## Conceptual Physics (12th Edition)

In water, the wavelengths of light are compressed (see Figure 28.25). In Figure 29.18, the bright fringes are seen when there is a full-wavelength difference in path lengths from the two slits. With a compressed wavelength, one does not have to go as far from the centerline to have a full-wavelength path difference, so the pattern is compressed and the fringes are closer together. The widely-spaced fringes are seen when the experiment is carried out in air. Alternatively, the equation at the bottom of page 550 explains this. $$\lambda = d sin \theta$$ In air, we have a longer wavelength, $\lambda$. The left-hand side of the equation is larger, so the right-hand side must be larger too. The distance between the slits, d, is fixed. The longer wavelength in air gives a more spread-out pattern, i.e., $\theta$ increases, because sin $\theta$ increases.