 #1
nao113
 65
 13
 Homework Statement:

Calculate the coherent length of the following LED.
Center wavelength, λ= 940 nm Width, Δλ= 30 nm
 Relevant Equations:
 I put the equation in the picture below
Answer:
Hi @nao113. You haven't actually asked a question! But a few thoughts anyway….Homework Statement:: Calculate the coherent length of the following LED.
Center wavelength, λ= 940 nm Width, Δλ= 30 nm
Relevant Equations:: I put the equation in the picture below
View attachment 302561
Answer:
View attachment 302562
so the answer will be 14726.7 nm?Hi @nao113. You haven't actually asked a question! But a few thoughts anyway….
Is Δλ the FWHM (full width at half maximum) or the halfwidth? Let’s assume it is the FWHM.
The coherence (not ‘coherent’) length, L, In a medium of refractive index 1 is given by:
##L = C \frac {λ^2}{Δλ}##
e.g. see https://en.wikipedia.org/wiki/Coherence_length
You have taken C=1 but typically C≈ ½. See above link.
You have been given data to 2 significant figures but your final answer has 6 significant figures  lose 1 mark in an exam’!
That would still lose a mark (even if correct) due to excess significant figures!so the answer will be 14726.7 nm?
Thank you very much for your answer. Can you please tell me what the correct one is? Actually, I also tried this calculator https://www.calculatoratoz.com/en/coherencelengthofwavecalculator/Calc31767 and just like what you said it uses C = 1/2That would still lose a mark (even if correct) due to excess significant figures!
However, from your attachment it appears that in your course, you are:
 using Δλ to mean the FWHM;
 using C=1 in the equation ##L = C \frac {λ^2}{Δλ }## (technically not correct, but never mind).
So, based on your course material, your original answer (in Post #1) would be correct if you rounded it properly.
Can you tell me what your Post #1 and Post #3 answers are when correctly rounded to the appropriate number of significant figures? If you are not sure how to do that, at least try! If wrong, I will help.Thank you very much for your answer. Can you please tell me what the correct one is? Actually, I also tried this calculator https://www.calculatoratoz.com/en/coherencelengthofwavecalculator/Calc31767 and just like what you said it uses C = 1/2