Q142 / 160 · TS EAMCET 2023 (12 May Shift 2) · Physics
If the slit width is $2 \mathrm{~mm}$ and wavelength of light used is $4000 Å$, then Fresnel distance is nearly
- A$2 \mathrm{~mm}$
- Correct answerB$10 \mathrm{~m}$
- C$20 \mathrm{~km}$
- D$2 \mu \mathrm{m}$
Solution
(b) Slit width $\mathrm{a}=2 \mathrm{~mm}=2 \times 10^{-3} \mathrm{~m}$. $\lambda=4000 \mathrm{~A}^{\circ}=4 \times 10^{-7} \mathrm{~m}$.
Fresnel distance: $\mathrm{Z}_{\mathrm{F}}=\frac{\mathrm{a}^2}{\lambda}=\frac{4 \times 10^{-6}}{4 \times 10^{-7}}=10$
$$
\mathrm{Z}_{\mathrm{F}}=10 \mathrm{~m}
$$
Hence, the correct answer is option: 'B'.