Amplitude vs Wavelength: What Is the Difference?

Amplitude is the maximum displacement from equilibrium, read up the vertical axis. Wavelength is the distance between matching points, read along the horizontal axis. Amplitude is independent of frequency and medium; wavelength depends on both. Turning a note up does not change it.

AP Physics: Unit 14 (topics 14.1 Properties of Wave Pulses and Waves, 14.2 Periodic Waves). Amplitude is AP Physics 2 essential knowledge 14.1.A.6 in Topic 14.1: the maximum displacement of a wave from its equilibrium position. Its sub-points allow a pressure amplitude for a longitudinal wave, the maximum increase or decrease in pressure from equilibrium pressure (14.1.A.6.i), tie loudness to amplitude (14.1.A.6.ii), and state that the energy carried by a wave increases with increasing amplitude (14.1.A.6.iii). Wavelength is essential knowledge 14.2.A.1.vi in Topic 14.2: the distance between successive corresponding positions, such as peaks or troughs, on a wave. The relation between wavelength, speed and frequency is 14.2.A.3, lambda = v/f, printed on the AP Physics 2 sheet, and the independence that separates the pair is 14.2.A.1.iii, the amplitude of a wave is independent of the period and the frequency of that wave. Essential knowledge 14.2.A.2 supplies the two example equations printed on the sheet, x(t) = A cos(omega t) = A cos(2 pi f t) and y(x) = A cos(2 pi x / lambda), where the same A appears in both and only one contains lambda. Frequency and pitch are 14.2.A.1.v and energy increasing with frequency is 14.2.A.1.iv. Intensity is defined only in Topic 14.3, at 14.3.A.3.i as power transferred per unit area and 14.3.A.3.ii as the average power per unit area over one period, both under 14.3.A.3 about polarization reducing intensity; there is no intensity equation on the sheet and no intensity entry in the symbol list of its waves, sound, and optics block, checked by rendering the appendix page, and the CED nowhere states that intensity goes as the square of the amplitude. The only quantitative amplitude relation in the AP framework is AP Physics 1 essential knowledge 7.4.A.4.ii for a spring and object system, E_total = (1/2) k A^2, alongside 7.3.A.2 that changing the amplitude of a system exhibiting SHM will not change its period. Electromagnetic waves oscillate fields rather than matter (14.4.A.1) and are categorised by wavelength (14.4.A.3). None of Topics 14.1, 14.2 or 14.3 carries a boundary statement. Unit 14 is weighted at 12 to 15 percent of the multiple-choice section over a suggested 14 to 23 class periods; AP Physics 1 has no waves unit, and its Unit 7, Oscillations, is 5 to 8 percent over a suggested 5 to 10 class periods.

The distinction, stated once

On a snapshot of a wave these two are measured on the same drawing, at right angles to each other, which is exactly why they get swapped.

Amplitude is measured up. AP Physics 2 essential knowledge 14.1.A.6 states it plainly: amplitude is the maximum displacement of a wave from its equilibrium position. Symbol $A$ . It is a single distance from the flat line to the top of a crest, not from the top of a crest to the bottom of a trough.

Wavelength is measured along. Essential knowledge 14.2.A.1.vi gives it as the distance between successive corresponding positions, such as peaks or troughs, on a wave. Symbol $\lambda$ . It runs parallel to the direction the wave travels, and it is the repeat distance of the pattern.

So far they sound like two sides of one rectangle. The physics separates them at once, and it separates them by dependence rather than by direction.

Wavelength is tied to the frequency and to the medium. The AP Physics 2 equation sheet prints the relevant equation for 14.2.A.3,

\lambda = \frac{v}{f}

and the CED states the dependence in words: the wavelength is proportional to the wave's speed and inversely proportional to the wave's frequency. Change the note, or carry the wave into a new material, and the wavelength moves.

Amplitude is tied to neither. Essential knowledge 14.2.A.1.iii is the sentence that does the work on this page: the amplitude of a wave is independent of the period and the frequency of that wave. Nothing on the equation sheet relates $A$ to $\lambda$ , to $f$ , or to $v$ , and that absence is not an oversight; it is the physics.

The consequence is the thing worth carrying out of here. You can turn a note louder without changing the note. Amplitude and wavelength are set by different knobs, and the fact that you read them off the same picture is a fact about the picture.

