A small coin is resting on the bottom of a beaker filled with liquid. A ray of light from the coin strikes the surface of the liquid and moves along its surface. How fast is light travelling in the liquid?
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Question

A small coin is resting on the bottom of a beaker filled with liquid. A ray of light from the coin strikes the surface of the liquid and moves along its surface. How fast is light travelling in the liquid?

Options

G1 Physics · Accepted Answer

Step 1: The emerging ray grazes the surface, so incidence in liquid is the critical angle C. Step 2: From the diagram the ray forms a 3–4–5 triangle; sin C = 3/5 (verify the triangle legs on the source diagram). Step 3: At critical incidence, sin C = v_liquid/c. Thus v_liquid = (3/5)(3 × 10⁸) = 1.8 × 10⁸ m/s. Final answer: (D) 1.8 × 10⁸ m/s.