thetaθ is the angle between the diffracted ray and normal to the grating, and
dd being the distance from the center of one slit to the center of the adjacent slit.
Condition for constructive interference, maximum, is the distance bcbc be integral multiple of wavelength lambdaλ.
In Delta abc, angle bac=theta, from geometry.
:. "distance " bc=d.sintheta
Grating formula becomes
d.sintheta_m=mlambda, where m is integer representing the maximum of interest.
In the problem, it is first order maxima m=1
Therefore the expression becomes
d.sintheta_1=1.lambda=lambda, inserting the given quantities
Wavelength lambda=1.00xx10^-6cdot sin 70.0^@
=1.00xx10^-6cdot 0.93969
=9.3969xx10^-7 m
