The correct answer is (a) Doppler effect.
The Doppler effect is the change in frequency of a wave for an observer moving relative to the source of the wave. It is named after Christian Doppler, who proposed the concept in 1842.
The Doppler effect can be observed for all types of waves, including sound waves, light waves, and radio waves. In the case of light waves, the Doppler effect is responsible for the redshift of light from distant galaxies. Redshift is the phenomenon of light from distant galaxies being shifted towards the red end of the visible spectrum. This is caused by the expansion of the universe, which causes the light waves to be stretched out.
The Stark effect is the splitting of spectral lines in the presence of an electric field. It is named after Johannes Stark, who discovered the effect in 1913.
The Stark effect is caused by the interaction of the electric field with the electrons in the atom. The electric field causes the electrons to be displaced from their equilibrium positions, which changes the energy levels of the atom. This results in the splitting of the spectral lines.
The Zeeman effect is the splitting of spectral lines in the presence of a magnetic field. It is named after Pieter Zeeman, who discovered the effect in 1896.
The Zeeman effect is caused by the interaction of the magnetic field with the electrons in the atom. The magnetic field causes the electrons to be deflected from their paths, which changes the energy levels of the atom. This results in the splitting of the spectral lines.
The Raman effect is the scattering of light by a substance, accompanied by a change in the wavelength of the light. It is named after Chandrasekhara Venkata Raman, who discovered the effect in 1928.
The Raman effect is caused by the interaction of the light with the molecules in the substance. The light can interact with the molecules in two ways: it can either be scattered elastically, without any change in wavelength, or it can be scattered inelastically, with a change in wavelength. The inelastic scattering is caused by the interaction of the light with the vibrational and rotational energy levels of the molecules.