According to the theory of special relativity proposed by Albert Einstein, the speed of light in a vacuum is constant and does not change regardless of the observer's motion. This fundamental principle is known as the postulate of the constancy of the speed of light.
Therefore, if an observer moves closer to a beam of light emitted from an object, the speed of light in their frame of reference remains the same. However, there are other effects that occur due to relative motion between the observer and the source of light.
One such effect is called the Doppler effect. The Doppler effect causes a shift in the observed frequency (or wavelength) of a wave, including light, when there is relative motion between the source and the observer. The effect can be observed with sound waves, such as when a siren of a moving vehicle sounds different as it approaches or recedes.
In the case of light, if an observer moves closer to a source of light, the frequency of the light waves appears to increase. This means that the light appears bluer, which is called a blue shift. On the other hand, if the observer moves away from the source, the frequency appears to decrease, resulting in a red shift.
It's important to note that while the frequency and color of the light may change due to the Doppler effect, the speed of light itself remains constant.