• The change in the observed frequency of a wave as the result of the relative motion between two bodies
• All waves can undergo the Doppler effect, including electromagnetic waves and sound waves
• For a moving source, the observer perceives a change in the frequency and the wavelength of the wave, but no change in its speed

• When a moving source produces a wave that reflects off a moving object and is received by the source again
• Echolocation: The different wave properties of the echo allow species like bats and dolphins to locate the object or prey the waves reflect off

• There must be a relative difference in the velocity of the source and the observer
• Scenario 1: The source is moving, and sound is reflected off a stationary object
• Scenario 2: The source is stationary, and sound is reflected off a moving object

• Light will always appear to be travelling at the speed of light, regardless of the reference frame
• Equation for relative changes in frequency/wavelength: Δf/f = Δλ/λ ≈ v/c, where c is the speed of light
• Spectral Lines: the wavelengths of light emitted by different elements losing electrons
• Redshift: if the light wave is moving away from us, the wavelength appears to be longer (red end of the spectrum)
• Blueshift: If the light wave is moving towards us, the wavelength appears to be shorter (blue end of the spectrum)