AP Physics 2: Algebra-Based Practice Quiz: Thin-Film Interference
Written by AP Content Team, Verified for 2026 AP Exams, Last updated: May 2026
Test your understanding with short quizzes. This quiz has 10 questions to check your progress.
Question 1 of 10
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A) It must be significantly larger than the light's wavelength.
B) It must be comparable to the light's wavelength.
C) It must be uniform and perfectly flat.
D) It is irrelevant to the phenomenon.
Correct Answer: B
The provided content explicitly states that thin-film interference occurs when light interacts with a medium whose thickness is comparable to the light’s wavelength.
A) The bending of a straw's appearance in a glass of water.
B) The separation of white light into a spectrum by a prism.
C) The colorful patterns seen on a soap bubble.
D) The focusing of light by a magnifying glass.
Correct Answer: C
The content lists soap bubbles and oil films as practical examples of thin-film interference, where the varying thickness of the film causes different colors of light to interfere constructively.
A) The intensity of the incident light.
B) The angle at which the light strikes the film.
C) The relative indices of refraction of the two materials.
D) The color (wavelength) of the incident light.
Correct Answer: C
According to the provided content, the phase change of a reflected ray specifically depends on the relative indices of refraction of the materials at the interface.
A) the oil chemically reacting with the water.
B) the dispersion of light as it passes through the oil.
C) the interference of light waves reflecting from the top and bottom surfaces of the oil film.
D) the polarization of light by the oil's molecular structure.
Correct Answer: C
This phenomenon is a classic example of thin-film interference. Light reflects from both the top surface of the oil and the bottom (oil-water) surface. These two reflected rays interfere, and because the oil film's thickness varies, different wavelengths (colors) interfere constructively at different points, creating the colorful pattern.
A) They create constructive interference to enhance the brightness of the transmitted light.
B) They filter out specific colors by absorbing them.
C) They create destructive interference for reflected light, minimizing glare and maximizing transmission.
D) They polarize the incoming light to reduce reflections from non-metallic surfaces.
Correct Answer: C
Antireflection coatings are a practical application of thin-film interference. They are designed so that light waves reflecting from the outer and inner surfaces of the coating interfere destructively, which cancels out the reflection and allows more light to pass through the lens.
A) Light is completely absorbed by the film.
B) Light is partially reflected and partially transmitted at the film's surfaces.
C) Light is scattered in random directions by the film.
D) Light changes its frequency upon entering the film.
Correct Answer: B
The description of thin-film interference implies that light rays reflect from both the top and bottom surfaces of the film. This requires the light to be partially reflected and partially transmitted at the first surface, which is the fundamental behavior of light interacting with a thin film.
A) The change in light's speed within the film.
B) The potential for a phase change upon reflection at one or both surfaces.
C) The polarization of light upon reflection.
D) The absorption of specific wavelengths by the soap material.
Correct Answer: B
The overall interference pattern depends on the total phase difference between the two reflected rays. This total difference is a combination of the phase difference from the extra path traveled by one ray and any phase changes that occur upon reflection, which depend on the relative indices of refraction.
A) A thin layer of oil on a puddle.
B) A soap bubble floating in the air.
C) An antireflective coating on eyeglasses.
D) A thick, clear glass window pane.
Correct Answer: D
Thin-film interference requires the thickness of the medium to be comparable to the wavelength of light. A thick glass window pane is many thousands of times thicker than the wavelength of visible light, so interference effects are not observable.
A) Reflection and interference.
B) Diffraction and polarization.
C) Refraction and dispersion.
D) Absorption and scattering.
Correct Answer: A
Light rays reflect from the top and bottom surfaces of the film. These reflected rays then interfere with each other, leading to the observed patterns. Therefore, reflection and interference are the two key phenomena.
A) the light source is changing color.
B) the refractive index of the soap is constantly changing.
C) the thickness of the soap film is not uniform and is changing due to gravity and air currents.
D) your viewing angle changes the polarization of the reflected light.
Correct Answer: C
The specific color that interferes constructively at any point on the bubble depends on the film's thickness at that point. As the soap drains due to gravity or is moved by air, the thickness changes, causing the observed colors to shift and swirl. This links the example of a soap bubble to the principle of thickness-dependent interference.