The potential difference that must be applied to stop the fastest photoelectrons emitted by a nickel surface, having work functions 5.01 eV, when ultraviolet light of 200 nm falls on it, must be

A source S₁ is producing, 10¹⁵ photons per second of wavelength 5000 $$\mathop A\limits^ \circ $$. Another source S₂ is producing 1.02 $$ \times $$ 10¹⁵ photons per second of wavelength 5100 $$\mathop A\limits^ \circ $$. Then, (power of S₂)/(power of S₁) is equal to

The number of photo electrons emitted for light of a frequency $$\upsilon $$ (higher than the threshold frequency $${\upsilon _0}$$) is proportional to

Monochromatic light of wavelength 667 nm is produced by a helium neon laser. The power emitted is 9 mW. The number of photons arriving per sec. on the average at a target irradiated by this beam is