2 collection screen divided by 4 pi r^2, r = 10km. This will be a small number.

Obviously be careful with units and carry lots of sig figs (round off at the very end).