Know that thermal radiation is infrared radiation and that all objects emit this radiation

2.3.3 Radiation

Learning Objective

Know that thermal radiation is infrared radiation, that it can travel through empty space, and that all objects emit it.

Key Concepts

• Every body with a temperature above 0 K emits electromagnetic radiation.
• In the temperature range we experience daily the strongest part of the spectrum lies in the infrared (IR) region.
• The amount and the “colour” (wavelength distribution) of the radiation depend on:

• Temperature – higher T → more radiation and the peak shifts to shorter wavelength.
• Surface area – a larger area radiates more.
• Surface colour/texture (emissivity) – dark, rough surfaces have a high emissivity (≈ 1) and therefore emit (and absorb) more IR than bright, smooth surfaces, which reflect a larger fraction of the incident IR.

Why is it called “thermal” radiation?

Thermal radiation is the electromagnetic energy that a body loses because its particles are moving (thermal kinetic energy). The term “thermal” links the radiation to the temperature of the object; the energy is carried away by photons, not by the kinetic motion of the particles themselves.

Infrared Radiation

Infrared (IR) radiation has wavelengths from about 700 nm – 1 mm. It lies between visible light and microwaves in the electromagnetic spectrum.

Thermal Radiation and the Need for a Medium

Because it is electromagnetic, thermal radiation can travel through vacuum. Unlike conduction or convection, it does not require a material medium. (Example: the Sun’s energy reaches Earth through the vacuum of space.)

Emissivity, Colour, Texture and Reflection

• Emission – High‑emissivity surfaces (black, matte) radiate strongly.
• Absorption – The same surfaces absorb most of the IR that falls on them.
• Reflection – Low‑emissivity surfaces (bright, shiny) reflect a large fraction of incident IR, so they appear “cooler” because less energy is emitted.

Quantitative Relationships (IGCSE level)

• Stefan‑Boltzmann proportionality:
  

  \(P \propto A\,T^{4}\)   (Power radiated is proportional to surface area and the fourth power of absolute temperature).
  

  You only need to state the proportionality; the constant \(σ\) is not required for the exam.

• Wien’s Displacement Law (qualitative):
  

  \(\displaystyle \lambda_{\text{max}} \approx \frac{2.9\times10^{-3}}{T}\;\text{m·K}\)
  

  Peak wavelength moves to shorter values as temperature rises.

Experiments

1. Good vs. Bad Emitters (qualitative)

1. Place a piece of black cardboard and a piece of white cardboard side‑by‑side under an identical heat lamp.
2. Put an infrared thermometer (or a regular thermometer in a sealed tube) behind each card to record the temperature of the air behind the card.
3. After a few minutes note the temperature rise.
4. Observation: the air behind the black card becomes hotter – the black surface is a better IR emitter (and absorber) than the white one.

2. Quantitative extension (AO3 skill)

• Record the temperature every 30 s for 3 min and calculate the rate of temperature rise, \(\Delta T/\Delta t\), for each card.
• Compare the two rates – the larger value corresponds to the better emitter.

3. Good vs. Bad Absorbers

1. Cover two identical metal plates with black paint (one) and aluminium foil (the other).
2. Place a thermometer on the surface of each plate and shine the same heat lamp on both.
3. Measure the temperature of the plates after a fixed time.
4. Observation: the black‑painted plate reaches a higher temperature – it absorbs more IR than the reflective foil plate.

Everyday Applications (linked to the syllabus)

• Heating of objects – A hot pan radiates IR to the surrounding air; the rate rises sharply as the pan gets hotter.
• Radiative heating of a room – Infrared heaters emit IR that is absorbed directly by walls, furniture and people, raising the room temperature without heating the air first.
• Insulation of buildings – Walls covered with low‑emissivity (reflective) foil lose less heat by radiation, keeping interiors warm.
• Negative consequence – Poorly insulated windows with high emissivity radiate heat outwards at night, causing unwanted heat loss.
• Thermal‑imaging cameras – Detect the IR emitted by objects and produce a temperature map (used in medicine, firefighting, night‑vision).
• Infrared remote controls – Use IR LEDs to send signals because the radiation is invisible and can be easily detected by a receiver.
• Radiative cooling – At night, surfaces emit IR to the clear sky, allowing temperatures to fall below daytime levels.
• Sun‑Earth energy balance – The Sun’s visible radiation is absorbed by Earth; the Earth then re‑emits the energy as IR, influencing climate.

Common Misconceptions

• “Only hot objects emit radiation.” – Even cold objects emit IR; the intensity is just much smaller.
• “Infrared is the same as heat.” – Infrared is one way heat can be transferred. Heat can also move by conduction and convection.
• “All surfaces radiate equally.” – Emissivity varies with colour and texture; dark, rough surfaces are better emitters (and absorbers) than bright, smooth ones.
• “Radiation needs air to travel.” – Because it is electromagnetic, thermal radiation can travel through vacuum, as demonstrated by the Sun’s energy reaching Earth.

Summary

All objects above absolute zero emit electromagnetic radiation. In everyday temperature ranges the peak of this emission lies in the infrared part of the spectrum, which is why we call it *thermal radiation*. The emitted power increases sharply with temperature (\(P\propto T^{4}\)), grows with surface area, and is strongly affected by emissivity (colour/texture). Thermal radiation does not require a medium, so it can transfer energy through empty space. Understanding these ideas explains phenomena such as the warmth felt from a fire, the operation of IR remote controls, the design of reflective insulation, and the functioning of thermal‑imaging devices.