Doppler effect for light (redshift and blueshift) (Physics HL)

The big idea: Split the light from a distant galaxy through a spectrometer and the familiar spectral lines sit further toward the red end than the same lines measured in a lab — the galaxy is racing away from us. This wavelength change from a moving light source is the Doppler effect for light.

Moving away → wavelength looks longer (toward the red end) → a redshift.

Moving toward you → wavelength looks shorter (toward the blue end) → a blueshift.

New words — redshift and blueshift: Wavelength (λ) is the length of one wave; red light has a long wavelength, blue light a short one.

Redshift = the wavelength is stretched longer (source receding).

Blueshift = the wavelength is squashed shorter (source approaching).

Spot it: Red = Receding (away, longer λ). Blue = approaching (toward, shorter λ).

The bigger the shift, the faster the source is moving.

For a source moving much slower than light, the fraction the wavelength shifts by equals the fraction the frequency shifts by, and both equal v ÷ c. This is given in the data booklet.

How to read it: Δλ/λ is the fraction by which the wavelength has shifted (e.g. 0.001 = 0.1%).

That fraction equals v/c — the source's speed as a fraction of the speed of light.

So a tiny shift means a small speed; a big shift means a big speed.

How this is tested — light Doppler is almost always astronomical:

The classic trap: Δλ is the change (observed − lab), not the whole wavelength — divide the small change by the original λ.

A rotating star has BOTH shifts at once: One edge of a spinning star turns toward us (that edge is blueshifted, shorter λ); the other edge turns away (redshifted, longer λ). The edge speed v is the star's rotation speed at its surface.