Side by side

	Amplitude	Wavelength
Symbol	$A$	$\lambda$
CED definition	14.1.A.6, the maximum displacement of a wave from its equilibrium position	14.2.A.1.vi, the distance between successive corresponding positions such as peaks or troughs
Direction on a snapshot graph	Vertical, up the displacement axis	Horizontal, along the position axis
Always a length?	No. For a longitudinal wave it may be a pressure, 14.1.A.6.i	Yes, always a distance in metres
Depends on the frequency	No, 14.2.A.1.iii	Yes, inversely, through $\lambda = v/f$
Depends on the medium	Not by any CED statement	Yes, through the speed
Changes at a boundary	The CED makes no statement	Yes, since $v$ changes and $f$ does not, 14.3.A.1.iv
Changes the period of an oscillator	No, AP Physics 1 EK 7.3.A.2	Not applicable
Sets the loudness of a sound	Yes, 14.1.A.6.ii	No
Sets the pitch of a sound	No	Yes, with the speed fixed, since pitch follows frequency, 14.2.A.1.v
Sets the category of light	No	Yes, 14.4.A.3
Appears in the interference relations	No	Yes, in all of them, as $m\lambda$
Energy statement in the CED	Increases with amplitude, 14.1.A.6.iii, qualitative	Increases with frequency, 14.2.A.1.iv, qualitative

The row that ends most confusions is always a length? Amplitude is a length only when the thing being displaced is a position. Essential knowledge 14.1.A.6.i allows a completely different unit for a longitudinal wave: the amplitude of a longitudinal pressure wave may be determined by the maximum increase or decrease in pressure from equilibrium pressure, which is a pressure in pascals. Wavelength has no such variation. Compression to compression is still a distance in metres.

The same point in a harder case: an electromagnetic wave has no material to displace at all. Essential knowledge 14.4.A.1 says it consists of oscillating electric and magnetic fields, so what has an amplitude is a field, not a piece of matter. The CED does not assign that amplitude a unit, and you will not be asked to. Its wavelength, meanwhile, is the quantity 14.4.A.3 uses to sort the whole spectrum.

Reading both off one picture, and the two factor-of-two errors

Draw the snapshot once and label it properly and this pair stops being difficult. Displacement is on the vertical axis, position along the medium is on the horizontal axis, and the flat line through the middle is the equilibrium position.

Amplitude is measured from the middle line, not across the whole wave. The distance from the top of a crest to the bottom of a trough is $2A$ . Halving that is the correction, and forgetting to halve it is the first of the two standard errors.

Wavelength is measured between matching points, not between any two features. Crest to the next crest is $\lambda$ . Crest to the next trough is $\lambda/2$ , because a trough is not a corresponding position to a crest. Doubling that is the correction, and forgetting to double it is the second standard error.

Notice the symmetry: one error halves a quantity, the other doubles it, and both come from measuring across the wrong pair of features on the same drawing. The CED's own phrasing guards against both. It says maximum displacement from its equilibrium position for amplitude, and successive corresponding positions for wavelength. Those two qualifying phrases are not decoration.

One more axis check, because the picture has a twin. A displacement-against-time graph, taken at one location, also shows a wavy line with the same shape. Its vertical extent is still the amplitude, because amplitude does not care which horizontal axis you chose. Its horizontal repeat is the period, not the wavelength, because time axes do not carry lengths. So amplitude can be read off either graph; wavelength can be read off only one of them.

Essential knowledge 14.2.A.2 puts the two graphs side by side with two equations, both printed on the sheet:

x(t) = A\cos(\omega t) = A\cos(2\pi f t) \qquad y(x) = A\cos\!\left(\frac{2\pi x}{\lambda}\right)

The same $A$ multiplies both. Only one of them contains $\lambda$ .

The case that separates them: same note, twice as loud

This is the case to hold in mind, because it is the one where the two quantities are visibly independent.

A guitar string is plucked gently and then hard. Both times the string is the same string, at the same tension, so the wave speed is the same by 14.1.A.3.ii. Both times it vibrates at the same frequency, because the frequency of a string is set by its length, tension and mass per length and not by how hard you hit it.

	Gentle pluck	Hard pluck
Wave speed	Unchanged	Unchanged
Frequency	Unchanged	Unchanged
Wavelength	Unchanged, since $\lambda = v/f$	Unchanged
Amplitude	Small	Large
Pitch you hear	The same note	The same note, 14.2.A.1.v
Loudness you hear	Quiet	Loud, 14.1.A.6.ii
Energy carried	Less	More, 14.1.A.6.iii

Every row about the shape along the string is unchanged, and every row about the size across it changed. That is the separation, and it is why an instrument can play the same note at any volume.

