Question

Two coherent monochromatic light beams of intensities $I$ and $4I$ are superposed. The maximum and minimum possible intensities in the resulting beam are:

• A. $5I$ and $I$
• B. $5I$ and $3I$
• C. $9I$ and $I$
• D. $9I$ and $3I$

Answer 1

The correct option is C $9I$ and $I$

Given intensities of the beams are $I_1=I,I_2=4I$
Maximum intensity
$I_{\text{max}}={(\sqrt{I_1}+\sqrt{I_2})}^2={(\sqrt{I}+\sqrt{4I})}^2=9I$
Minimum intensity
$I_{\text{min}}={(\sqrt{I_1}-\sqrt{I_2})}^2={(\sqrt{I}-\sqrt{4I})}^2=I$