Physics - Spectrum

Dispersion of Light: The phenomenon of splitting of a beam of white light into its constituent seven colors (VIBGYOR) when passed through a transparent medium like a glass prism. This happens because different colors of light travel at different speeds in glass, though they travel at the same speed $c = 3 \times 10^8 \text{ m/s}$ in vacuum.

Cause of Dispersion: The refractive index $\mu$ of a medium depends on the wavelength $\lambda$ of light. According to Cauchy's relation, $\mu \approx A + \frac{B}{\lambda^2}$. Since $\lambda_{red} > \lambda_{violet}$, the refractive index for red light is less than that for violet light ($\mu_{red} < \mu_{violet}$), causing red to deviate the least and violet to deviate the most.

Electromagnetic Spectrum: The complete range of electromagnetic waves arranged in order of increasing frequency or decreasing wavelength. The sequence is: Radio waves, Microwaves, Infrared (IR), Visible light, Ultraviolet (UV), X-rays, and Gamma rays ($\gamma$-rays).

Visible Spectrum: The narrow band of the EM spectrum visible to the human eye, ranging from approximately $4000 \text{ \AA}$ ($400 \text{ nm}$) for violet to $8000 \text{ \AA}$ ($800 \text{ nm}$) for red light.

Infrared Radiation: These are heat radiations with wavelengths longer than red light ($> 8000 \text{ \AA}$). They are detected using a thermopile or a blackened bulb thermometer. They do not affect ordinary photographic film but affect special infrared films.

Ultraviolet Radiation: These have wavelengths shorter than violet light ($< 4000 \text{ \AA}$). They can be detected by their chemical effect on silver chloride ($AgCl$) solution, which turns violet and then black. UV rays are absorbed by the Ozone layer in the atmosphere.

Scattering of Light: The process where light is absorbed by atmospheric particles and re-emitted in all directions. According to Rayleigh's law of scattering, the intensity of scattered light $I$ is inversely proportional to the fourth power of the wavelength: $I \propto \frac{1}{\lambda^4}$. This explains why the sky appears blue (shorter wavelengths scatter more) and the sun appears red at sunrise/sunset (longer wavelengths reach the observer).