Question

The threshold wavelength for photo electric emission from a material is $5000\mathring{A}.$ Photo electrons will be emitted when the material is illuminated with monochromatic radiation from-

• A. $25\text{W}-$ neon lamp
• B. $100\text{W}-$ infrared lamp
• C. $50\text{W}-$ ultra - violet lamp
• D. $50\text{W}-$ sodium vapour lamp

Answer 1

The correct option is C $50\text{W}-$ ultra - violet lamp

For photo electric emission, the incident light wavelength must be less than the threshould wavelength i.e.,

$\lambda >{\lambda }_{\text{incident}}$

Only ultra - violet light's wavelength is less than the threshould wave length i.e., $5000\mathring{A}$

<!--td {border: 1px solid #ccc;}br {mso-data-placement:same-cell;}--> Hence, $\left(C\right)$ is the correct answer.

Hence photo - electric emission will take place for only ultra - violet lamp.