Redshift as Doppler Shift in Light

Steps:

• Recall the Doppler effect: light from a receding source stretches, increasing wavelength.
• Identify redshift: this stretching shifts light toward red (longer wavelengths).
• Evaluate choices: check for accurate description of wavelength increase from moving-away galaxies.
• Select the option matching the definition precisely.

Why B is correct:

• Redshift is defined as the increase in observed wavelength due to the Doppler effect for galaxies receding from Earth, as per the formula $z = \frac{\Delta \lambda}{\lambda}$, where $z > 0$ indicates expansion.

Why the others are wrong:

• A: Incorrectly states increase in frequency; receding sources decrease frequency.
• C: Wrong; redshift makes light appear red, not blue, upon reaching Earth.
• D: Misleading; emission is not inherently red—it's the observed shift that reddens it.

Final answer: B