Run it the other way for the mirror image. Press the string at the twelfth fret and pluck it with exactly the same force. Now the frequency doubles, so the wavelength halves, and the amplitude is roughly what it was. Different note, same loudness.

Two knobs, two independent effects. Essential knowledge 14.2.A.1.iii is the licence for treating them that way: the amplitude of a wave is independent of the period and the frequency of that wave.

The same independence shows up for a single oscillator rather than a wave, and there the CED is even more direct. AP Physics 1 essential knowledge 7.3.A.2 says changing the amplitude of a system exhibiting SHM will not change the period of that system. Pull the mass twice as far and it takes exactly as long to come back.

Energy, stated as carefully as the CED states it

This is where a plausible half-remembered formula does real damage, so it is worth being precise about which energy statements exist.

For a wave, the CED is qualitative and stops there. Essential knowledge 14.1.A.6.iii says the energy carried by a wave increases with increasing amplitude. Essential knowledge 14.2.A.1.iv says the energy of a wave increases with increasing frequency. Both are directions of dependence with no formula attached, and no wave-energy equation appears on the AP Physics 2 equation sheet.

Intensity is defined, and it is also qualitative. Essential knowledge 14.3.A.3.i says intensity is a measure of the amount of power transferred per unit area, and 14.3.A.3.ii says the intensity of a wave is the average power per unit area over one period of the wave. Both sit under 14.3.A.3, which is about polarization reducing a wave's intensity, so intensity arrives in the boundary-behaviour topic rather than in the periodic-waves topic. There is no intensity equation on the sheet and no entry for intensity in the symbol list of its waves, sound, and optics block, which was checked by rendering the appendix page.

So do not write that intensity is proportional to amplitude squared. It is a true statement in physics generally and it is not in the AP Physics 2 CED, which relates amplitude to energy only by the word "increases". An AP answer that leans on the square is quoting a relation the course did not give it, and an AP answer that says "the energy increases with amplitude" is quoting 14.1.A.6.iii exactly.

One quantitative energy statement does exist, and it is not about waves. AP Physics 1 essential knowledge 7.4.A.4.ii says changing the amplitude of a system exhibiting SHM will change the maximum potential energy of the system and therefore the total energy of the system, and gives the relevant equation for a spring and object system:

E_{\text{total}} = \tfrac{1}{2}kA^2

That is an oscillator, not a travelling wave, and the square is real there because the spring potential energy is $U_s = \tfrac{1}{2}k(\Delta x)^2$ . Double the amplitude of a spring oscillator and the stored energy goes up by a factor of four, with the period untouched by 7.3.A.2. Use that relation where the CED puts it and nowhere else.

Wavelength, for its part, appears in no energy statement at all. It appears in the interference and diffraction relations, where the question is not how much energy but where it lands.

When it costs a mark

Calling the crest-to-trough distance the amplitude. It is $2A$ . Essential knowledge 14.1.A.6 says maximum displacement from the equilibrium position, so the measurement starts at the middle line.

Calling the crest-to-trough distance the wavelength. It is $\lambda/2$ . The definition asks for successive corresponding positions, and a trough does not correspond to a crest.

Saying a louder sound travels faster or arrives sooner. Amplitude is not in 14.1.A.3, which lists the type of wave and the properties of the medium and nothing else.

Saying a louder sound is a higher note. Loudness follows amplitude, 14.1.A.6.ii. Pitch follows frequency, 14.2.A.1.v. They are separate statements in separate topics.

Writing an intensity proportional to amplitude squared. Not in the AP Physics 2 CED, which gives intensity only as power per unit area at 14.3.A.3.i and 14.3.A.3.ii, and gives the amplitude to energy link only as "increases" at 14.1.A.6.iii.

Using $E = \tfrac{1}{2}kA^2$ for a wave. That relation is AP Physics 1 essential knowledge 7.4.A.4.ii, for a spring and object system. There is no wave equivalent printed anywhere.

Assuming a longitudinal wave's amplitude is a distance. It may be, but 14.1.A.6.i explicitly allows it to be a maximum increase or decrease in pressure from equilibrium pressure, so check the units the question is using before substituting.

Reading a wavelength off a displacement-against-time graph. That graph shows a period. Converting it into a wavelength needs the wave speed, which the graph does not contain.

