Clyner82

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

1) Calculate the percentage uncertainty in the number of lines per metre suggested by this marking.

The answer I got was 1/3.5x10^3 (x100%) = 0.029% but the answer is 2.9%. What am I doing wrong?

2) State the absolute uncertainty in the value of the spacing?

The answer is +- 0.01mm but I don t know how this is got either.

Any help would be great.

ChipB

Given 3.5 x 10^3 the possible range of "true" measurement is anywhere from 3.45 x 10^3 to 3.55 x 10^3. That's a range of possible error of 0.1 x 10^3. So as a percentage:

0.1 x 10^3/3.5 x 10^3 = 2.9%

Note that you actually can ignore the powers of ten:

0.1/3.5 = 2.9%

Clyner82

Thank you so much for explaining this to me

topsquark

I'm betting that you are using a calculator with a % symbol. If you hit the % key it automatically divides your answer by 100. You don't need the % key for anything, really.

-Dan

__________________
Do not meddle in the affairs of dragons for you are crunchy and taste good with ketchup.

See the forum rules here.