- #1
nao113
- 68
- 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….nao113 said: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?Steve4Physics said: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 half-width? 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!nao113 said: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/coherence-length-of-wave-calculator/Calc-31767 and just like what you said it uses C = 1/2Steve4Physics said:That 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.nao113 said: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/coherence-length-of-wave-calculator/Calc-31767 and just like what you said it uses C = 1/2
The coherent length is a measure of how far light waves can travel before they lose their phase relationship with each other. In other words, it is the distance over which light waves maintain their coherence.
Knowing the coherent length of an LED is important because it determines the maximum distance over which the light emitted from the LED will remain coherent. This information is crucial for applications such as optical communications and interferometry.
The coherent length of an LED can be calculated by dividing the speed of light by the frequency of the light emitted by the LED. This gives the distance that the light can travel in one cycle, which is the coherent length.
The coherent length of an LED can be affected by various factors such as the quality of the LED's fabrication, the temperature of the LED, and the materials used in the LED's construction. Additionally, any external factors such as vibrations or magnetic fields can also impact the coherent length.
The coherent length of an LED can be increased by improving the quality of the LED's fabrication and using materials with higher refractive indices. Additionally, keeping the LED at a lower temperature and minimizing external factors can also help increase the coherent length.