Final answer:
The width of the central maximum in a single-slit diffraction pattern becomes narrower when the pattern is immersed in water because the wavelength of light decreases in water, which increases the angle of the minima.
Step-by-step explanation:
When a single slit diffraction pattern is immersed in water, the width of the central maximum is affected due to the change in the wavelength of light within the medium. The wavelength of light in a medium is given by λ' = λ/n, where λ is the original wavelength in vacuum and n is the refractive index of the medium. Since water has a refractive index greater than 1, the wavelength of light in water decreases.
This change in wavelength affects the diffraction pattern according to the diffraction formula d sin(θ) = mλ', where d is the width of the slit, θ is the angle of the minima, m is the order of the minima, and λ' is the new wavelength of light in water. The angle of the minima is inversely proportional to the wavelength, so if the wavelength decreases, the angle of the minima increases. As a result, the width of the central maximum becomes narrower when a diffraction pattern is immersed in water without changing any other parameter.
Figure 17.11 shows a single-slit diffraction pattern with a bright central maximum and dimmer and thinner maxima on either side. The location of the minima determines the boundaries of these maxima, which in turn is altered when immersed in water due to the change in wavelength of light.