This diagram demonstrates the Doppler Effect for light, showing how the motion of a star relative to an observer affects the wavelength of light received, resulting in blueshift or redshift.

Let’s solve each part step by step, using the definition of the Doppler Effect provided in the text:

> “When the observer and the source of light are moving toward each other, the light is shifted to shorter wavelengths (blueshifted). When the observer and the source of light are moving away from each other, the light is shifted to longer wavelengths (redshifted).”

Also note: The diagrams show a star (light source) and an observer (telescope or person). The arrows indicate the direction the star is moving. The wavy lines represent light waves traveling from the star to the observer.

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Part 1: Motion of Source

#### Question 1: Shorter Wavelengths (Blueshift)

a) In which situation will the observer receive light that is shifted to shorter wavelengths?

→ Shorter wavelengths = blueshift → occurs when source and observer are moving TOWARD each other.

Look at the diagrams:

- A: Star is not moving → no Doppler shift.
- B: Star is moving UP → perpendicular to line of sight to observer (who is below). So, no radial motion toward or away → no Doppler shift.
- C: Star is moving DOWN → directly toward the observer (who is below). So, motion toward → blueshift.
- D: Star is moving RIGHT → perpendicular to line of sight → no radial component → no Doppler shift.

✔ So, only C has motion toward the observer → shorter wavelengths.

b) Will this light be blueshifted or redshifted for this case?

→ Since it’s shifted to shorter wavelengths, it’s blueshifted.

c) What direction is the star moving relative to the observer for this case?

→ In C, the star is moving downward, directly toward the observer.

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#### Question 2: Longer Wavelengths (Redshift)

a) In which situation will the observer receive light that is shifted to longer wavelengths?

→ Longer wavelengths = redshift → occurs when source and observer are moving AWAY from each other.

Looking again:

- A: No motion → no shift.
- B: Moving UP → directly away from observer (who is below).
- C: Toward → blueshift.
- D: Moving right → perpendicular → no radial motion.

✔ So, only B has motion away from observer → longer wavelengths.

b) Will this light be blueshifted or redshifted for this case?

→ Redshifted.

c) What direction is the star moving relative to the observer for this case?

→ In B, the star is moving upward, directly away from the observer.

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#### Question 3: No Doppler Shift at all

> In which of the situations shown (A–D) will the observer receive light that is not Doppler Shifted at all? Explain your reasoning.

→ Doppler shift occurs only if there is a radial velocity component — that is, motion directly toward or away from the observer.

- A: Star is stationary → no motion → no Doppler shift.
- B: Moving directly away → redshift → has Doppler shift.
- C: Moving directly toward → blueshift → has Doppler shift.
- D: Moving horizontally (right) → perpendicular to line of sight → no radial component → no Doppler shift.

✔ So, A and D have no Doppler shift.

Explanation:
- In A, the star is not moving at all, so the wavelength received is unchanged.
- In D, although the star is moving, its motion is perpendicular to the line connecting it to the observer. There is no component of motion toward or away from the observer, so the distance between source and observer isn’t changing → no Doppler effect.

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## ✔ Final Answers:

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1)

a) C
b) Blueshifted
c) Downward (toward the observer)

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2)

a) B
b) Redshifted
c) Upward (away from the observer)

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3)

Situations A and D will have no Doppler shift.

Reasoning:
- In A, the star is stationary, so there is no relative motion to cause a shift.
- In D, the star moves perpendicular to the line of sight to the observer, so there is no radial velocity component (no motion toward or away), and thus no Doppler shift occurs. Only motion along the line of sight causes Doppler shifts.

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Let me know if you’d like a visual explanation or analogy!