A 5 W isotropic source emits monochromatic light of wavelength 5000 Å. At a distance 0.5 m it liberates photoelectrons from a photosensitive surface. If the source is moved to 1.0 m, by what factor is the number of photoelectrons liberated reduced?
Physics · Photoelectric effect and dual nature of matter
JEE Mains Advanced

Question

A 5 W isotropic source emits monochromatic light of wavelength 5000 Å. At a distance 0.5 m it liberates photoelectrons from a photosensitive surface. If the source is moved to 1.0 m, by what factor is the number of photoelectrons liberated reduced?

Physics · Photoelectric effect and dual nature of matter

G1 Physics · Accepted Answer

Intensity from an isotropic point source varies as 1/r². Moving from 0.5 m to 1.0 m reduces intensity, and hence the photoelectron rate, by (1/0.5)² = 4. Required factor: 4.