Physics · Wave Optics · NEET
In a denser medium light moves slower (v2 is less than v1). Picture a wide plane wavefront hitting the surface at an angle. One edge of the wavefront reaches the surface first and enters the slow medium, so it advances only a short distance while the other edge is still racing in the fast medium. Because one side lags, the whole wavefront pivots. Since the rays are always perpendicular to the wavefront, the new direction is turned toward the normal. That is why r is less than i (bends toward normal) when going rarer to denser.
It is the speed change, not the surface material by itself. Huygens wave theory shows the amount of bending depends only on the ratio of speeds: sin i / sin r = v1 / v2 = n2 / n1. The surface is just where the two speeds meet. If both media had the same speed there would be no bending at all, even at a boundary.
No. Frequency stays the same because it is set by the source. In the denser medium the speed drops, so the wavelength shortens (v = f times lambda). Same frequency, smaller wavelength, slower speed. Only the speed and wavelength change, and that is exactly what causes the wavefront to bend. See the next concept, why wavelength changes but frequency stays same in a medium.
Away from the normal. In the rarer medium light speeds up, so the edge that crosses first now runs ahead instead of lagging. The wavefront tilts the other way and r becomes larger than i. If i is large enough that r would need to be more than 90 degrees, the light cannot leave and total internal reflection happens.
Each point on the wavefront acts as a source of secondary spherical wavelets. In time t, wavelets in medium 1 spread a distance v1 t, and wavelets in medium 2 spread a smaller distance v2 t. The new wavefront is the surface just touching all these wavelets. Because the wavelets in medium 2 are smaller, the tangent surface is tilted, and drawing the geometry gives Snell's law sin i / sin r = v1 / v2. This is the Huygens refraction of a plane wave proof.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Only the ratio of speeds in the two media. sin i / sin r = v1 / v2 = n2 / n1. Bigger speed drop means more bending toward the normal.
The light interacts with the atoms of the medium, which absorb and re-emit it with tiny delays. The net effect is a slower wave speed. A slower speed shortens the wavelength while frequency stays fixed.
No. The incident ray and refracted ray are on opposite sides of the surface, but the incident ray, refracted ray and the normal all lie in one plane, and the normal is the reference line for measuring i and r.
If light bends toward the normal, r is less than i, so n21 = sin i / sin r is greater than 1. That means medium 2 is optically denser than medium 1.
No bending in direction. If i = 0 the wavefront is parallel to the surface, both edges cross at the same time, so there is no lag and no turn. The speed and wavelength still change, but the ray goes straight in.
