What is “Red Shift” and “Blue Shift” ?

A blueshift is any decrease in wavelength (increase in frequency); the opposite effect is referred to as redshift. In visible light, this shifts the color from the red end of the spectrum to the blue end. The term also applies when photons outside the visible spectrum (e.g. x-rays and radio waves) are shifted toward shorter wavelengths, as well as to shifts in the de Broglie wavelength of particles. Blueshift is most commonly caused by relative motion toward the observer, described by the Doppler effect. An observer in a gravity well will also see infalling radiation gravitationally blueshifted, described by General Relativity in the same way as gravitational redshift. In a contracting universe, cosmological blueshift would be observed; the expanding universe gives a cosmological redshift, and the expansion is observed to be accelerating.
redshift happens when light or other electromagnetic radiation from an object is increased in wavelength, or shifted to the red end of the spectrum. In general, whether or not the radiation is within the visible spectrum, "redder" means an increase in wavelength – equivalent to a lower frequency and a lower photon energy, in accordance with, respectively, the wave and quantum theories of light.

In physics, redshift happens when light or other electromagnetic radiation from an object is increased in wavelength, or shifted to the red end of the spectrum. In general, whether or not the radiation is within the visible spectrum, "redder" means an increase in wavelength – equivalent to a lower frequency and a lower photon energy, in accordance with, respectively, the wave and quantum theories of light.

A blueshift is any decrease in wavelength (increase in frequency). the opposite effect is referred to as redshift. In visible light, this shifts the color from the red end of the spectrum to the blue end. The term also applies when photons outside the visible spectrum (e.g. x-rays and radio waves) are shifted toward shorter wavelengths, as well as to shifts in the de Broglie wavelength of particles.

A redshift occurs whenever a light source moves away from an observer.Doppler blueshift is caused by movement of a source towards the observer.