3 questions, one for each idea where we can. Answer them, then see which ideas to fix.
Question 1 of 3
In each diagram, the arrow shows the direction in which a wave travels and the other lines are drawn to represent the wave. In which diagram are the lines the wave fronts of a plane wave?
Answer and reasoning
ADiagram Q A student who draws wave fronts along the direction of travel, like rays, picks this. Lines parallel to the direction of travel are rays; wave fronts are perpendicular to it.
BDiagram PCorrect The wave fronts of a plane wave are flat, parallel to one another and perpendicular to the direction of travel. In a diagram they appear as straight, parallel lines drawn across the direction of travel, as in P.
CDiagram R A student who thinks a plane wave spreads out across a flat surface, like ripples on a pond, picks this. Circular wave fronts spread out from a point; a plane wave's wave fronts are straight and parallel.
DDiagram S A student who thinks a plane wave is an oscillation drawn in one flat plane picks this. A single sine curve is a graph of the field along the line of travel; 'plane' refers to the flat shape of the wave fronts.
A spacecraft far beyond Earth's atmosphere sends radio signals to a receiving station on Earth. Which statement about how the signals cross the space between them is correct?
Answer and reasoning
AThey are carried by the thin gas in space, as every wave needs a medium. A student who thinks every wave needs a medium picks this. Mechanical waves such as sound do, but electromagnetic waves do not: their fields exist in a vacuum, and they would reach Earth even if space were perfectly empty.
BThey travel as sound waves, which the receiving station then plays aloud. A student who thinks radio waves are sound waves picks this. Sound needs a medium and cannot cross the vacuum of space; the signals are electromagnetic waves, which a receiver converts into an electrical signal and a loudspeaker into sound.
CThey travel more slowly than light, as radio waves have a lower frequency. A student who thinks an electromagnetic wave's speed depends on its frequency picks this. All electromagnetic waves travel at the same speed in a vacuum, c = 3.00 × 10⁸ m/s; a lower frequency means a longer wavelength, not a lower speed.
DThey travel as electromagnetic waves, which do not need a medium.Correct Radio waves are electromagnetic waves: oscillating electric and magnetic fields that can exist in empty space. They need no medium, so they cross the near-vacuum of space and reach Earth.
X-rays and radio waves are both electromagnetic waves. Which property distinguishes the two categories?
Answer and reasoning
ATheir speed in a vacuum, which is much greater for X-rays A student who thinks higher-frequency electromagnetic waves travel faster picks this. Every electromagnetic wave travels at c = 3.00 × 10⁸ m/s in a vacuum; X-rays differ from radio waves in wavelength and frequency.
BTheir amplitude, which is much greater for X-rays A student who links X-rays' high energy with a large amplitude picks this. The category is set by wavelength; a weak X-ray beam is still X-rays and a strong radio signal is still radio waves.
CTheir nature: X-rays are particles, and radio waves are waves A student who thinks X-rays are particles rather than light-like waves picks this. X-rays are electromagnetic waves, as radio waves are; they sit at the short-wavelength end of the same spectrum.
DTheir wavelength, which is much shorter for X-raysCorrect The categories of electromagnetic waves are characterized by their wavelengths. X-rays have far shorter wavelengths (and so far higher frequencies) than radio waves; in a vacuum both travel at c.
In preparation: 0 of 3 sections compiled and reviewed. The rest show key terms and common mistakes from our question bank until they are.
14.4.A.1 Electromagnetic wave Fix
Electromagnetic wave
A wave made of an oscillating electric field and an oscillating magnetic field. At every point the two fields are perpendicular to each other and to the direction in which the wave travels, and the wave transfers energy from one place to another.
Electric and magnetic fields of the wave, E⃗ and B⃗
Vector fields that oscillate in magnitude and direction as the wave passes. For a wave traveling along the x-axis whose electric field oscillates along the y-axis, the magnetic field oscillates along the z-axis. Units: E in N/C (= V/m), B in T.
Transverse wave
A wave in which the disturbance is perpendicular to the direction of propagation. Electromagnetic waves are transverse because both oscillating fields are perpendicular to the direction of travel.
Wave front
A line or surface joining neighboring points of a wave that are at the same stage of their oscillation, such as all the points at a crest. Rays are drawn perpendicular to wave fronts, in the direction of travel.
