Question

A diffraction grating has 600 grooves per mm. You illuminate it with a red laser beam (633 nm). How many bright spots will you see on the wall?

Confused how to approach this problem without the distance the beam is shot at from the screen (L). Any help is appreciated.

Answer 1

the applicable equation:

d*sin(theta)=m*lambda

where d=slit width=1 mm/600 =1.667*10^(-6) m

theta is the angle made by the light ray.

lambda=wavelength=633 nm=633*10^(-9) m

m=order of the bright fringe on either side of central bright fringe

now , maximum value of theta is 90 degrees

hence d*sin(90)=m*lambda

==>m=d/lambda=2.633

so on either side of the central bright fringe, there will be 2 bright fringes.

hence total number of bright fringe visible=2+2+1 (1 is for central bright fringe)=5

so you will see total 5 bright spots on the wall.