Question

The threshold frequency of a photosensitive surface is $5\times {10}^{14}$Hz. Then which of the following will produce photoelectric effect from the same surface?

• A. Sodium vapour lamp
• B. Ruby laser
• C. He-Ne laser
• D. Both (b) and (c)

Answer 1

Answer: (A)

Sodium vapour lamp

Wavelength of light given by sodium vapour lamp $\lambda =589nm$
Thus frequency of light $\nu =\frac{c}{\lambda }$
$⟹\nu =\frac{3\times {10}^8}{589\times {10}^{-9}}=5.09\times {10}^{14}Hz$
which is greater than the threshold frequency of the given photosensitive surface.
Since photoelectric effect takes place from the metal surface only if the frequency of incident photon of light is greater than the threshold frequency of the metal surface.
Here this condition is satisfied, so sodium vapour lamp will produce the photoelectric effect from the given photosensitive surface.