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MCQEasyJEE 2023Reflection & Spherical Mirrors

JEE Physics 2023 Question with Solution

When one light ray is reflected from a plane mirror with 30∘30^\circ angle of reflection, the angle of deviation of the ray after reflection is:

  • A

    140∘140^\circ

  • B

    130∘130^\circ

  • C

    120∘120^\circ

  • D

    110∘110^\circ

Answer

Correct answer:C

Step-by-step solution

Standard Method

Given: Angle of reflection r=30∘r = 30^\circ.

Find: The angle of deviation after reflection from a plane mirror.

According to the law of reflection,

i=ri = r

Therefore,

i=30∘i = 30^\circ

The angle of deviation δ\delta between the incident ray and the reflected ray is

δ=180∘−2i\delta = 180^\circ - 2i

Substituting i=30∘i = 30^\circ,

δ=180∘−2(30∘)\delta = 180^\circ - 2(30^\circ) δ=180∘−60∘=120∘\delta = 180^\circ - 60^\circ = 120^\circ

Therefore, the angle of deviation is 120∘120^\circ. The correct option is C.

Common mistakes

  • Using the angle of reflection directly in a wrong formula such as δ=180∘−r\delta = 180^\circ - r. This is incorrect because deviation for reflection from a plane mirror depends on the angle between the incident and reflected rays, so use δ=180∘−2i\delta = 180^\circ - 2i with i=ri = r.

  • Confusing the angle with the mirror surface and the angle with the normal. The law of reflection uses angles measured from the normal, not from the mirror surface. Always check how the angle is defined before substitution.

  • Assuming deviation is 2i2i instead of 180∘−2i180^\circ - 2i. That gives the smaller angle between the ray directions in a different interpretation. For plane-mirror reflection, use the standard deviation expression shown in the solution.

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