Final answer:
The gray shades in a Doppler spectrum correspond to varying velocities of objects causing blueshifts or redshifts in light or sound wavelengths. These shifts are more accurately measured in spectral lines than in the continuous spectrum.
Step-by-step explanation:
The gray shades of a Doppler spectrum relate to the variations in the velocity of objects, such as blood within an artery or stars moving in space. These velocities result in a Doppler shift, which changes the wavelength of light or sound being emitted by the moving object. When an object approaches, the wavelengths shorten and shift toward the blue end of the spectrum; this is called a blueshift. Conversely, when an object recedes, the wavelengths lengthen and shift toward the red end of the spectrum, known as a redshift. The Doppler shift in visible light becomes noticeable only when an object's speed is extremely high, but it can be accurately measured in the spectral lines—either absorption lines or emissions—where it can indicate the star or other celestial body's velocity relative to the observer.