Edexcel IGCSE Physics 8 16p Red Shift Equation Questions

Rearrange the redshift equation, read wavelength changes from galaxy data, and convert units before finding an unknown value.

Exam points

Substitute reference wavelength and its change into Δλ/λ₀ = v/c to calculate galaxy recession speed.
Calculate observed wavelength by adding Δλ to the reference wavelength after applying the redshift relation.
Read wavelength change from galaxy data or a graph, then apply Δλ/λ₀ = v/c.

[Maximum number: 3]

Between 1929 and 1931, physicists Hubble and Humason investigated the red-shift of light from galaxies at different distances from the Earth.

The distance unit they used is the megaparsec (Mpc).

The graph shows some of the results of their investigation.

Figure for Question Edexcel IGCSE Physics 8 16p Red Shift Equation Questions question 1 — Edexcel IGCSE Physics

The reference wavelength of the light used in this investigation is 660 nm.
Use information from the graph to determine the velocity of a galaxy at a distance of 0.75 Mpc.

[speed of light = 300000 km/s]

velocity = ............ km/s

Marking points:
- change in wavelength $=0.03\ \mathrm{nm}$
- substitution or rearrangement
- evaluation

Example:
$\Delta\lambda=0.03\ \mathrm{nm}$ $\frac{0.03}{660}=\frac{v}{300000}$ $v=14\ \mathrm{km\,s^{-1}}$

Accept:
- ECF from 8(b)(ii), correct within $\frac12$ small square
- ECF from incorrect reading of $\Delta\lambda$ if clear from working
- $13.6\ldots\ \mathrm{km\,s^{-1}}$

Marking guidance:
- deduct 1 mark for power-of-ten error
- award change in wavelength if seen anywhere in working

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Edexcel IGCSE Physics 8 16p Red Shift Equation Questions question 1