5.15P Explain that all bodies emit radiation, that the intensity and…

The idea

Every body — a star, a kettle, a person, an ice cube — emits electromagnetic radiation because its particles have thermal energy. The emission is not limited to visible light: it spreads across a range of wavelengths, and the shape of that spread is set by the body's temperature. Raise the temperature and two things change together: the total intensity (power emitted per unit area) increases, and the wavelength distribution shifts so that the peak moves towards shorter wavelengths. A cool body such as a human at about 310 K peaks in the infrared, which is why thermal cameras see people in the dark; a hot filament at around 2500 K glows visibly. The statement is about emission from all bodies, not only hot ones, and about how temperature controls both intensity and the spread of wavelengths.

What earns the marks

• States that every body emits electromagnetic radiation regardless of its temperature.
• Explains that a higher temperature gives a greater intensity of emitted radiation.
• Explains that a higher temperature shifts the wavelength distribution so the peak emission is at a shorter wavelength.
• Recognises that emission covers a continuous range of wavelengths rather than a single wavelength.
• Applies the idea to a named example, such as infrared emission from a warm object or visible glow from a hot filament.

Common mistakes and how to put them right

• Claims that only hot objects emit radiation.

  Put it right by noting that all bodies emit, with cooler bodies emitting mainly in the infrared.

• Assumes a hotter body emits only at one wavelength.

  Put it right by describing a continuous distribution whose peak shifts to shorter wavelengths as temperature rises.

• States that raising temperature shifts the peak to longer wavelengths.

  Put it right by saying the peak moves to shorter wavelengths.

• Treats intensity and wavelength distribution as unrelated to temperature.

  Put it right by linking both to the body's temperature.

Exam tips

• Read the question for the word 'all' — options that restrict emission to hot bodies are wrong.
• Link each change to temperature explicitly: higher temperature means greater intensity and a peak at shorter wavelength.
• Use a familiar example such as a person seen by a thermal camera or a lamp filament to check an option makes physical sense.
• Reject options that describe emission at a single wavelength, because real emission is spread over a range.