Given that,
Central maximum = 1 cm
Distance from the window shade to the wall =4 m
We know that,
The visible range of the sun light is 400 nm to 700 nm.
(a). We need to calculate the average wavelength
Using formula of average wavelength
Put the value into the formula
(b). We need to calculate the diameter of the pinhole
Using formula for diameter
Put the value into the formula
Hence, (a). The average wavelength 550 nm.
(b). The diameter of the pinhole is 0.537 mm.