Topic 3.5: Doppler Effect Questions

Which is correct about the light received from a galaxy that is moving towards Earth?

A microphone on a trolley moves along the line joining it to a stationary loudspeaker. The loudspeaker emits 900 Hz and the microphone records 950 Hz.

State the direction of motion of the trolley.

A cyclist rides at constant speed along a straight path from loudspeaker X towards loudspeaker Y. The loudspeakers emit 600 Hz and 700 Hz, and the cyclist hears both at the same frequency.

Explain which frequency Y emits.

A bird call of constant frequency is emitted by a bird flying directly towards a stationary microphone.

Identify whether the frequency recorded by the microphone is higher than, lower than, or equal to the emitted frequency, and identify what happens to the wavelength of the sound reaching the microphone.

A spectral line that has a laboratory wavelength of 587.6 nm is observed at 587.5 nm in the light from a star.

Identify whether the star is approaching or receding, and explain how you decided.

A train travels east at constant speed on a straight track between two sirens, one west of it and one east of it. The sirens emit 380 Hz and 420 Hz, and a passenger hears both at the same frequency.

Explain which frequency the west siren emits.

A star moves directly away from Earth. Which is correct about the light received on Earth?

A listener on a bicycle rides along a straight road that passes through a stationary bell tower. The bell rings at 520 Hz and she hears 490 Hz.

State the direction in which she is riding relative to the tower.

State what is meant by the Doppler effect for sound.

State what is meant by a redshift, and state what it tells you about the motion of the light source.

A boat sails at constant speed straight towards a lighthouse horn, away from a harbour horn behind it. The horns emit 250 Hz and 280 Hz, and the crew hear both at the same frequency.

Explain which frequency the lighthouse horn emits.

A detector on a drone records exactly the frequency that a stationary buzzer emits, although the drone is moving.

State the direction in which the drone is moving relative to the line joining it to the buzzer.

A sound source moves at constant velocity from observer Y towards observer X along the line joining them. Both observers are at rest.

I. The frequency heard at Y is lower than at X.
II. The wavelength at Y is longer than at X.
III. The speed of the sound at X is greater than at Y.

Which statements are correct?

Astronomers compare light from the two sides of a spinning star. From the side whose surface Q moves directly away from us, a known line appears at a slightly longer wavelength than a lab source gives.

Explain this observation.

A sodium line measured at 589.0 nm in the laboratory appears at 586.6 nm in the light of a nearby galaxy.

Calculate the velocity of the galaxy relative to Earth.

A train sounding a horn moves at constant speed along a straight track towards an observer O who stands on the track far ahead.

I. O hears a higher frequency than the horn emits.
II. The wavelength reaching O is shorter than when the train is at rest.
III. The frequency O hears rises steadily as the train gets closer.

Which statements are correct?

Two galaxies, X and Y, are observed.

The light from galaxy X shows a redshift of Δλ/λ = 0.010, and the light from galaxy Y shows a redshift of Δλ/λ = 0.030.

Deduce which galaxy is moving away from Earth faster, and state how many times faster it is moving.

A car horn sounds a steady note of frequency 480 Hz. The car drives directly towards a stationary pedestrian at 28 m s⁻¹. The speed of sound in air is 340 m s⁻¹.

Light of emitted wavelength λ0 comes from sources moving away at speeds v much less than c.

Which describes the graph of the observed wavelength against v?

A spectral line has a laboratory wavelength of 410.0 nm.

In the spectrum of a distant galaxy the same line is observed at 420.0 nm.

Determine the speed of the galaxy relative to Earth.