Answer: 12.9 km
Explanation:
θ = 1.22 * (λ/d), where
θ = angle of resolution
λ = wavelength
d = width of slit
Also, the radius of the beam on the moon is
r = lθ, were
l = distance from the earth to the moon, = 384*10^6 m
if we substitute the relation for θ, we should have
r = (1.22 * lλ) / d
Now, we substitute the values into the equation
r = (1.22 * 384*10^6 * 550*10^-9) / 0.04
r = 258 / 0.04
r = 6450 m
Diameter of the beam, d = 2r
d = 2 * 6450
d = 12900 m or 12.9 km