A light ray travels from air into water with angle of incidence 30 degrees. Using Snell's law, what is the angle in water?

A light ray travels from air into water with angle of incidence 30 degrees. Using Snell's law, what is the angle in water?

• A. 18°
• B. 22.1°
• C. 30°
• D. 45°

Answer: (B)

When light crosses a boundary between two media, its direction changes according to Snell's law: n1 sin θ1 = n2 sin θ2. Here, air has a refractive index about 1.00 and water about 1.33, so sin θ2 = (n1/n2) sin θ1 = (1/1.33) sin 30°. Since sin 30° = 0.5 and 1/1.33 ≈ 0.752, sin θ2 ≈ 0.376. The angle in water is θ2 ≈ arcsin(0.376) ≈ 22.1°. Light slows and bends toward the normal in the denser medium, so the angle becomes smaller than the incident angle. That’s why the angle in water is about 22.1°, matching the given result.

When light crosses a boundary between two media, its direction changes according to Snell's law: n1 sin θ1 = n2 sin θ2. Here, air has a refractive index about 1.00 and water about 1.33, so sin θ2 = (n1/n2) sin θ1 = (1/1.33) sin 30°. Since sin 30° = 0.5 and 1/1.33 ≈ 0.752, sin θ2 ≈ 0.376. The angle in water is θ2 ≈ arcsin(0.376) ≈ 22.1°. Light slows and bends toward the normal in the denser medium, so the angle becomes smaller than the incident angle. That’s why the angle in water is about 22.1°, matching the given result.