A laser beam of wavelength λ=632.8 nm shines at normal incidence on the reflective side of a compact disc. The tracks of tiny pits in which information is coded onto the CD are 1.60 μm apart. For what non-zero angles of reflection (measured from the normal) will the intensity of light be maximum?

We use the formula for the diffraction grating, dsinθ=nλ, where n is an integer, λ = wavelength of laser beam = 632.8 nm = 6.328 × 10⁻⁷, d = width of slit = 1.60 μm = 1.60 × 10⁻⁶ m, θ = angle of reflection.

So, dsinθ=nλ ⇒ sinθ = nλ/d = n6.328 × 10⁻⁷/ 1.60 × 10⁻⁶= 0.3955n

The intensity of light is maximum at integer values of n.

So, when n =1, sinθ₁ = 1 × 0.3955 = 0.3955 ⇒ θ₁ = sin⁻¹(0.3955) = 23.297° ≅ 23.30° . when n =2, sinθ₂ = 2 × 0.3955 = 0.7910 ⇒ θ₂ = sin⁻¹(0.791) = 52.279° ≅ 52.28°. when n =3, sinθ₃ = 3 × 0.3955 = 1.1865. Since sinθ cannot be greater than 1, sinθ₃ = 1.1865 is invalid. So, the non-zero angles of reflection (measured from the normal) where the intensity of light will be maximum are θ₁ = 23.30° when n =1 and θ₂ = 52.28° when n =2