Question

A single slit Fraunhoffer diffraction pattern is formed with white light. For what wavelength of light the third secondary maximum in the diffraction pattern coincides with the second secondary maximum in the pattern for red light of wavelength 6500 $\stackrel{o}{A}$

• A. 4400 $\stackrel{o}{A}$
• B. 4100 $\stackrel{o}{A}$
• C. 4642.8 $\stackrel{o}{A}$
• D. 9100 $\stackrel{o}{A}$

Answer 1

The correct option is C 4642.8 $\stackrel{o}{A}$

$x=\frac{(2n+1)\lambda D}{2a}$
For red light, $x=\frac{(4+1)D}{2a}\times 6500\stackrel{o}{A}$
For other light, $x=\frac{(6+1)D}{2a}\times \lambda \stackrel{o}{A}$
$x$ is same for each.
$\therefore$ 5 $\times$ 6500 = 7 $\times \lambda$ $\Rightarrow \lambda$ = $\frac{5}{7}\times$ 6500 = 4642.8 $\stackrel{o}{A}$