A diffraction grating has N lines per unit length and is placed at 90° to monochromatic light of
wavelength λ.
What is the expression for θ, the angle to the normal to the grating at which the third order
diffraction peak is observed?
http://xtremepapers.net/CIE/International%20A%20And%20AS%20Level/9702%20-%20Physics/9702_s08_qp_1.pdf
Q.28, 36, 37 and 38. i will be very thankful. kindly answer

Question

A diffraction grating has  N lines per unit length and is placed at 90° to monochromatic light of 
wavelength λ. 
What is the expression for  θ, the angle to the normal to the grating at which the third order 
diffraction peak is observed?
http://xtremepapers.net/CIE/International%20A%20And%20AS%20Level/9702%20-%20Physics/9702_s08_qp_1.pdf
Q.28, 36, 37 and 38. i will be very thankful. kindly answer

anonymous · Answer

ok, so i think i got this, its been a while since physics 101. 

Diffraction grating basically comes down to a rather simple equation.$$n \sin \Theta =m \lambda$$

so, just manipulate the answer mathematically, and you get d, since the order is to the third.