Redshift and blueshift refer to the change in the observed frequency or wavelength of light relative to its source due to the relative motion between the source and the observer. While light is indeed an electromagnetic wave, its interaction with the motion of objects can cause a shift in its observed frequency.
The phenomenon of redshift and blueshift is a consequence of the Doppler effect. When a light source and an observer are moving closer together, the observed frequency of light appears higher, resulting in a blueshift. Conversely, when the source and the observer are moving away from each other, the observed frequency appears lower, resulting in a redshift.
It's important to note that the actual wavelength or frequency of light itself is not changing, but rather what the observer perceives due to their relative motion. This effect can be observed not only with light but also with other waves, such as sound.
The Doppler effect applies to any wave, including electromagnetic waves like light. It is a fundamental principle in physics and has been observed and measured in various contexts, such as the redshift of light from distant galaxies due to the expansion of the universe (cosmological redshift) or the blueshift of light from objects moving towards us, such as stars with high radial velocities.
In summary, the redshift or blueshift of light occurs due to the relative motion between the light source and the observer, causing a change in the observed frequency or wavelength of the light, even though the light itself remains an electromagnetic wave.