Answer
a. The fringes on the screen become more tightly packed.
b. The fringes on the screen spread out, and become more loosely packed.
Work Step by Step
a. Light traveling in water has a shorter wavelength than the same light traveling in air. Equation 24-3b, $sin \theta = \frac{m \lambda}{D}$, predicts the location of the minima for nonzero integer values of m. Decreasing the wavelength $\lambda$ in a single-slit diffraction experiment will decrease the angle $\theta$ at which each minimum is seen, decreasing the spacing of the fringes.
b. Light traveling in a vacuum has a longer wavelength than the same light traveling in air. Equation 24-3b, $sin \theta = \frac{m \lambda}{D}$, predicts the location of the minima for nonzero integer values of m. Increasing the wavelength $\lambda$ in a single-slit diffraction experiment will increase the angle $\theta$ at which each minimum is seen, increasing the spacing of the fringes.