Blueshift is observed when
a. a luminous object is stationary compared to an observer
b. a luminous object travels alongside an observer
c. a distant luminous object travels rapidly towards an observer
d. a distant luminous object travels rapidly away from an observer
c. a distant luminous object travels rapidly towards an observer
d. a distant luminous object travels rapidly away from an observer
The correct answer is option c. Blueshift is observed when a distant luminous object (such as a galaxy or star) travels rapidly towards an observer.
To understand why this causes blueshift, we need to know a bit about the Doppler effect. The Doppler effect is the change in frequency or wavelength of a wave as perceived by an observer moving relative to the source of the wave. It is commonly experienced with sound waves, like the change in pitch of an ambulance siren as it approaches and then passes by. However, it also applies to light waves.
When an object emitting light is moving towards an observer, the wavelengths of the light waves get compressed, increasing their frequency. As a result, the light is shifted towards the higher frequency, or the bluer end of the electromagnetic spectrum. This phenomenon is called blueshift.
On the other hand, when an object is moving away from an observer, the wavelengths of the light waves get stretched, reducing their frequency. The light is then shifted towards the lower frequency, or the redder end of the electromagnetic spectrum. This is known as redshift.
So, in the given options, only option c describes the scenario where a distant luminous object travels rapidly towards an observer, causing blueshift.