Question:medium

The light ray is incident at angle of $60^{\circ}$ on a prism of angle $45^{\circ}$. When the light rays falls on the other surface at $90^{\circ}$, the refractive index of the material of prism $\mu$ and the angle of deviation $\delta$ are given by

Show Hint

When a ray emerges normally, $e = 0$ and $r_2 = 0$.
Updated On: May 3, 2026
  • $\mu = \sqrt{2},\delta = 30^{\circ}$
  • $\mu = 1.5,\delta = 15^{\circ}$
  • $\mu = \frac{\sqrt{3}}{2},\delta = 30^{\circ}$
  • $\mu = \sqrt{\frac{3}{2}},\delta = 15^{\circ}$
Show Solution

The Correct Option is D

Solution and Explanation

The problem involves determining the refractive index and angle of deviation for a light ray passing through a prism. Here’s how to solve the question step-by-step:

Step 1: Understanding the Problem

We have a prism with an angle of \(45^{\circ}\). A light ray is incident at an angle of \(60^{\circ}\), and it emerges at \(90^{\circ}\) on the other surface. We need to find the refractive index \(\mu\) of the prism's material and the angle of deviation \(\delta\).

Step 2: Applying Snell's Law at First Surface

The incident angle \(i = 60^{\circ}\) and prism angle \(A = 45^{\circ}\).

Using Snell's Law, at the first surface:

\(\mu = \frac{\sin(i)}{\sin(r_1)}\)

Where \(r_1\) is the angle of refraction at the first surface.

Step 3: Total Internal Reflection

The light ray should satisfy the prism's condition:

\(r_2 = 90^{\circ} - A = 90^{\circ} - 45^{\circ} = 45^{\circ}\)

Using the prism formula, \(A = r_1 + r_2\), substitute the values:

\(45^{\circ} = r_1 + 45^{\circ}\)

Solving gives \(r_1 = 0^{\circ}\).

Step 4: Calculating Refractive Index

Using Snell's Law:

\(\mu = \frac{\sin(60^{\circ})}{\sin(0^{\circ}+45^{\circ})} = \frac{\sqrt{3}/2}{1/\sqrt{2}}\)

Calculating gives \(\mu = \sqrt{\frac{3}{2}}\).

Step 5: Calculating Angle of Deviation

The angle of deviation, \(\delta\), is given by:

\(\delta = i + e - A\)

Given that the emergent angle (e) is \(90^{\circ}\),

\(\delta = 60^{\circ} + 45^{\circ} - 45^{\circ} = 15^{\circ}\).

Conclusion

Thus, the refractive index \(\mu\) is \(\sqrt{\frac{3}{2}}\) and the angle of deviation \(\delta\) is \(15^{\circ}\), which corresponds to the correct answer:

\(\mu = \sqrt{\frac{3}{2}},\delta = 15^{\circ}\)

Was this answer helpful?
0