Doppler Effect for Receding Light Source

Steps:

• Identify the phenomenon: light from a star moving away experiences the Doppler effect, causing redshift.
• Recall the effect: for a receding source, observed wavelengths stretch longer than emitted ones.
• Apply to light: the star's motion relative to Earth increases the observed wavelength.
• Conclude: no change to other properties like speed; wavelength is the key increase.

Why D is correct:

• The relativistic Doppler formula for a receding source, λ_observed = λ_emitted × √[(1 + v/c)/(1 - v/c)], shows wavelength increases (v > 0).

Why the others are wrong:

• A: Amplitude relates to light intensity, unaffected by relative motion.
• B: Frequency decreases as wavelength increases (c = fλ, constant c).
• C: Speed of light remains constant in vacuum per special relativity.

Final answer: D