A beam of light is incident from air to water at an angle of 30$^o$. Find the angle of refraction if the refractive index of water is 4/3

Analyze the question: Light moves from air into water. The angle of incidence is i = 30°, and the refractive index of water is n = 4/3. We need the angle of refraction r.

Concept: Snell's law says

n = sin i / sin r

so sin r = sin i / n.

Solution:

1. sin 30° = 0.5
2. sin r = 0.5 ÷ (4/3) = 0.5 × 3/4 = 0.375
3. r = sin⁻¹(0.375)

To find the angle, use known values:

• sin 22° ≈ 0.3746
• sin 22.02° ≈ 0.375

So r ≈ 22°.

Check the other options:

• sin 15° = 0.259
• sin 18° = 0.309
• sin 24° = 0.407

None of these is close to 0.375, so only 22° fits.

Answer: C. 22°.

Tip: When light goes from air into a denser medium, it bends toward the normal, so r must be smaller than i. That means r is less than 30°. Always compute sin r = sin i / n, then match it to the nearest sine value. Learn that sin 30° = 0.5, because it appears in many refraction questions.