Reading assignment: 37.1-37.3
The figure on the left is from your textbook. The figure on the right is
from the Wikipedia article on
double slit experiment, and more closely corresponds to what Young actually
did. What is the purpose of S1 in the right figure? (This is subtle, but give it
your best guess.)
The figure on the left requires that the light going through slit 1 is always perfectly in phase with the light going through slit 2. (Notice the wave fronts on the left.) You can do that with a laser, or (as Young did) with regular light by having a single slit prior to the double slit setup. Notice how slit "a" on the right guarantees that the light reaching "b" and "c" will always be in phase.
Consider a two-slit interference pattern like those above. What do you suppose
would happen if the entire setup were immersed in water?
The interference max and min are set by the wavelength of the light. See section 37.2. In water, the wavelength is reduced by the index of refraction; therefore the pattern of max & min will shrink.