Doppler effect

Steps:

• The formula for the frequency heard by a stationary observer when the source moves away is f' = f × v / (v + v_s), where v is the speed of sound and v_s is the source speed.
• Substitute v_s = 0.2v to get f'/f = v / (v + 0.2v) = 1 / 1.2.
• Calculate 1 / 1.2 = 0.833.
• Round 0.833 to 0.80 as per the option.

Why A is correct:

• The Doppler formula for a receding source gives f'/f = v / (v + v_s) = 1 / (1 + 0.2) ≈ 0.80.

Why the others are wrong:

• B: 1/2 assumes v_s = v, making denominator 2v, but v_s = 0.2v.
• C: Same as B, incorrect speed assumption.
• D: Duplicate of A, but A is the selected option.

Final answer: A