physics

Radiometer

Radiometer is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Radiometer rather than just read about it. In short: A radiometer is an instrument for measuring radiometric quantities such as radiant flux (power), irradiance, or radiance. Definitions typically limit radiometry to optical radiation, but some definitions include other kinds of electromagnetic radiation.

Key takeaways

Radiometer belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Radiometer to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Radiometer from memory before moving on to harder problems.

Reference excerpt

A radiometer is an instrument for measuring radiometric quantities such as radiant flux (power), irradiance, or radiance. Definitions typically limit radiometry to optical radiation, but some definitions include other kinds of electromagnetic radiation. Radiometers typically measure infrared radiation, visible radiation (light), ultraviolet radiation, or some combination of these. Microwave radiometers operate in the microwave wavelengths. A roentgenometer is a radiometer for measuring the intensity of X-rays or gamma radiation. While the term radiometer can refer to any device that measures electromagnetic radiation (e.g., light), the term is often used to refer specifically to a Crookes radiometer ("light-mill"), a device invented in 1873 in which a rotor (having vanes which are dark on one side, and light on the other) in a partial vacuum spins when exposed to light. A common misbelief (one originally held even by Crookes) is that the momentum of the absorbed light on the black faces makes the radiometer operate. If this were true, however, the radiometer would spin away from the non-black faces, since the photons bouncing off those faces impart more momentum than the photons absorbed on the black faces. Photons do exert radiation pressure on the faces, but those forces are dwarfed by other effects. The currently accepted explanation depends on having just the right degree of vacuum, and relates to the transfer of heat rather than the direct effect of photons. A Nichols radiometer demonstrates photon pressure. It is much more sensitive than the Crookes radiometer and it operates in a complete vacuum, whereas operation of the Crookes radiometer requires an imperfect vacuum. The MEMS radiometer can operate on the principles of Nichols or Crookes and can operate over a wide spectrum of wavelength and particle energy levels.

See also Active cavity radiometer Bolometer Copernicus Net radiometer Photon rocket Pyranometer Radiation pressure Radiometry Solar sail Spectroradiometer

References

Illustrations

Worked examples

Example 1 — a first encounter with Radiometer

Start with the simplest possible case. Write down what Radiometer claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Radiometer before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Radiometer ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Radiometer

In research: Radiometer appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Radiometer in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Radiometer is common in secondary-school and first-year university syllabi. It links to neighbouring topics Electromagnetic radiation meters, Radiometry, so understanding it makes those chapters shorter.
In everyday life: Look for Radiometer outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Radiometer in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Radiometer means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Radiometer out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Radiometer in simple terms?

Why does Radiometer matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Radiometer?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Radiometer.