The
physical manifestation of the
Doppler Effect on
light emitted by an object as that object moves towards the observer, as opposed to the
red shift observed when an object is moving away.
When an object is moving towards the observer, the object's
electromagnetic spectrum is shifted towards the blue end of the
electromagnetic spectrum. Thus, blue shift can be detected by analyzing
emission lines and
absorption lines in the
spectra of astronomical objects that are moving toward the Earth, and comparing them to the spectra of objects of similar chemical composition. (For example, a rotating object displays blue shift on one side, and
red shift on the other).
The faster an object approaches the observer, the more the radiation emmited by the object is shifted:
microwaves become
infrared, infrared becomes
visible light, visible light turns into
ulraviolet, ultraviolet becomes
X-rays, X-rays become
gamma rays.
Not only are emission and absorption lines shifted, the object's overall electromagnetic
frequency distribution due to
black body radiation is also shifted. To an osberver, an approaching object is
hotter than one that is stationary relative to the object, or moving away!
Due to
the expansion of the universe, as well as the fact that other effects besides motion cause observable red shift, objects exhibiting blue shift are much harder to find than ones exhibiting red shift.