Cracking the SAT Physics Subject Test

where y measures the vertical displacement along the screen from the center of the
screen (y = 0, the point directly across from the midpoint of the slits). The bright
fringe directly opposite the midpoint of the slits—the central maximum—will
have the greatest intensity, the bright fringes with m = ±1 will have a lower
intensity, those with m = ±2 will be fainter still, and so on. If more than two slits
are cut in the barrier, the interference pattern becomes sharper, and the distinction
between dark and bright fringes becomes more pronounced. Barriers containing
thousands of tiny slits per centimeter—called diffraction gratings—are used
precisely for this purpose.

Questions 4-5

For the experimental setup we’ve been studying, assume that d = 1

mm, L = 6.0 m, and that the light used has a wavelength of 600 nm.

4. How far above the center of the screen will the first bright fringe
   appear?


5. What would happen to the interference pattern if the slits were
   moved closer together?

Here’s How to Crack It

4. The central maximum corresponds to m = 0 (y 0 = 0). The first maximum
   above the central one is labeled y 1 (since m = 1), and the first maximum
   below the central one is labeled y−1 (that is, m = −1). The other bright fringes
   on the screen are labeled accordingly.