Answer
A blue shift for light refers to the Doppler effect shifting the received light toward shorter wavelengths (the blue end of the visible spectrum). A blue shift occurs when the source and the observer approach each other.
A red shift for light refers to the Doppler effect shifting the received light toward longer wavelengths (the red end of the visible spectrum). A red shift occurs when the source is receding (moving away) from the observer, or the observer is moving away from the source.
Work Step by Step
This is discussed on page 366.