Plane wave
A wave whose wave fronts are flat planes, parallel to one another and perpendicular to the direction of travel. Light far from its source, or in a laser beam, is commonly modeled as a plane wave.
Students often think The electric and magnetic fields of an electromagnetic wave oscillate along the same line, in the same plane. In fact No. They are mutually perpendicular: if the electric field oscillates along the y-axis for a wave traveling along the x-axis, the magnetic field oscillates along the z-axis.
Students often think The fields of an electromagnetic wave oscillate back and forth along the direction of travel, like the air in a sound wave. In fact No. Both fields oscillate perpendicular to the direction of travel; an electromagnetic wave is transverse, while sound in air is longitudinal.
14.4.A.2 Propagation without a medium Fix
Propagation without a medium
Electromagnetic waves need no medium: their oscillating fields can exist in empty space, so they travel through a vacuum (at c = 3.00 × 10⁸ m/s, the same for every category). Mechanical waves such as sound need a medium.
Students often think Every wave, including light and radio waves, needs a medium, so electromagnetic waves crossing space must be carried by something that fills it. In fact No. Electromagnetic waves are oscillating electric and magnetic fields, which can exist in empty space, so they need no medium. Light and radio waves cross the vacuum of space; sound cannot.
Students often think Radio waves are sound waves: the sound travels from the transmitter to the radio. In fact No. Radio waves are electromagnetic waves. A radio receiver detects them and uses them to drive a loudspeaker, which then produces sound waves in the air near the listener.
14.4.A.3 Electromagnetic spectrum Fix
Electromagnetic spectrum
The full range of electromagnetic waves, divided into categories by wavelength. Because every category travels at c in a vacuum, λ = c/f, so a longer wavelength also means a lower frequency.
Order of the categories
In order of decreasing wavelength (increasing frequency): radio waves, microwaves, infrared, visible, ultraviolet, X-rays and gamma rays, spanning wavelengths from kilometers to picometers.
Colors of visible light
Visible light is divided into colors by wavelength; in order of decreasing wavelength: red, orange, yellow, green, blue and violet.
Light and electromagnetic radiation
'Light' usually means visible electromagnetic waves, but electromagnetic waves of all wavelengths are sometimes called light or electromagnetic radiation. 'Radiation' here means energy carried by electromagnetic waves, not particles from radioactive materials.
Students often think Electromagnetic waves are sorted into categories by their amplitude: X-rays are 'bigger' waves than radio waves. In fact No. The categories are set by wavelength (or frequency), not by amplitude. A weak X-ray beam and a strong radio signal are still X-rays and radio waves.
Students often think X-rays and gamma rays are a different kind of thing from light: they are particles given off as 'radiation', not electromagnetic waves. In fact No. X-rays and gamma rays are electromagnetic waves, like light and radio waves, differing from them only in wavelength.
5 more questions. Every wrong answer here is a real mistake students make, and you see why it is wrong as soon as you answer.
Question 1 of 5
The diagram shows the electric field E⃗ of an electromagnetic wave at one instant, and the direction in which the wave travels. Along which line does the magnetic field of the wave oscillate, and why?
Answer and reasoning
AAlong the y-axis, parallel to E⃗, since the fields oscillate together A student who thinks the two fields oscillate along the same line picks this. The fields do oscillate at the same frequency, but they are perpendicular to each other: with E⃗ along y and travel along x, B⃗ oscillates along z.
BAlong the x-axis, the direction of travel, as the air does in a sound wave A student who pictures the fields oscillating along the direction of travel, as in a sound wave, picks this. Both fields of an electromagnetic wave oscillate perpendicular to the direction of travel; the wave is transverse.
CAlong the z-axis, perpendicular to both E⃗ and the direction of travelCorrect The electric and magnetic fields of an electromagnetic wave are mutually perpendicular, and both are perpendicular to the direction of travel. E⃗ oscillates along the y-axis and the wave travels along the x-axis, so the magnetic field oscillates along the z-axis.
DIn circles around the x-axis, as the field around a current does A student who carries over the circular field around a current-carrying wire picks this. The magnetic field of the wave oscillates back and forth along one line, the z-axis, perpendicular to E⃗ and to the direction of travel.
