JEE Advanced · Physics · 24. Ray Optics
Two identical concave mirrors each of focal length \(f\) are facing each other as shown in the schematic diagram. The focal length \(f\) is much larger than the size of the mirrors. A glass slab of thickness \(t\) and refractive index \(n_0\) is kept equidistant from the mirrors and perpendicular to their common principal axis. A monochromatic point light source \(S\) is embedded at the center of the slab on the principal axis, as shown in the schematic diagram. For the image to be formed on \(S\) itself, which of the following distances between the two mirrors is/are correct:

- A \(4 f+\left(1-\frac{1}{n_0}\right) t\)
- B \(2 f+\left(1-\frac{1}{n_0}\right) t\)
- C \(4 f+\left(n_0-1\right) t\)
- D \(2 f+\left(n_0-1\right) t\)
Answer & Solution
Correct Answer
(B) \(2 f+\left(1-\frac{1}{n_0}\right) t\)
Step-by-step Solution
Detailed explanation

\(\begin{aligned} & \frac{S-t}{2}+\frac{t}{2 n_0}=2 f \\ & S+t\left(\frac{1}{n_0}-1\right)=4 f \\ & S=4 f+\left(1-\frac{1}{n_0}\right) t\end{aligned}\)
Also
\(\begin{aligned} & \frac{\mathrm{S}-\mathrm{t}}{2}+\frac{\mathrm{t}}{2 \mathrm{n}_0}=\mathrm{f} \\ & \mathrm{S}=2 \mathrm{f}+\left(1-\frac{1}{\mathrm{n}_0}\right) \mathrm{t}\end{aligned}\)
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