CBSE 2025, Set 3

Question

**Assertion (A):** A ray of light is incident normally on the face of a prism. The emergent ray will graze along the opposite face of the prism when the critical angle at glass-air interface is equal to the angle of the prism.

**Reason (R):** The refractive index of a prism depends on angle of the prism. A). Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). B). Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). C). Assertion (A) is true, but Reason (R) is false. D). Both Assertion (A) and Reason (R) are false.

Edvaya · Accepted Answer

The correct answer is Assertion (A) is true, but Reason (R) is false.. **Step 1: Check Assertion (A)**
When a ray is incident normally on the first face, the angle of incidence $i = 0$ and angle of refraction $r_1 = 0$. For a prism, the angle $A = r_1 + r_2$, so $r_2 = A$. The ray reaches the second face at an angle of incidence $r_2$. For the ray to graze the opposite face, it must strike at the critical angle $i_c$. Therefore, $r_2 = i_c$. Thus, $A = i_c$. (Assertion is **True**).

**Step 2: Check Reason (R)**
The refractive index of a material is an intrinsic property that depends on the nature of the material and the wavelength of light used. It does NOT depend on the physical angle of the prism. (Reason is **False**).

Thus, A is true, but R is false.

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Physics > Optics > Ray Optics And Optical Instruments

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