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If a stationary object emits light at a certain wavelength in all directions, you will measure the same wavelength {\lambda} of light, no matter where you are. However, for moving objects different observers will observe different wavelengths ({\lambda}'s).

approaching side: observer will measure a shorter {\lambda} than emitted
receding side: observer will measure a longer {\lambda}

This is known as the Doppler Effect.

When the source of light is moving away from the observer the wavelength of the emitted light will appear to increase. We call this a “redshift”.

When the source of light is moving toward the observer the wavelength of the emitted light will appear to decrease. We call this a “blueshift”.

The Doppler Shift can be used to calculate an objects velocity. Here is how:

{\frac{\Delta\lambda}{{\lambda}_0}}=\frac{\text{v}}{\text{c}}

where {\Delta\lambda} is the wavelength shift and

{\Delta\lambda}={\lambda}_{\text{observed}}-{\lambda}_{\text{rest}}

{\lambda}_{\text{rest}}={\text{wavelength}} if source is not moving (measured in a lab)

{\text{v}}={\text{velocity of source}}

{\text{c}}={\text{speed of light}}

The faster the source is moving, the larger the amount of shift – either blueshift or redshift.

Some real life examples of Doppler Effect are the use of Doppler Radar for weather purposes, airline or submarine radar system, radar gun used by Law Enforcement Officers, etc.

In astronomy, we use the patterns of absorption lines in stars to measure the direction and amount of the shift seen in the spectrum. So, the absorption lines have two uses:

to tell what elements are present in the star, and
to tell how fast is the star moving or rotating.

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