---
title: "Continuous Spectrum — AP Physics 2 Definition & Exam Guide"
description: "A continuous spectrum is radiation emitted at all wavelengths in a range, like a blackbody's glow. Its shape depends only on temperature, tested in Topic 15.4."
canonical: "https://fiveable.me/ap-physics-2-revised/key-terms/continuous-spectrum"
type: "key-term"
subject: "AP Physics 2"
unit: "Unit 15"
---

# Continuous Spectrum — AP Physics 2 Definition & Exam Guide

## Definition

A continuous spectrum is electromagnetic radiation with intensity spread smoothly across a whole range of wavelengths, with no gaps. In AP Physics 2, it's what an ideal blackbody emits, and the spectrum's shape depends on only one thing: the object's temperature (Topic 15.4).

## What It Is

A continuous spectrum is [electromagnetic radiation](/ap-physics-2-revised/key-terms/electromagnetic-radiation "fv-autolink") that covers an unbroken range of wavelengths. Plot [intensity per unit wavelength](/ap-physics-2-revised/unit-15/4-blackbody-radiation/study-guide/jcWRJUVghpDEK71c "fv-autolink") versus wavelength and you get a smooth curve, not isolated spikes. Every wavelength in the range shows up with some intensity.

In [AP Physics 2](/ap-physics-2-revised "fv-autolink"), this term lives in **Topic 15.4 (Blackbody Radiation)**. Any matter with internal thermal energy spontaneously converts some of it into electromagnetic energy. A blackbody is the idealized version, an object that absorbs all radiation hitting it and, if it sits in equilibrium at constant temperature, must emit energy back out. The punchline the CED hammers: a blackbody emits a continuous spectrum that depends *only* on its temperature. Not its material, not its color, not its shape. Hotter objects emit more total intensity and their peak shifts toward shorter wavelengths. That's why a heating piece of metal glows red, then orange, then white.

## Why It Matters

This is the heart of learning objective **15.4.A**, which asks you to describe the electromagnetic radiation an object emits because of its [temperature](/ap-physics-2-revised/unit-9/1-kinetic-theory-of-temperature-and-pressure/study-guide/wWjb2NGJDLNmMhB3 "fv-autolink"). The continuous spectrum is also where classical physics famously broke. Classical wave theory could not reproduce the measured blackbody curve, and fixing that failure (Planck treating energy as quantized) launched [modern physics](/ap-physics-2-revised/unit-15 "fv-autolink"), which is the whole story of Unit 15. So this isn't just vocabulary. It's the opening scene of quantum mechanics, and the exam expects you to read and interpret intensity-vs-wavelength graphs, predict how the curve changes with temperature, and recognize that the spectrum's shape is set by temperature alone.

## Connections

### [Planck's Law (Unit 15)](/ap-physics-2-revised/key-terms/plancks-law)

[Planck's law](/ap-physics-2-revised/key-terms/plancks-law "fv-autolink") is the equation behind the continuous spectrum's curve. Classical physics predicted the wrong shape; Planck got the right one by assuming energy comes in discrete chunks. The continuous spectrum is the graph, Planck's law is the math that finally explained it.

### [Wien's Law (Unit 15)](/ap-physics-2-revised/key-terms/wiens-law)

[Wien's law](/ap-physics-2-revised/key-terms/wiens-law "fv-autolink") tells you where the continuous spectrum peaks. Hotter object means the peak wavelength slides toward the short (blue) end. It's how one number, temperature, locates the high point of the whole curve.

### [Stefan-Boltzmann Law (Unit 15)](/ap-physics-2-revised/key-terms/stefan-boltzmann-law)

While Wien's law gives the peak's location, the [Stefan-Boltzmann law](/ap-physics-2-revised/key-terms/stefan-boltzmann-law "fv-autolink") gives the total power radiated, the area under the continuous spectrum curve. It scales with temperature to the fourth power, so doubling temperature multiplies emitted power by 16.

### Thermal Energy and Equilibrium (Unit 9)

Blackbody radiation is a thermodynamics idea wearing modern-physics clothes. An object in equilibrium at constant temperature absorbs and emits energy at equal rates, which is exactly why a perfect absorber must also be a perfect emitter.

## On the AP Exam

Continuous spectrum questions show up as multiple-choice items in two main flavors. The first is identification, where a stem describes an object that absorbs all incoming radiation and emits a spectrum depending only on temperature, and you name it a blackbody. The second asks what determines the spectrum, and the answer is temperature alone, not material or surface properties. You should also be ready to interpret intensity-vs-wavelength graphs, comparing curves for two temperatures by their peaks and total area. No released FRQ has used the phrase verbatim, but blackbody graph interpretation supports the kind of model-based reasoning Unit 15 free-response questions reward, especially explaining why classical physics failed to predict the curve.

## continuous spectrum vs Line (discrete) spectrum

A continuous spectrum has intensity at every wavelength in a range; a line spectrum has only specific, separated wavelengths. The source is the giveaway. Hot dense matter radiating because of its temperature (a blackbody) gives a continuous spectrum. Individual atoms jumping between quantized energy levels emit or absorb only certain photon energies, producing discrete lines. If an exam question shows sharp spikes, think atomic transitions. If it shows a smooth curve, think blackbody.

