A ray of light, travelling in the direction 1 2 i ^ + 3 j ^ , is incident on a plane mirror. After…

A ray of light, travelling in the direction 12i^+3 j^, is incident on a plane mirror. After reflection, it travels along the direction 12i^-3 j^. The angle of incidence is
  1. 30°
  2. 45°
  3. 60°
  4. 75°

Solution

α = angle between vectors
cosα=12 i^+3 j^·12 i^-3 j^i^+3 j^i^-3 j^
          = 1 4 1 - 3 1 × 1 = - 2 4 = - 1 2
α=120°=2π3
∴   π-α=2θ
π = 2 π 3 = 2 θ
θ=π3×2=π6=30°

Asked in: JEE Advanced 2013 (Paper 1)

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