Physics 2 · Geometric and Physical Optics · 14 min read · Updated 2026-05-11
Interference and Diffraction — AP Physics 2
AP Physics 2 · Geometric and Physical Optics · 14 min read
1. Fundamentals of Interference and Diffraction★★☆☆☆⏱ 3 min
Interference occurs when two or more coherent light waves (with constant phase difference) superpose, producing a resultant wave with amplitude dependent on the phase difference of the incoming waves. Diffraction is the bending of light as it passes through a narrow aperture or around an obstacle, an effect that only exists because light acts as a wave. Diffracted waves from different parts of the aperture naturally superpose, producing interference patterns.
This topic makes up approximately 10-15% of the total AP Physics 2 exam score, appearing in both multiple-choice (MCQ) and free-response (FRQ) sections, and provides foundational evidence for the wave nature of light.
2. Double-Slit Interference★★☆☆☆⏱ 4 min
Young's double-slit experiment was the first definitive proof that light acts as a wave. In this setup, coherent monochromatic light passes through two narrow slits separated by distance $d$, and projects onto a screen a distance $L$ away from the slits. For any point on the screen, the path difference between light from the two slits is $\Delta r = d\sin\theta$.
Constructive interference (bright fringe) occurs when the path difference is an integer multiple of the wavelength:
For small angles ($\theta < 10^\circ$), $\sin\theta \approx \tan\theta = \frac{y}{L}$, where $y$ is the vertical distance from the central maximum to the fringe. This simplifies to the fringe spacing formula (distance between adjacent bright fringes):
\Delta x = \frac{\lambda L}{d}
Exam tip: Always convert all length units to the same base unit (usually meters) before plugging values into formulas; unit mismatches are the most common avoidable mistake on this topic.
3. Single-Slit Diffraction★★★☆☆⏱ 4 min
When light passes through a single narrow slit of width $a$, each point within the slit acts as a secondary wave source, and these secondary waves interfere to produce a diffraction pattern. The pattern features a wide, very bright central maximum, followed by much dimmer, narrower maxima on either side, with dark minima between each maximum. Unlike double-slit interference, single-slit maxima are not equally spaced.
The condition for dark minima (zero intensity) in single-slit diffraction is:
Note that $m=0$ is not a minimum, as all waves interfere constructively at the central maximum. For small angles, the distance from the central maximum to the first minimum on one side is $y = \frac{\lambda L}{a}$. The full width of the central maximum (distance between the two first minima on opposite sides of center) is:
W = \frac{2\lambda L}{a}
A key relationship: decreasing the slit width $a$ increases the width of the central maximum, meaning more diffraction occurs for smaller apertures.
Exam tip: Never mix up $a$ (single slit width) and $d$ (slit separation for double-slit/grating); swapping these gives an answer off by multiple orders of magnitude.
4. Diffraction Gratings★★★☆☆⏱ 4 min
A diffraction grating is an optical device with hundreds or thousands of equally spaced parallel slits etched into a glass or metal surface. It produces much sharper, brighter, and better-separated principal maxima than a double-slit, making it ideal for measuring the wavelength of unknown light or splitting white light into its component wavelengths to produce a spectrum.
For a grating with $N$ lines per unit length, the separation between adjacent slits is $d = \frac{1}{N}$. The condition for principal (bright) maxima is identical to the double-slit constructive interference condition:
Since $\sin\theta$ can never be greater than 1, the maximum possible order $m$ is the largest integer less than $\frac{d}{\lambda}$. Any order with $m > \frac{d}{\lambda}$ does not exist, as it would require a diffraction angle larger than 90°.
Exam tip: Always check if your calculated $\sin\theta$ is greater than 1; if it is, that order does not exist, and you need to pick the next smaller integer order.
5. AP-Style Problem Solving Practice★★★★☆⏱ 5 min
Common Pitfalls
Why: The symbols are both lowercase letters, so students often confuse what each variable measures.
Why: Students memorize that the first minimum is at $y = \lambda L/a$ from center, so they stop there instead of doubling for full width between the two minima.
Why: Students get used to small angles in basic double-slit experiments and automatically use the approximation regardless of angle size.
Why: Problems usually give lines per mm or lines per cm, so students just plug the given number in without inverting or converting units.
Why: Students memorize the conditions backwards or confuse them with thin film interference rules.