In AP Physics 2, intensity is the average power a wave transfers per unit area over one period (measured in W/m²). It is proportional to the square of the wave's amplitude, and at a boundary the incident intensity splits between a reflected wave and a transmitted wave.
Intensity measures how much energy a wave delivers, per second, per square meter. Think of it as the wave's "power density." A bright flashlight and a dim one can have the same frequency, but the bright one pumps more power through every square meter it hits. Mathematically, intensity is the average power per unit area over one full period of the wave, with units of watts per square meter.
The relationship you'll actually use is that intensity is proportional to amplitude squared. Double the amplitude and you quadruple the intensity. In Topic 14.3, intensity is the bookkeeping tool for boundaries. When a wave hits the boundary between two media, part of its intensity reflects and part transmits, and the two pieces must add up to the incident intensity because energy is conserved. The frequency never changes at a boundary, but the intensity gets divided up.
Intensity lives in Topic 14.3 (Boundary Behavior of Waves and Polarization) in Unit 14: Waves, Sound, and Physical Optics, supporting learning objective 14.3.A, describing the interaction between a wave and a boundary. The CED says a wave crossing into a new medium produces both a reflected and a transmitted wave, and intensity is how you quantify that split. It's also the quantity that polarizers cut down and that interference redistributes, so it threads through the entire optics unit. Then it shows up again in modern physics, where the photoelectric effect forces you to separate intensity (how many photons arrive) from frequency (how much energy each photon carries). That distinction is one of the most-tested conceptual traps in the course.
Keep studying AP® Physics 2 Unit 14
Boundary Behavior of Waves (Unit 14)
At a boundary, incident intensity splits into reflected plus transmitted intensity, and energy conservation means the pieces sum to the original. If 64% transmits, exactly 36% reflects. Whether that reflected wave flips depends on the speed change, not on how the intensity divides.
Phase Shift and Interference (Unit 14)
When two waves overlap, you add amplitudes first, then square to get intensity. Two identical waves perfectly in phase give four times the intensity of one wave, not two times, because intensity goes as amplitude squared. The phase shift between waves decides whether intensity piles up or cancels.
Polarization (Unit 14)
A polarizer is basically an intensity filter. Unpolarized light loses half its intensity passing through one polarizer, and each additional polarizer cuts intensity based on the angle between filters. Polarization questions are really intensity questions in disguise.
Photoelectric Effect (Unit 15)
Increasing light intensity means more photons per second, so more electrons get ejected. But each electron's maximum kinetic energy depends only on frequency. The 2018 FRQ leaned on exactly this idea, varying frequency while tracking maximum kinetic energy. Cranking intensity never gives individual electrons more energy.
Multiple-choice questions use intensity as the energy-accounting variable at boundaries. A classic stem gives you the transmitted percentage (say 64%) and asks about the reflected wave, where you need both the energy math (36% reflects) and the inversion rule (the wave speeds up in medium B, so the reflection is not inverted). Other stems give acoustic impedances of two media and ask for the transmitted or reflected intensity fraction, like a sound wave hitting a water-steel boundary. Interference questions test whether you know to add amplitudes before squaring, so two equal waves 90° out of phase produce twice the intensity of one wave, not some naive sum. On FRQs, intensity shows up in the photoelectric context, like the 2018 long-answer question on monochromatic light hitting a metal, where you have to argue that frequency, not intensity, controls maximum kinetic energy.
Amplitude is the maximum displacement of the wave; intensity is the power per area it carries. They're linked but not interchangeable, because intensity is proportional to amplitude squared. Doubling amplitude quadruples intensity. This matters most in interference problems, where you must add amplitudes (with phase) first and only then square to find the resultant intensity. Adding intensities directly gives the wrong answer.
Intensity is the average power a wave transfers per unit area over one period, measured in W/m².
Intensity is proportional to amplitude squared, so doubling a wave's amplitude quadruples its intensity.
At a boundary between two media, the incident intensity splits between a reflected wave and a transmitted wave, and the fractions must add to 100% by energy conservation.
Frequency never changes when a wave crosses a boundary, even though intensity gets divided between reflection and transmission.
In interference problems, add the amplitudes first (accounting for phase shift), then square to find intensity; two in-phase identical waves give 4 times the single-wave intensity.
In the photoelectric effect, intensity controls how many electrons are ejected, while frequency controls each electron's maximum kinetic energy.
Intensity is the average power a wave delivers per unit area over one period, in W/m². It scales with the square of the wave's amplitude and tells you how the wave's energy splits at a boundary between two media.
No. Higher intensity means more photons per second, which ejects more electrons, but each electron's maximum kinetic energy depends only on the light's frequency. The 2018 FRQ tested exactly this by varying frequency and tracking maximum kinetic energy.
Amplitude is the wave's maximum displacement; intensity is the power per area, and intensity is proportional to amplitude squared. In interference problems you add amplitudes first, then square, which is why two identical in-phase waves give 4I₀, not 2I₀.
It splits. Part of the incident intensity reflects and part transmits, and the two fractions add to 100% because energy is conserved. If 64% of the intensity transmits into the new medium, 36% reflects back.
No. The CED is explicit that frequency stays the same when a wave moves between media. Intensity divides between the reflected and transmitted waves, and wavelength changes with the new wave speed, but frequency is fixed by the source.
Connect this key term to the AP exam workflow: review the course, practice questions, and check related study tools.
Review units, study guides, and course resources.
Check this vocabulary in multiple-choice context.
Apply key concepts in written AP responses.
Estimate the exam score you are working toward.
Review the highest-yield facts before practice.
Put the full course together before test day.