Question

Monochromatic light from a helium-neon laser (wavelength=632.8 nm) shines at a right angle onto the surface of a diffraction grating that contains 130 960 lines/m. Find the angle at which one would observe the first-order maximum.

formula: (a+b)Sin theta = n wavelength
(a+b) = 1
____ m; n=1

wavelength=632.8*10^-9 m
=6.328*10^-7m

Sin theta=6.32^8 * 10^-7 * 130960
= 6.328*10^-7*130960*10^5
theta= 4.7536deg
= 4.75

is this done correctly?
130960

Answer 1

I don't know what a and b are supposed to be in your diffrection grating formula. The correct form of the quation is
d (sin i + sinr) = m*lambda
where i and r are the angles of incidence and reflection.
In your case sin i = 0, m = 1, and d is the grating line separation, which is your case is 1/(130960m^-1) = 7.636*10^-6 m.
Therefore
d sin r = 632.8*10^-9 m
sin r = 632.8*10^-9*130960 = 0.08287
r = 4.75 degrees

So you got the right answer, anyway