Redshift as Doppler Effect in Astronomy

Steps:

• Recall redshift: phenomenon where light from distant objects stretches to longer wavelengths due to expansion or motion.
• Identify blanks: Word 1 describes change (increase/decrease), word 2 is property (wavelength/frequency), word 3 is motion (receding/approaching).
• Match to sentence: Redshift means "increase in the observed wavelength of electromagnetic radiation emitted from receding stars and galaxies."
• Confirm with Doppler principle: Receding sources cause wavelength increase (redshift), approaching cause decrease (blueshift).

Why C is correct:

• Redshift is defined as the increase in observed wavelength (λ_observed > λ_emitted) for light from receding sources, per the relativistic Doppler formula: z = (λ_observed - λ_emitted)/λ_emitted > 0.

Why the others are wrong:

• A: Increase in frequency indicates blueshift for approaching sources, opposite of redshift.
• B: Decrease in frequency for approaching sources describes blueshift, not redshift.
• D: Increase in wavelength for approaching sources would be blueshift, contradicting redshift's receding motion.

Final answer: C