A wavefront is a surface connecting all points of a wave that are at the same phase (like all the crests), and it is always perpendicular to the light rays that show the wave's direction of travel.
A wavefront is a surface made of every point on a wave that's in the same phase of its cycle at the same moment. Picture dropping a pebble in a pond. Each expanding circular ripple is a wavefront, and every point on that circle is doing the exact same thing at the exact same time. For light from a point source, wavefronts are expanding spheres. Far from the source, those spheres are so huge that a small piece of one looks flat, which is why we treat laser light as having flat (plane) wavefronts.
The wavefront's best friend is the light ray. Per the CED (13.1.A), a light ray is a straight line drawn perpendicular to the wavefront, pointing in the direction the wave travels. So rays and wavefronts are two pictures of the same wave. Rays are the simplified picture you use in geometric optics (Unit 13), where you can ignore the wave nature of light. Wavefronts are the picture you need in Unit 14, because interference and diffraction only make sense when you track how wavefronts from different slits overlap, reinforce, and cancel.
Wavefronts sit at the seam between the two optics units. In Unit 13 (Geometric Optics), learning objective 13.1.A defines a ray as perpendicular to the wavefront, and the CED explicitly warns that rays alone can't explain the spreading of light. That warning is the setup for Unit 14, where wavefronts take over. In Topic 14.7, diffraction is described as multiple wavefronts spreading through a single opening and interfering (14.7.A). In Topic 14.8, the bright and dark fringes of the double-slit pattern come from constructive and destructive interference of wavefronts originating from each slit, controlled by the path length difference ΔD (14.8.A). If you understand wavefronts, you understand exactly when the ray model works and when it breaks, which is one of the big conceptual moves the exam tests.
Keep studying AP® Physics 2 Unit 13
Light ray (Unit 13)
Rays and wavefronts are perpendicular partners describing the same wave. The ray tells you where the energy is going; the wavefront tells you the wave's phase. Every ray diagram you draw in Unit 13 has invisible wavefronts crossing each ray at 90 degrees.
Path length difference and ΔD (Unit 14)
In double-slit problems, the wavefronts leaving each slit travel different distances to a point on the screen. That path length difference decides whether the wavefronts arrive in phase (bright fringe) or out of phase (dark fringe). The whole d sin θ = mλ equation is really a wavefront-phase bookkeeping tool.
Diffraction through a single slit (Unit 14)
When a wavefront squeezes through an opening comparable in size to its wavelength, it doesn't stay flat. It spreads, and the spread-out portions of the wavefront interfere with each other to produce the single-slit pattern. This is the spreading the CED says rays can't explain.
Coherent light (Unit 14)
Interference patterns require coherent sources, meaning the wavefronts keep a constant phase relationship over time. A laser works for double-slit experiments precisely because it produces coherent, monochromatic wavefronts that interfere in a stable, predictable way.
Wavefronts show up most often in multiple-choice questions testing the geometry between rays and wavefronts. Expect stems like a spherical wave expanding from a point source (far away, the wavefronts are nearly flat planes and the rays are nearly parallel), or two laser beams traveling in different directions (their wavefronts are oriented differently, each perpendicular to its own beam). You should be able to (1) define a wavefront as a surface of equal phase, (2) draw or identify wavefronts perpendicular to rays in any diagram, and (3) explain double-slit and single-slit patterns as wavefronts from each slit interfering based on path length difference. No released FRQ has required the word verbatim, but free-response explanations of interference patterns in Unit 14 are stronger and more precise when you describe wavefronts overlapping in or out of phase rather than vaguely saying "the light interferes."
A ray and a wavefront are perpendicular to each other, not the same thing. The ray is an arrow showing the direction the wave travels; the wavefront is the surface of equal phase the wave carries along with it. Geometric optics (Unit 13) gets away with rays alone, but interference and diffraction (Unit 14) require wavefronts because phase is what creates bright and dark fringes. If a question involves spreading, fringes, or path differences, think wavefronts. If it involves mirrors, lenses, and angles, think rays.
A wavefront is a surface connecting all points of a wave that are at the same phase at the same instant.
Light rays are always perpendicular to wavefronts and point in the direction the wave travels.
Far from a point source, spherical wavefronts look approximately flat, so the rays are approximately parallel.
Rays are enough for geometric optics in Unit 13, but interference and diffraction in Unit 14 require thinking about wavefronts.
Bright and dark fringes in slit experiments come from wavefronts arriving in phase or out of phase, which depends on their path length difference.
Diffraction is most pronounced when the opening is comparable in size to the wavelength, because that's when the wavefront spreads the most.
A wavefront is a surface made of all points on a wave that share the same phase, like a line connecting all the crests. For light, wavefronts are perpendicular to the rays and move in the direction of travel.
No. They're perpendicular to each other. The ray is an arrow showing direction of travel; the wavefront is the equal-phase surface the wave carries. The CED (13.1.A) defines the ray as the line perpendicular to the wavefront.
Near a point source, wavefronts are expanding spheres. Far away, any small section of that giant sphere is nearly flat, so the wavefronts look like parallel planes and the rays look like parallel lines. This is why laser beams are modeled with plane wavefronts.
Each slit acts as a source of new wavefronts. Where wavefronts from the two slits arrive at the screen in phase (path length difference equals mλ), you get a bright fringe; where they arrive out of phase, you get a dark fringe. That's the physics behind d sin θ = mλ.
In geometric optics (Unit 13), like reflection off mirrors and refraction through lenses, rays alone work fine. The moment light spreads through small openings or produces fringes (Topics 14.7 and 14.8), you need the wavefront picture, because rays can't explain interference.
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