Question

Q.1 Single Choice
A light ray in water strikes the water surface at an angle of 60° to the normal. The path of the ray
after striking is


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Answer 1

Given : A light ray in water strikes the water surface at an angle of 60° to the normal.

To find : The path of the ray after striking is...

solution : A light in water strikes the surface of water at an angle 60° to the normal.

from Snell's law,

μ₁sinθ₁ = μ₂sinθ₂

here, μ₁ = refractive index of water = 4/3

θ₁ = incident angle = 60°

μ₂ = refractive index of air = 1

θ₂ = refracted angle

so, 4/3 × sin60° = 1 × sinθ₂

⇒4/3 × √3/2 = sinθ₂

⇒sinθ₂ = 2/√3 [ not possible because sine can't be greater than 1. ]

here we see critical angle, C = sin¯¹ {1/μ}

= sin¯¹(3/4) = 48.59° > 60°

we know, if a beam of light strikes the surface at an angle which is greater than its critical angle, light doesn't refract but it reflects the same medium. this phenomenon is known as total internal reflection.

so the path of the ray after striking is shown in figure. i.e., light ray reflects in the same medium.