A monochromatic beam of constant power and wavelength $600 nm$ gives a saturation photocurrent $I_{1}$ on a metal. The wavelength is changed to $400 nm$ at the same power (still above threshold), and the fraction of photons that eject an electron stays the same. The new saturation current is

A $\frac{3}{2} I_{1}$
B $\frac{2}{3} I_{1}$
C $I_{1}$
D $\frac{4}{9} I_{1}$

Answer: B

Photons per second = P/(hc/

λ

), proportional to

λ

at fixed power. Current ratio = 400/600 = 0.667 = 2/3. Shorter wavelength means more energetic photons, hence fewer photons per second and a smaller current.

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A monochromatic beam of constant power and wavelength 600 nm gives a saturation photocurrent I1​ on a metal. The wavelength is changed to 400 nm at the same power (still above threshold), and the fraction of photons that eject an electron stays the same. The new saturation current is

Answer: B

More questions on Quantum theory of light and photoelectric effect

A monochromatic beam of constant power and wavelength $600 nm$ gives a saturation photocurrent $I_{1}$ on a metal. The wavelength is changed to $400 nm$ at the same power (still above threshold), and the fraction of photons that eject an electron stays the same. The new saturation current is