Really Quick A2 Doppler Shift Question

I have this question:



I know how to do it, using the $\Delta \lambda = \frac{v}{c} \lambda$.

I don't get why it would be closer to the blue end of the spectrum though, surely it's red shift.

That leaves me stunned, so what values would I use in the equation?

Thanks,

-f8.

use the 20nm in place of the delta lambda, divided by the 420nm they gave you in the question and multiply it by the speed of light c. to get v the velocity they want!

you should know this rule about pastpapers no matter how hard you study edexcel will always invent questions that don't support the theories you know! So even if you are not convinced with what they are suggesting just use the formula you know!!

I realised that, but isn't it only in abstract cases like orbits and stuff? I didn't think they'd give us a question like that.

Anyways, the mark scheme said use $\lambda = 400nm$ or $440nm$. How odd!

I have this question:



I know how to do it, using the $\Delta \lambda = \frac{v}{c} \lambda$.

I don't get why it would be closer to the blue end of the spectrum though, surely it's red shift.

That leaves me stunned, so what values would I use in the equation?

Thanks,

-f8.

It says 20nm nearer the blue end of the spectrum, so the wavelength has effectively decreased, so the initial wavelength must've been 420+20nm = 440nm, so you use 440nm as your wavelength in the equation (without any shifting) and 20nm for your difference in wavelengths.

Also the galaxy is moving towards us since there's a blue shift.

yeah, the light was emitted with wavelength 440nm, which decreased to 420nm (which is "nearer the blue end" of the EM spectrum) on observation at the Earth due to the Doppler shift.

If the initial wavelength was 400nm, it would've moved 20nm nearer the red end of the spectrum.