uncertainties a level physics question

A student has a diffraction grating that is marked 3.5x10^3 per m.

Calculate the percentage uncertainty in the number of lines per metre suggested by this marking. (1 mark)
Answer is 2.9%

-can someone please explain how I am meant to know that the absolute uncertaintie is meant to be 100 when it doesn't give it to me in the question ? or do I always assume absolute uncertainty to be 100 ?

(edited 1 year ago)

Reply 1

sigmagrindset49

The number of significant figures which a value is given to tells you about how accurately that value is known. The question gives the value as 3.5x10^3 per m. This means you should assume the uncertainty to be 0.1x10^3 per m = 100 per m. This is because the uncertainty is always given by the last digit given in the data.

If the value in the question was 3.54x10^3 per m, then the absolute uncertainty would be 0.01x10^3 per m = 10 per m because that corresponds to the last digit in the data given.