Expecting the amplitude to change when the frequency does. Essential knowledge 14.2.A.1.iii rules it out. A graph can pair any amplitude with any frequency, and exam questions do.

When they look alike, and why that lulls you

Three things keep this pair looking like one idea.

They are drawn on the same axes. Every wave diagram a student sees carries both, one going up and one going across, which encourages the sense that they are two dimensions of a single object. They are not: one of them changes when you cross into glass and the other has no CED statement about crossing at all.

Both are often in metres. For a transverse mechanical wave they genuinely are, so a unit check will not catch a swap. It will catch a swap for a sound wave measured as a pressure, and for light, where the amplitude is a field.

Both look bigger when the wave looks bigger. A drawing scaled up on a page increases both. A physical wave with more energy increases only one of them, and which one depends on where the energy went: more amplitude by 14.1.A.6.iii, or more frequency by 14.2.A.1.iv, and only the second touches the wavelength.

There is also one genuine link between them worth stating so it is not mistaken for a bigger one. On a standing wave, the amplitude at a point depends on where the point sits in the pattern, and where the point sits is measured in wavelengths: nodes half a wavelength apart, antinodes halfway between them. So wavelength decides where the amplitude is large. It still does not decide how large. Turn the driver up and every antinode grows while every node stays exactly where it was. See standing vs traveling waves for that pattern in full.

The question that separates them in one step: would this change if I shook harder without changing the rate? If yes, it is the amplitude. If no, it is the wavelength.

Where this sits on the AP exam

Amplitude is defined in Topic 14.1, Properties of Wave Pulses and Waves and wavelength in Topic 14.2, Periodic Waves, both in Unit 14, Waves, Sound, and Physical Optics, which the CED weights at 12 to 15 percent of the multiple-choice section over a suggested 14 to 23 class periods. Intensity is defined in Topic 14.3. None of the three topics carries a boundary statement, checked page by page.

Amplitude also appears in AP Physics 1, where the current framework has no waves unit at all. It turns up in Unit 7, Oscillations, weighted at 5 to 8 percent over a suggested 5 to 10 class periods, as the amplitude of an oscillating object rather than of a wave, in Topic 7.3 and Topic 7.4. Wavelength does not appear in AP Physics 1 at all, and its equation sheet prints no $\lambda$ , which was checked by rendering the appendix page rather than assumed.

The suggested skills for Topic 14.1 are 1.C, create qualitative sketches of graphs that represent features of a model or the behavior of a physical system; 2.C, compare physical quantities between two or more scenarios or at different times and locations in a single scenario; 3.B, apply an appropriate law, definition, theoretical relationship, or model to make a claim; and 3.C, justify or support a claim using evidence from experimental data, physical representations, or physical principles or laws. Topic 14.2 lists 1.A, create diagrams, tables, charts, or schematics, in place of 1.C, and the other three are identical.

Skill 1.C is the one this pair is examined through most often: sketch what happens to the graph when the amplitude doubles, or when the frequency doubles, and the two sketches are different in ways that test exactly the independence above.

For the definitions on their own, see amplitude and wavelength. For the pair on the horizontal axis, see frequency vs wavelength. For the arithmetic of getting one from another, the wave speed, frequency and wavelength guide owns that procedure.

Reading both quantities off one snapshot

A snapshot of a wave on a string at one instant shows the string displaced to a maximum of $+8.0$ cm above the equilibrium line and a minimum of $-8.0$ cm below it. Successive crests sit at $x = 1.2 \ \mathrm{m}$ and $x = 4.8 \ \mathrm{m}$ . The wave has a frequency of $5.0 \ \mathrm{Hz}$ . (a) State the amplitude. (b) State the wavelength. (c) State the distance from a crest to the next trough. (d) Find the wave speed and the period.

(a) Essential knowledge 14.1.A.6 defines amplitude as the maximum displacement from the equilibrium position, so $A = 8.0 \ \mathrm{cm} = 0.080 \ \mathrm{m}$ . It is not $16$ cm; that is the crest-to-trough distance, which is $2A$ .
(b) Essential knowledge 14.2.A.1.vi asks for the distance between successive corresponding positions, and two crests are corresponding positions: $\lambda = 4.8 - 1.2 = 3.6 \ \mathrm{m}$ .
(c) A trough is half a cycle from a crest, so the crest-to-trough distance is $\lambda/2 = 1.8 \ \mathrm{m}$ . Reporting $1.8$ m as the wavelength is the standard halving error.
(d) Rearranging $\lambda = v/f$ gives $v = \lambda f = (3.6)(5.0) = 18 \ \mathrm{m/s}$ , and $T = 1/f = 1/5.0 = 0.20 \ \mathrm{s}$ .
Check the two measurements are independent: the $8.0$ cm was read up the vertical axis and never entered the speed calculation, and the $3.6$ m was read along the horizontal axis and never entered the amplitude. Nothing in this problem connects them, which is 14.2.A.1.iii in practice.

