When a light ray enters into glass medium from water at an angle of incidence 0°, what would be the angle of refraction?
- A.90°
- B.45°
- C.0°
- D.The ray will not enter at all
Correct answer
Explanation
The correct answer is C, 0°. An angle of incidence of 0° means the ray falls along the normal, and a ray along the normal passes into the next medium without bending.
Snell's law says n1 sin i = n2 sin r. When i is 0°, sin i is zero, so sin r must also be zero whatever the refractive indices of water (about 1.33) and glass (about 1.5) are, and r is 0°. The ray keeps its direction. Only its speed changes: light slows down as it enters the optically denser glass. For any other angle, a ray going from water into glass bends towards the normal.
Option A is wrong, because a refracted angle of 90° happens only at the critical angle, when light goes from a denser to a rarer medium. Option B is wrong, because nothing in the situation produces a 45° bend. Option D is wrong, because total internal reflection needs light travelling from a denser to a rarer medium at more than the critical angle, and glass is denser than water.
Exam tip: along the normal, no bending; rarer to denser, bends towards the normal.
Practice Questions
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