The diagram shows the electric field vectors at points along the path of an electromagnetic wave at one instant, and the direction in which the wave travels. Which claim about the wave, with its reasoning, is correct?
Answer and reasoning
AIt is transverse, as the field oscillates across the direction of travel.Correct The field vectors all point up or down, perpendicular to the direction of travel, and the magnetic field is perpendicular to the direction of travel as well. A wave whose disturbance is perpendicular to its direction of travel is transverse.
BIt is longitudinal, as it carries energy along the direction of travel. A student who classifies a wave by the direction its energy moves picks this. Every wave carries energy along its direction of travel; a wave is longitudinal only if its disturbance is along that direction, and these field vectors are perpendicular to it.
CIt is transverse, as the light moves along a path that zigzags up and down. A student who reads the up-and-down pattern as the path of the light picks this. The light travels straight along the line; the arrows show how the field's value varies from point to point along it at one instant.
DIt is transverse, as its electric field is perpendicular to its magnetic field. A student who confuses the fields' perpendicularity to each other with the meaning of transverse picks this. The fields are perpendicular to each other, but a wave is transverse because its disturbance is perpendicular to the direction of travel, which is what the diagram shows.
Which ranking of the wavelengths of four categories of electromagnetic waves, from longest to shortest, is correct?
Answer and reasoning
AMicrowave > infrared > ultraviolet > X-rayCorrect In order of decreasing wavelength the spectrum runs radio waves, microwaves, infrared, visible, ultraviolet, X-rays and gamma rays. Microwaves are the longest of these four and X-rays the shortest.
BX-ray > ultraviolet > infrared > microwave A student who reverses the order of the spectrum picks this. X-rays have the highest frequencies of these four and so, by λ = c/f, the shortest wavelengths; microwaves have the longest.
CMicrowave > ultraviolet > infrared > X-ray A student who reads 'ultra' as meaning a longer wavelength than 'infra' picks this. Infrared lies beyond the red (long-wavelength) end of the visible spectrum and ultraviolet beyond the violet (short-wavelength) end, so infrared is longer.
DInfrared > ultraviolet > X-ray > microwave A student who takes the prefix 'micro' to mean the shortest wavelengths picks this. Microwaves are 'micro' only compared with radio waves; their wavelengths are longer than those of infrared, ultraviolet and X-rays.
How does the wavelength of orange light in a vacuum compare with the wavelength of blue light in a vacuum?
Answer and reasoning
ABlue light has the longer wavelength. A student who reverses the order of the spectrum picks this. Blue is nearer the violet (short-wavelength) end of the visible spectrum and orange nearer the red (long-wavelength) end.
BThey are equal, as both travel at c. A student who thinks equal speeds mean equal wavelengths picks this. Both travel at c in a vacuum, but orange light has a lower frequency, so by λ = c/f its wavelength is longer.
COrange light has the longer wavelength.Correct In order of decreasing wavelength the colors are red, orange, yellow, green, blue and violet, so orange light has a longer wavelength than blue light (and a lower frequency).
DIt depends on how bright each beam is. A student who thinks brightness changes the wavelength picks this. The wavelength sets the color; a dimmer or brighter orange beam is still orange and has the same wavelength.
A physics textbook uses the term 'electromagnetic radiation'. Which waves does this term include?
Answer and reasoning
AThe particles and rays that radioactive materials give off A student who reads 'radiation' as nuclear radiation picks this. In 'electromagnetic radiation' the word means energy carried by electromagnetic waves of any wavelength; alpha and beta particles are not electromagnetic waves at all.
BWaves of every wavelength, from radio waves to gamma raysCorrect Electromagnetic waves of all wavelengths, visible light included, are collectively called electromagnetic radiation (and sometimes light). Radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays all qualify.
CThe invisible waves, such as ultraviolet, but not visible light A student who thinks 'radiation' means only invisible waves picks this. Visible light is electromagnetic radiation too; it lies between infrared and ultraviolet on the same spectrum.
DInfrared waves, which are the kind of radiation that carries heat A student who equates radiation with heat radiation picks this. Infrared is one category of electromagnetic radiation; the term covers every category, from radio waves to gamma rays.
Compiled from the AP Physics 2 Course and Exam Description (effective Fall 2024, 2026 reissue) and our question bank · Specialist review in progress. How these pages are made · Free, no account
