Physics · Wave Optics · NEET
Frequency is decided by the source that produced the light, not by the medium it travels through. At the boundary between two media, the electric field of the incoming wave forces the atoms of the second medium to oscillate. These atoms act like driven oscillators, so they vibrate at the same frequency as the incoming light and re-emit light of that same frequency. If the frequency suddenly changed at the surface, the wave crests on the two sides would not match up in time, which is not physically allowed. So the number of crests arriving per second must equal the number leaving per second, keeping f constant.
Use the wave equation v = f x lambda. In a denser medium the speed v of light is smaller (light interacts more with the closely packed atoms). Since f is fixed by the source and cannot change, the only quantity left to adjust is the wavelength. So lambda must shrink in the same proportion as the speed. The exact rule is lambda_medium = lambda_vacuum / n, where n is the refractive index. For water (n = 4/3), the wavelength becomes three-quarters of its value in air.
No. Colour is determined by frequency, and frequency does not change. A beam that looks yellow in air is still yellow inside water, even though its wavelength inside water is shorter. This is a very common trap: students think shorter wavelength means the colour shifts toward blue, but colour is tied to frequency, which is unchanged. What we call the wavelength of a colour (for example 589 nm for sodium yellow) is always its wavelength in vacuum or air.
Light is an electromagnetic wave. When it enters a medium, its oscillating electric field makes the electrons in the atoms vibrate. These vibrating electrons re-radiate their own tiny waves, which combine with the original wave. The combined wave travels forward more slowly than light in vacuum. The denser the medium (higher refractive index), the stronger this effect, so the slower the light. This is why v = c / n, and light is slowest in the densest optical medium.
No, the energy of the light wave does not fall just because its speed drops. The energy of each photon is E = h x f, and since frequency f stays the same, the photon energy stays the same. The intensity may change a little because some light is reflected at the surface, but the reduction in speed itself carries no loss of energy. NCERT states this directly in Example 10.1(b): a reduction in speed does not mean a reduction in the energy carried by the light.
Group the three quantities into 'source-controlled' and 'medium-controlled'. Frequency f and photon energy E = h x f are source-controlled, so they never change on refraction. Speed v and wavelength lambda are medium-controlled, so they both change together (both fall in a denser medium). One line to memorise: 'Frequency and energy stay; speed and wavelength change.'
In a double slit experiment, when light of wavelength 400 nm was used, the angular width of the first minima formed on a screen placed 1 m away was found to be 0.2 degree. What will be the angular width of the first minima if the entire apparatus is immersed in water? (n_water = 4/3)
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Frequency stays the same. Wavelength and speed both decrease. Frequency is fixed by the source, so it cannot change on refraction.
lambda_medium = lambda_vacuum / n, where n is the refractive index of the medium. For water, n = 4/3, so the wavelength becomes three-quarters of its air value.
Yes. Speed becomes v = c / n, so it is smaller in a denser (higher n) medium. In vacuum the speed is the maximum, c = 3 x 10^8 m/s.
Colour depends on frequency, and frequency does not change on refraction. So a yellow beam stays yellow in water even though its wavelength is shorter there.
No. Photon energy is E = h x f, and frequency f is unchanged, so the energy per photon is unchanged. Slowing down alone does not reduce the light's energy.