$A = 8.0 \ \mathrm{cm}$ , $\lambda = 3.6 \ \mathrm{m}$ , crest to next trough $= 1.8 \ \mathrm{m}$ , $v = 18 \ \mathrm{m/s}$ and $T = 0.20 \ \mathrm{s}$ .

Turn one knob at a time on the same string

Start from the wave above: $A = 8.0$ cm, $\lambda = 3.6$ m, $f = 5.0$ Hz, $v = 18$ m/s on a string whose tension is not altered. Treat two changes separately. (a) The end is shaken twice as far, giving $A = 16.0$ cm at the same rate. (b) The end is shaken at $10 \ \mathrm{Hz}$ with the original $8.0$ cm amplitude. For each, give the new amplitude, wavelength, frequency and wave speed.

(a) The string is unchanged, so 14.1.A.3 fixes the wave speed at $v = 18 \ \mathrm{m/s}$ . The rate of shaking is unchanged, so $f = 5.0 \ \mathrm{Hz}$ . Then $\lambda = v/f = 18/5.0 = 3.6 \ \mathrm{m}$ , unchanged. Only the amplitude moved, to $16.0 \ \mathrm{cm}$ .
By 14.1.A.6.iii the wave now carries more energy, and if this were sound, 14.1.A.6.ii would make it louder. The CED gives the direction and no factor, so "more energy" is the complete answer.
(b) The string is still unchanged, so $v = 18 \ \mathrm{m/s}$ again. Now $f = 10 \ \mathrm{Hz}$ , so $\lambda = 18/10 = 1.8 \ \mathrm{m}$ , halved. The amplitude stays at $8.0 \ \mathrm{cm}$ , by 14.2.A.1.iii.
Compare the two results. Doubling the amplitude changed one quantity out of four. Doubling the frequency changed two, the frequency and the wavelength, and left the amplitude alone. There is no change that moves the amplitude and the wavelength together while the medium is fixed.
Sanity check on (b): $v = \lambda f = (1.8)(10) = 18 \ \mathrm{m/s}$ , which matches the original speed as it must, since the string was never touched.

(a) $A = 16.0 \ \mathrm{cm}$ , $\lambda = 3.6 \ \mathrm{m}$ , $f = 5.0 \ \mathrm{Hz}$ , $v = 18 \ \mathrm{m/s}$ . (b) $A = 8.0 \ \mathrm{cm}$ , $\lambda = 1.8 \ \mathrm{m}$ , $f = 10 \ \mathrm{Hz}$ , $v = 18 \ \mathrm{m/s}$ .

Where amplitude does get a formula, and where it does not

A block of mass $0.50 \ \mathrm{kg}$ on a spring of spring constant $k = 200 \ \mathrm{N/m}$ oscillates with an amplitude of $5.0 \ \mathrm{cm}$ . (a) Find the total energy of the system. (b) Find the total energy if the amplitude is doubled to $10.0 \ \mathrm{cm}$ . (c) Find the period in each case. (d) State why this calculation cannot be transferred to a wave on a string.

(a) AP Physics 1 essential knowledge 7.4.A.4.ii gives the relevant equation for a spring and object system, $E_{\text{total}} = \tfrac{1}{2}kA^2$ . With $A = 0.050 \ \mathrm{m}$ : $E = \tfrac{1}{2}(200)(0.050)^2 = \tfrac{1}{2}(200)(0.0025) = 0.25 \ \mathrm{J}$ .
(b) With $A = 0.100 \ \mathrm{m}$ : $E = \tfrac{1}{2}(200)(0.100)^2 = \tfrac{1}{2}(200)(0.0100) = 1.0 \ \mathrm{J}$ . Doubling the amplitude quadrupled the energy, because the amplitude is squared.
(c) The AP Physics 1 sheet prints $T_s = 2\pi\sqrt{m/k}$ , which contains no amplitude: $T = 2\pi\sqrt{0.50/200} = 2\pi\sqrt{0.0025} = 2\pi(0.050) = 0.31 \ \mathrm{s}$ in both cases. Essential knowledge 7.3.A.2 says the same thing in words: changing the amplitude of a system exhibiting SHM will not change the period of that system.
(d) Because $E_{\text{total}} = \tfrac{1}{2}kA^2$ is stated for a spring and object system, and a wave has no spring constant. For a wave, AP Physics 2 gives only the direction of the dependence, 14.1.A.6.iii, that the energy carried by a wave increases with increasing amplitude, and prints no wave-energy equation on the sheet.
The same caution covers intensity. The CED defines it at 14.3.A.3.i and 14.3.A.3.ii as power per unit area, and never states that it goes as the square of the amplitude, so that relation should not appear in an AP Physics 2 answer.

