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A fish looking up through the water sees the outside world contained in a circular horizon. If the refractive index of water is 4/3 and the fish is 12 cm below the surface, the radius of this circle (in cm) is

Asked in JEE Main 2005 · Total internal reflection and optical fibres

• (1) 36√5
• (2) 4√5
• (3) 36√7
• (4) (36)/(√7)

Step-by-step solution

Why the other options are wrong

• (1) Uses √5 in place of √7 and multiplies instead of dividing.
• (2) Takes cos C = (√5)/3 with a wrong Pythagorean step.
• (3) Multiplies by √7 instead of dividing, i.e. uses cot C in place of tan C with a factor slip.

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