Final answer:
When an optical apparatus for single-slit diffraction is immersed in water, the diffraction pattern becomes narrower because the wavelength of light decreases due to the higher refractive index of water compared to air.
Step-by-step explanation:
When the optical apparatus used in a single-slit diffraction experiment is immersed in water, the diffraction pattern changes due to the alteration in the medium through which light is traveling. In particular, light travels more slowly in water than in air because water has a higher refractive index. This causes the wavelength of light to decrease in water compared to its wavelength in air. As a result of the reduced wavelength, the diffraction pattern becomes narrower when the optical apparatus is immersed in water. Therefore, the correct answer to what happens to the diffraction pattern of a single slit when the entire optical apparatus is immersed in water is: a) The diffraction pattern becomes narrower.
Diffraction occurs when waves pass through a small opening or around an obstacle, and it is what allows waves to bend around corners and spread out. Interference, on the other hand, is a phenomenon in which waves combine to form a new wave pattern. In the context of light, interference and diffraction can both produce patterns of light and dark bands, but they arise from different physical processes.