Spectrum
The band of seven colours obtained on a screen after dispersion is called a spectrum. This phenomenon occurs when white light passes through a prism or a droplet of water. The white light is made up of different colors, each with a different wavelength. When this light passes through the prism or droplet, it is refracted, or bent, and the different colors separate.

Dispersion
Dispersion is the process by which white light separates into its component colors. It occurs because different colors have different wavelengths and therefore different speeds when passing through a medium. As the light enters the prism or droplet, it is refracted, or bent, by an angle that depends on its wavelength. This results in the colors separating and forming a spectrum on the screen.

Formation of a Spectrum
When white light passes through a prism, it is refracted twice - once when entering the prism and once when leaving it. As a result, the different colors of light are bent at different angles, causing them to spread out. The shortest wavelength, violet light, is bent the most, while the longest wavelength, red light, is bent the least. The other colors, such as blue, green, yellow, orange, and indigo, are bent at varying degrees between violet and red. When these colors are projected onto a screen, they form a band of seven colors, which is known as a spectrum.

Applications
The formation of a spectrum has various applications in science and everyday life:
1. Spectroscopy: Scientists use spectra to study the properties of different substances. Each substance absorbs or emits light at specific wavelengths, creating a unique spectral pattern that can be analyzed to identify the substance.
2. Astronomy: By analyzing the spectra of stars and galaxies, astronomers can determine their composition, temperature, and other important characteristics.
3. Rainbows: The formation of a rainbow is also a result of dispersion. When sunlight passes through raindrops in the atmosphere, it undergoes dispersion, creating a beautiful display of colors.
4. Optics: Understanding the behavior of light and its interaction with different materials is crucial in the design and development of optical devices such as lenses, prisms, and cameras.

In conclusion, the band of seven colors obtained on a screen after dispersion is called a spectrum. This phenomenon occurs when white light is refracted and the different colors separate due to their varying wavelengths. The formation of a spectrum has various applications in science and everyday life.

Band of seven color is called prism. And dispersion of all seven color is called spectrum.