Doppler effect for light (redshift and blueshift)
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Question
What is the Doppler effect for light?
Answer
The change in the **wavelength (and frequency)** of light you receive when its **source moves toward or away** from you.
Question
What is a redshift?
Answer
The observed wavelength is **stretched longer** (shifted toward red) because the source is **receding** (moving away).
Question
What is a blueshift?
Answer
The observed wavelength is **squashed shorter** (shifted toward blue) because the source is **approaching** (moving toward you).
Question
Doppler-shift equation for light?
Answer
$\dfrac{\Delta f}{f} = \dfrac{\Delta\lambda}{\lambda} \approx \dfrac{v}{c}$ — **given** in the data booklet (valid for v ≪ c).
Question
How do you find the source speed from a wavelength shift?
Answer
Rearrange to **v = (Δλ ÷ λ) × c**, where Δλ = observed − laboratory wavelength and c = 3.0 × 10⁸ m s⁻¹.
Question
What does Δλ mean?
Answer
The **change** in wavelength: observed wavelength − laboratory (true) wavelength — NOT the whole wavelength.
Question
Red = ? and Blue = ? (memory aid)
Answer
**Red = Receding** (away, longer λ); **Blue = approaching** (toward, shorter λ).
Question
Why are distant galaxies redshifted?
Answer
The **Universe is expanding**, so distant galaxies are **receding** from us — their light is shifted to longer (redder) wavelengths.
Question
A rotating star — what shifts do its two edges show?
Answer
The **approaching edge is blueshifted** (shorter λ) and the **receding edge is redshifted** (longer λ) at the same time.
Question
Does a bigger shift mean a faster or slower source?
Answer
A **bigger** shift means a **faster** source — Δλ is proportional to v (Δλ/λ = v/c).
Question
When is Δλ/λ = v/c valid?
Answer
Only when the source speed **v is much smaller than c** (the speed of light).
Question
Most common mistake in a Doppler-of-light calculation?
Answer
Using the **whole observed wavelength** instead of the **change Δλ** (observed − lab) on the top of the fraction.
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