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 and that all objects emit this radiation.

Key Concepts

• All matter with a temperature above absolute zero emits electromagnetic radiation.
• The dominant wavelength of the radiation depends on the temperature of the object (Wien’s displacement law).
• At everyday temperatures the peak of the emitted spectrum lies in the infrared region of the electromagnetic spectrum.

Why is it called “thermal” radiation?

Thermal radiation is the electromagnetic energy released by a body as a result of its temperature. The term “thermal” links the radiation to the internal kinetic energy of the particles in the material.

Infrared Radiation

Infrared (IR) radiation has wavelengths from about \$700\ \text{nm}\$ to \$1\ \text{mm}\$. It lies between visible light and microwaves in the electromagnetic spectrum.

All Objects Emit Infrared Radiation

According to the law of black‑body radiation, any object with temperature \$T > 0\ \text{K}\$ emits a continuous spectrum of radiation. The intensity \$I(\lambda,T)\$ at wavelength \$\lambda\$ is given by Planck’s law:

\$\$

I(\lambda,T)=\frac{2hc^{2}}{\lambda^{5}}\frac{1}{\exp\!\left(\frac{hc}{\lambda k_{\mathrm{B}}T}\right)-1}

\$\$

where \$h\$ is Planck’s constant, \$c\$ is the speed of light, and \$k_{\mathrm{B}}\$ is Boltzmann’s constant.

Wien’s Displacement Law

The wavelength \$\lambda_{\text{max}}\$ at which the emission is strongest is inversely proportional to the absolute temperature:

\$\$

\lambda_{\text{max}} = \frac{b}{T}

\$\$

with \$b \approx 2.898\times10^{-3}\ \text{m·K}\$. For typical room temperature (\$T \approx 300\ \text{K}\$), \$\lambda_{\text{max}} \approx 9.7\ \mu\text{m}\$, which lies in the infrared region.

Examples of Everyday Objects

Implications for Everyday Life

1. Thermal imaging cameras detect the infrared radiation emitted by objects to produce a temperature map.
2. Heat loss from buildings occurs mainly through infrared radiation; insulating materials reduce this loss.
3. Remote controls use infrared LEDs to transmit signals because infrared is not visible to the eye.

Common Misconceptions

• “Only hot objects emit radiation.” – Even a cold object (e.g., ice at 273 K) emits infrared radiation, though the intensity is much lower.
• “Infrared is the same as heat.” – Infrared radiation is one way that heat can be transferred; heat can also be transferred by conduction and convection.

Summary

All objects with a temperature above absolute zero emit electromagnetic radiation. At the temperatures encountered in daily life, the peak of this emission lies in the infrared part of the spectrum, which is why we refer to it as thermal radiation. Understanding this concept is essential for topics such as heat transfer, remote sensing, and everyday technologies that rely on infrared.