Light refracts when traveling from air into glass because light

Question

BlackTom AI · Accepted Answer

When light passes from a less optically dense medium (air) into a more optically dense medium (glass), its propagation speed decreases; this change in speed is the fundamental reason for refraction.

Relevant concepts:
- Wave relation: v = f·λ (speed = frequency × wavelength).
- Refractive index: n = c/v, so a larger n means a smaller v.
- Boundary conditions: the frequency f of a light wave remains the same across the boundary because the oscillation of the electromagnetic field must be continuous in time at the interface.
- Snell’s law (derived from change of wave speed): n1 sinθ1 = n2 sinθ2, which can be written as sinθ2/sinθ1 = v2/v1.

Step-by-step reasoning:
1. In air the speed v_air is approximately c (speed of light in vacuum); in glass v_glass = c/n_glass where n_glass > 1, so v_glass < v_air.
2. Frequency continuity: f_air = f_glass because the time dependence of the wave cannot abruptly change at the boundary.
3. Using v = f·λ, if f is constant and v decreases on entering glass, the wavelength λ must decrease proportionally: λ_glass = v_glass / f < λ_air.
4. The change in wavelength (and hence wavefront geometry) combined with the differing speeds on either side produces a change in propagation direction given by Snell’s law. Therefore light bends (refracts) when entering glass because it travels more slowly there.

Why the correct answer is correct:
- Option d states that light “travels slower in glass than in air.” That slower speed is the direct cause of refraction (with frequency unchanged), so this is the correct physical explanation.

Why the other options are incorrect:
- a (same speed): false — speed changes because glass has a higher refractive index.
- b and c (frequency differs): false — frequency remains constant across the boundary; only wavelength and speed change.

Conclusion: The correct answer is d. Light refracts entering glass because it travels slower in glass than in air.

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