## Key Takeaways

- A continuous spectrum has intensity spread smoothly across all wavelengths in a range, with no gaps or isolated lines.
- A blackbody is an idealized object that absorbs all radiation hitting it, and in equilibrium it must emit radiation back out.
- The continuous spectrum a blackbody emits depends only on its temperature, not on what the object is made of.
- Blackbody radiation is usually shown as a graph of intensity per unit wavelength versus wavelength, and you should be able to compare curves at different temperatures.
- Hotter objects emit more total intensity and their spectrum peaks at a shorter wavelength, which is why heated metal shifts from red toward white.
- Classical physics could not explain the shape of the blackbody spectrum, and resolving that failure kicked off quantum physics.

## FAQs

### What is a continuous spectrum in AP Physics 2?

It's electromagnetic radiation with intensity distributed across an unbroken range of wavelengths, the kind emitted by a blackbody. In Topic 15.4, the key fact is that this spectrum's shape depends only on the object's temperature.

### What determines the continuous spectrum a blackbody emits?

Temperature, and only temperature. The material, color, and shape of the object don't matter for an ideal blackbody, which is exactly the trap multiple-choice questions set with answer choices like 'composition' or 'surface texture.'

### How is a continuous spectrum different from a line spectrum?

A continuous spectrum covers every wavelength in a range and comes from thermal radiation (blackbodies). A line spectrum shows only specific wavelengths and comes from atoms jumping between quantized energy levels. Smooth curve means blackbody; sharp spikes mean atomic transitions.

### Does a continuous spectrum mean light isn't quantized into photons?

No. The radiation still comes as photons, each carrying energy set by its frequency. 'Continuous' describes the range of wavelengths emitted, not the energy of individual photons. In fact, Planck had to assume quantized energy to explain the continuous blackbody curve at all.

### Do all objects emit a continuous spectrum, or just blackbodies?

Any matter with internal thermal energy spontaneously converts some of it into electromagnetic radiation, so ordinary objects glow too. The blackbody is the idealized model AP Physics 2 uses, a perfect absorber whose emitted spectrum depends purely on temperature.

## Related Study Guides

- [15.4 Blackbody Radiation](/ap-physics-2-revised/unit-15/4-blackbody-radiation/study-guide/jcWRJUVghpDEK71c)

## Structured Data

```json
{"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/ap-physics-2-revised/key-terms/continuous-spectrum#resource","name":"Continuous Spectrum — AP Physics 2 Definition & Exam Guide","url":"https://fiveable.me/ap-physics-2-revised/key-terms/continuous-spectrum","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/ap-physics-2-revised/key-terms/continuous-spectrum#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-06-11T05:27:45.018Z","isPartOf":{"@type":"Collection","name":"AP Physics 2 Key Terms","url":"https://fiveable.me/ap-physics-2-revised/key-terms"},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/ap-physics-2-revised/key-terms/continuous-spectrum#term","name":"continuous spectrum","description":"A continuous spectrum is electromagnetic radiation with intensity spread smoothly across a whole range of wavelengths, with no gaps. In AP Physics 2, it's what an ideal blackbody emits, and the spectrum's shape depends on only one thing: the object's temperature (Topic 15.4).","url":"https://fiveable.me/ap-physics-2-revised/key-terms/continuous-spectrum","inDefinedTermSet":{"@type":"DefinedTermSet","name":"AP Physics 2 Key Terms","url":"https://fiveable.me/ap-physics-2-revised/key-terms"}},{"@type":"FAQPage","mainEntity":[{"@type":"Question","name":"What is a continuous spectrum in AP Physics 2?","acceptedAnswer":{"@type":"Answer","text":"It's electromagnetic radiation with intensity distributed across an unbroken range of wavelengths, the kind emitted by a blackbody. In Topic 15.4, the key fact is that this spectrum's shape depends only on the object's temperature."}},{"@type":"Question","name":"What determines the continuous spectrum a blackbody emits?","acceptedAnswer":{"@type":"Answer","text":"Temperature, and only temperature. The material, color, and shape of the object don't matter for an ideal blackbody, which is exactly the trap multiple-choice questions set with answer choices like 'composition' or 'surface texture.'"}},{"@type":"Question","name":"How is a continuous spectrum different from a line spectrum?","acceptedAnswer":{"@type":"Answer","text":"A continuous spectrum covers every wavelength in a range and comes from thermal radiation (blackbodies). A line spectrum shows only specific wavelengths and comes from atoms jumping between quantized energy levels. Smooth curve means blackbody; sharp spikes mean atomic transitions."}},{"@type":"Question","name":"Does a continuous spectrum mean light isn't quantized into photons?","acceptedAnswer":{"@type":"Answer","text":"No. The radiation still comes as photons, each carrying energy set by its frequency. 'Continuous' describes the range of wavelengths emitted, not the energy of individual photons. In fact, Planck had to assume quantized energy to explain the continuous blackbody curve at all."}},{"@type":"Question","name":"Do all objects emit a continuous spectrum, or just blackbodies?","acceptedAnswer":{"@type":"Answer","text":"Any matter with internal thermal energy spontaneously converts some of it into electromagnetic radiation, so ordinary objects glow too. The blackbody is the idealized model AP Physics 2 uses, a perfect absorber whose emitted spectrum depends purely on temperature."}}]},{"@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"AP Physics 2","item":"https://fiveable.me/ap-physics-2-revised"},{"@type":"ListItem","position":2,"name":"Key Terms","item":"https://fiveable.me/ap-physics-2-revised/key-terms"},{"@type":"ListItem","position":3,"name":"Unit 15","item":"https://fiveable.me/ap-physics-2-revised/unit-15"},{"@type":"ListItem","position":4,"name":"continuous spectrum"}]}]}
```