SAT II Physics questions on optical instruments are generally of two kinds. Either there
will be a quantitative question that will expect you to apply one of the two equations
we’ve learned, or there will be a qualitative question asking you to determine where light
gets focused, whether an image is real or virtual, upright or upside down, etc.
Wave Optics
As you may know, one of the weird things about light is that some of its properties can be
explained only by treating it as a wave, while others can be explained only by treating it as
a particle. The classical physics that we have applied until now deals only with the particle
properties of light. We will now take a look at some phenomena that can only be
explained with a wave model of light.
Young’s Double-Slit Experiment
The wave theory of light came to prominence with Thomas Young’s double-slit
experiment, performed in 1801. We mention this because it is often called “Young’s
double-slit experiment,” and you’d best know what SAT II Physics means if it refers to
this experiment. The double-slit experiment proves that light has wave properties
because it relies on the principles of constructive interference and destructive
interference, which are unique to waves.
The double-slit experiment involves light being shone on a screen with—you guessed it—
two very narrow slits in it, separated by a distance d. A second screen is set up a distance
L from the first screen, upon which the light passing through the two slits shines.
Suppose we have coherent light—that is, light of a single wavelength , which is all
traveling in phase. This light hits the first screen with the two parallel narrow slits, both
of which are narrower than. Since the slits are narrower than the wavelength, the light
spreads out and distributes itself across the far screen.