Question

The threshold wavelength for photoelectric emission from a material is 5200$\dot{A}$. Photoelectrons will be emitted when this material is illuminated with monochromatic radiation from a

• A. 50 watt infrared lamp
• B. 1 watt infra-red lamp
• C. 50 watt ultraviolet lamp
• D. 1 watt ultraviolet lamp

Answer 1

Answer: (C), (D)

1 watt ultraviolet lamp

By now you know that intensity is not going to affect photocurrent. If the energy of the photon Is not sufficient then no matter how many of it you shoot at the emitter , it will never eject a photo electron.

Threshold wavelength is 5200 angstrom. = $5.2\times {10}^7$m

Energy $=\frac{hc}{\lambda }$

Photo electrons come out only if they are hit by a photon of certain energy or higher.

The wavelength corresponding to that energy is termed as threshold wavelength.

Since energy is inversely proportional to wavelength, for photo electrons to come out the wavelength of incident light should be less than threshold wavelength.

Infrared wavelength = ${10}^{-4}$m

Ultraviolet wavelength = ${10}^{-8}$m

So infrared has more wavelength than threshold while ultraviolet has lesser.

So both c and d sources will emit photo electrons.