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###### Himanshu posted an Question
May 17, 2020 • 02:53 am 30 points
• IIT JAM
• Physics (PH)

# Optics a parallel beam of light comp....

Optics A parallel beam of light comp.... Answer is 0.001

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• ##### Ruby negi

grating element and grating spacing is same i.e e+d... don't get confuse... look this formula.. sometimes in ques, they ask to calculate width of grating.. so W=N(e+d) is width of grating... best regards.

• ##### Ruby negi

here is the solution dear, 0.001cm is the correct ans.. hope you will get it.. for any doubt please ask.

• ##### Abhishek singh Best Answer

The plane where the parallel beam of light after falling on a lens converges is called focal plane of that lens. In actual practice, the grating is always followed by a lens(I have attached a diagram for explaining that). For small angle approximation sin@ = tan@ = @, where @ is the angle. also, tan@ = Perpendicular/ Base. here the base length will be equal to focal length of lens(as we always put lens very close to grating and the focal plane is far enough from lens). I am attaching the necessary diagrams followed by solution of this question. Best regards.

read the solution atleast 2-3 times as it is conceptually rich.

• ##### Himanshu Pandey

also focal plane!!

• ##### Himanshu Pandey

how to solve it,main thing here Focal length is given what to do for that !!!!!

• ##### Dhairya sharma

see the attached.

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