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Light is incident from a medium into air at two possible angles of incidence (a) 20° and (b) 40°. In the medium light travels 3.0 cm in 0.2 ns. The ray will

Asked in JEE Main Online 2013 · Total internal reflection and optical fibres

• (1) suffer total internal reflection in both cases (a) and (b)
• (2) suffer total internal reflection in case (b) only
• (3) have partial reflection and partial transmission in case (b)
• (4) have 100% transmission in case (a).

Step-by-step solution

Why the other options are wrong

• (1) 20° is below the critical angle of 30°, so case (a) is refracted out.
• (3) 40° exceeds the critical angle of 30°, so there is no transmission in case (b).
• (4) Even below the critical angle some light is always reflected, so transmission is never 100%.

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