Question

If the sodium light (yellow color) in Young's double slit experiment, is replaced by red light, the fringe width will,

• A. decrease
• B. increase
• C. remain constant
• D. first increase then decrease

Answer 1

The correct option is B increase

Fringe width is given by,
$\beta =\frac{\lambda D}{d}$

Sodium light is of yellow color and,
${\lambda }_{red}>{\lambda }_{yellow}$

Since, $\beta$ is directly proportional to the wavelength of light, so fringe width will increase, if sodium light is replaced with red light.

Hence, $\left(B\right)$ is the correct answer.
$$\begin{array}{c}\text{Why this question:}\\ \text{Tip: Fringe width increases with increase in wavelength}\\ \text{of light used in YDSE. In case of VIBGYOR, fringe width}\\ \text{will be minimum for violet light and maximum}\\ \text{for red light}\end{array}$$