INTERFERENCE AND DIFFRACTION
As we learned in the preceding chapter, waves experience interference when they
meet, and whether they interfere constructively or destructively depends on their
relative phase. If they meet in phase (crest meets crest), they combine
constructively, but if they meet out of phase (crest meets trough), they combine
destructively. The key to the interference patterns we’ll study in the next section
rests on these points. In particular, if waves that have the same wavelength meet,
then the difference in the distances they’ve traveled determine whether they are in
phase. Assuming that the waves are coherent (which means that their phase
difference remains constant over time and does not vary), then if the difference in
their path lengths, ∆ℓ, is a whole number of wavelengths—0, ±λ, ±2λ, etc.—they’ll
arrive in phase at the meeting point. However, if this difference is a whole number
plus one-half a wavelength—± λ, ±(1 + )λ, ±(2 + )λ, etc.—then they’ll arrive
exactly out of phase. That is
Young’s Double-Slit Interference Experiment
The following figure shows light incident (shining) on a barrier that contains two
narrow slits (perpendicular to the plane of the page), separated by a distance d. On
the right is a screen whose distance from the barrier, L, is much greater than d. The
question is, what will we see on the screen? You might expect to see just two bright
narrow strips of light, directly opposite the slits in the barrier, but this is not what
happens.