$0.25 \ \mathrm{J}$ at $5.0$ cm and $1.0 \ \mathrm{J}$ at $10.0$ cm, a factor of four. The period is $0.31 \ \mathrm{s}$ in both cases. The squared relation belongs to a spring and object system in AP Physics 1 and has no wave counterpart in AP Physics 2.

Frequently asked questions

What is the difference between amplitude and wavelength?

Amplitude is the maximum displacement of the wave from its equilibrium position, measured up the vertical axis of a wave diagram, and AP Physics 2 defines it at essential knowledge 14.1.A.6. Wavelength is the distance between successive corresponding positions such as peak to peak, measured along the horizontal axis, defined at 14.2.A.1.vi. The deeper difference is dependence: wavelength equals wave speed divided by frequency, so it moves when either changes, while 14.2.A.1.iii states that amplitude is independent of the period and the frequency of the wave.

Is the amplitude the height from crest to trough?

No, that distance is twice the amplitude. Essential knowledge 14.1.A.6 defines amplitude as the maximum displacement from the equilibrium position, so it is measured from the flat middle line up to a crest, not from a trough all the way up to a crest. Halving a crest-to-trough measurement is the correction. The matching error on the other axis is measuring a crest to the next trough and calling it a wavelength; that distance is half a wavelength, because a trough is not a corresponding position to a crest.

Does changing the amplitude change the wavelength?

No. Essential knowledge 14.2.A.1.iii in AP Physics 2 states that the amplitude of a wave is independent of the period and the frequency of that wave, and wavelength equals wave speed divided by frequency, so with the medium unchanged the wavelength cannot move either. This is why a guitar string plucked hard and plucked gently produces the same note at two different volumes. Nothing on the AP Physics 2 equation sheet relates amplitude to wavelength, frequency or speed.

Does amplitude affect the energy of a wave?

Yes, and the AP Physics 2 CED states it qualitatively. Essential knowledge 14.1.A.6.iii says the energy carried by a wave increases with increasing amplitude, and 14.1.A.6.ii says the loudness of a sound increases with increasing amplitude. No formula is attached and no wave-energy equation appears on the equation sheet. The relation that energy or intensity goes as amplitude squared is not in the AP Physics 2 CED, so do not use it there. The squared relation the course does give is E = one half k A squared, for a spring and object system in AP Physics 1.

Is amplitude always measured in metres?

No. For a transverse mechanical wave it is a displacement in metres, but essential knowledge 14.1.A.6.i in AP Physics 2 says the amplitude of a longitudinal pressure wave may be determined by the maximum increase or decrease in pressure from equilibrium pressure, which is a pressure in pascals. For an electromagnetic wave the thing oscillating is a field rather than matter, by 14.4.A.1, so its amplitude is not a distance either. Wavelength, in contrast, is always a length.

Which one changes when a wave enters a new medium?

The wavelength. Essential knowledge 14.3.A.1.iv says the frequency does not change at a boundary, and the wave speed does, so wavelength equals speed over frequency forces the wavelength to change by the same factor as the speed. The AP Physics 2 CED makes no statement about what happens to the amplitude of the transmitted wave, so an exam answer should not claim to know. It does say, at 14.3.A.1.ii and 14.3.A.1.iii, whether the reflected wave is inverted, which is a phase statement rather than an amplitude one.

How do I tell amplitude and wavelength apart on a graph?

Check the horizontal axis first. Amplitude is always the vertical extent measured from the equilibrium line, so it can be read off either a displacement-against-position snapshot or a displacement-against-time trace. Wavelength is a horizontal repeat distance and can only be read off a graph whose horizontal axis is in metres. If the horizontal axis is in seconds, the repeat you are looking at is the period, and converting it to a wavelength requires the wave speed.