Question

The threshold wave length for photo electric emission from a material is 5,200 $A^0$. Photo electrons will be emitted when this material is illuminated with monochromatic radiation from a :

• A. 50 watt infrared lamp
• B. 1 watt infrared lamp
• C. 1 watt ultraviolet lamp
• D. 50 watt sodium vapour lamp

Answer 1

The correct option is C 1 watt ultraviolet lamp

Threshold wave length $=5,200A^0=520nm$
energy $=\frac{hc}{\lambda }$

$=\frac{1240}{520}eV(hc=1240eV-nm)$

$=2.38eV$

So, threshold energy is 2.38 eV

So, for ejection of electrons energy of incident rays should be more than 2.38 eV.

In the given option only ultraviolet rays has energy greater than 2.38 eV which is 3.1 eV to 124 eV and infrared, sodium vapour lamp has energy less than 2.38 eV.
So, only ultraviolet lamp can be used for photoelectric effect.