Transmission is the passage of light through a medium; when light hits a boundary between two media, some light transmits into the new medium while the rest is reflected or absorbed, and the transmitted ray keeps its original phase.
Transmission is what happens when light actually makes it through a boundary and keeps traveling in the new medium. Every time light hits an interface between two materials (air to glass, oil to water), the energy gets split three ways. Some light is transmitted into the new medium, some is reflected back, and some is absorbed. That three-way split is straight from the Essential Knowledge for Topic 14.9.
The detail the AP exam actually cares about is phase. A reflected ray can pick up a 180-degree phase change (it does when bouncing off a medium with a higher index of refraction). A transmitted ray never does. The phase of a wave does not change when it transmits into a new medium. So in any thin-film problem, only the reflections can flip the wave; the light passing through the film is phase-neutral. Keep in mind transmission isn't all-or-nothing. The same incoming beam produces both a transmitted ray and a reflected ray at the same surface, and that's exactly why thin films create interference in the first place.
Transmission lives in Unit 14: Waves, Sound, and Physical Optics, specifically Topic 14.9: Thin Film Interference, supporting learning objective 14.9.A (describe the behavior of light that interacts with a thin film). Thin-film interference only works because a film's top surface transmits part of the light down to the bottom surface, where it reflects and comes back up to interfere with the ray that reflected off the top. If you don't track which rays transmitted (no phase change) and which reflected (possible 180-degree flip), you'll pick the wrong condition for constructive versus destructive interference. This is also the physics behind antireflection coatings on glasses and camera lenses, which are engineered to maximize transmission by making the reflected rays cancel each other out.
Keep studying AP® Physics 2 Unit 14
Antireflection Coating (Unit 14)
An antireflection coating is transmission engineering. The coating's thickness is chosen so the two reflected rays interfere destructively, which means almost all the light transmits through the lens instead of bouncing off. Practice questions love asking for real-world examples, like coated eyeglasses and camera lenses.
Thin Film Interference (Unit 14)
This is the home topic. The colorful swirls on an oil film exist because light partially transmits through the film's top surface, reflects off the bottom, and travels back up to interfere with the top-surface reflection. The transmitted leg adds extra path length (2t inside the film), which sets the interference condition.
Reflection and Phase Change (Unit 14)
Reflection is transmission's partner at every boundary, and it's the only one of the pair that can flip phase. A 180-degree shift happens when light reflects off a higher-index medium; no shift happens off a lower-index medium. Transmitted light never shifts. That asymmetry is the whole trick of Topic 14.9.
Index of Refraction (Unit 14)
Inside the medium that light transmits into, the wavelength shrinks to λ/n. So when you count wavelengths along the transmitted path through a thin film, you have to use the wavelength in the film, not the wavelength in air. Forgetting this is one of the most common thin-film mistakes.
Transmission shows up in multiple-choice questions as part of the setup for thin-film interference. Typical stems ask what causes a phase shift in thin-film interference (answer: reflection off a higher-index medium, never transmission), what phenomenon explains the colors in oil films, or what real-world devices use antireflection coatings. You should be able to do three things with this term. First, state the three-way split at any boundary (transmitted, reflected, absorbed). Second, correctly assign phase changes only to reflections, using the relative indices of refraction. Third, combine the transmitted ray's extra path length (2t through the film, measured in the film's wavelength λ/n) with any reflection phase flips to decide whether interference is constructive or destructive. Quantitative analysis assumes light hits the film at normal incidence, meaning perpendicular to the surface, which is another detail MCQs have tested. No released FRQ has hinged on the word 'transmission' by itself, but it's baked into any thin-film interference question.
Transmission means light passes into the new medium at all. Refraction is the bending that the transmitted ray undergoes because light travels at a different speed in the new medium. So refraction is something that happens to transmitted light, not a separate fate for it. In thin-film problems you usually assume normal incidence (light hitting the film perpendicularly), so the transmitted ray doesn't visibly bend, and you only worry about transmission's path length and phase behavior.
When light hits a boundary between two media, some of it is transmitted, some is reflected, and some is absorbed, all at the same time.
Transmitted light never changes phase; only reflection can produce a phase change.
A reflected ray flips phase by 180 degrees when it bounces off a medium with a higher index of refraction, and it keeps its phase when it bounces off a lower-index medium.
Thin-film interference happens because one ray reflects off the film's top surface while another transmits through, reflects off the bottom, and travels back to interfere with the first.
Inside the film, the transmitted light's wavelength is λ/n, so the extra path length 2t must be compared to the wavelength in the film, not the wavelength in air.
Antireflection coatings work by making reflected rays cancel destructively, which maximizes the light that transmits through a lens.
Transmission is the passage of light through a medium. At any boundary between two media, the incoming light splits: some transmits into the new medium, some reflects, and some is absorbed. It's tested in Topic 14.9 (Thin Film Interference).
No. The phase of a wave does not change when it transmits into a new medium. The 180-degree phase flip only happens on reflection, and only when the light reflects off a medium with a higher index of refraction.
Not quite. Transmission means the light enters the new medium; refraction is the bending of that transmitted ray due to the speed change. In thin-film problems at normal incidence, the light transmits straight through without noticeable bending.
A boundary between two media always splits the light's energy. Part of the wave continues into the second medium (transmission) while part bounces back (reflection). This split at both surfaces of a thin film is exactly what creates the two interfering rays in oil-film colors and soap bubbles.
Antireflection coatings on glasses and camera lenses are designed to maximize transmission. The coating's thickness makes the rays reflected from its top and bottom surfaces interfere destructively, so reflection is minimized and nearly all the light passes through the lens.
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