Q. A slit of width $a$ is illuminated by red light of wavelength $6500\mathring{\mathrm{A}}.$ If the first minimum falls at $\theta = 30{^\circ},\ $ the value of $a$ is
A) $6.5 \times 10^{- 4}mm$
B) 1.3 micron
C) $3250\mathring{\mathrm{A}}$
D) $2.6 \times 10^{- 4}cm$
Explanation
(b) According to principle of diffraction
$a\sin\theta = n\lambda$
Where $n$ = order of secondary minimum
$\therefore a\sin{30{^\circ} = 1 \times \left( 6500 \times 10^{- 10} \right)}$
Or $a = 1.3 \times 10^{- 6}\ m$
Or $a = 1.3\ micron$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.