Radiation | Physics for GCSE/IGCSE - Year 11 PDF Download

Thermal Radiation

• Every object emits thermal radiation.
  • The emission of thermal radiation increases with the temperature of the object.
  • Thermal radiation corresponds to the infrared portion of the electromagnetic spectrum.
• Thermal radiation constitutes the sole means by which heat can traverse through a vacuum.
  • It is the mechanism through which heat from the Sun reaches us across the vacuum of space.
• The ability of an object to emit and absorb thermal radiation is influenced by its color.

Thermal Equilibrium

• When an object absorbs thermal radiation, it experiences a rise in temperature.
• With increasing temperature, the object also emits more thermal radiation.
  • A body's temperature elevates when it absorbs radiation at a faster rate than it emits radiation.
• Ultimately, the object will reach a state of equilibrium where it absorbs and emits radiation at an equal rate.
  • At this juncture, the object achieves thermal equilibrium.

• If an object receives energy at a slower rate than it releases energy, it will lose heat and cool down.
• Conversely, if the rate of energy transfer from an object is slower than the rate at which it gains energy, the object will warm up.
• This process invariably progresses towards achieving thermal equilibrium.

Effects of Different Surfaces

The emission of thermal radiation from an object is influenced by various factors:

• Surface Color: Dark-colored surfaces emit more radiation.
• Texture: Shiny surfaces emit less radiation.
• Surface Area: Objects with greater surface area emit more radiation due to increased area for emission.

• Black objects excel at absorbing thermal radiation. For instance, black clothing can make you feel warmer in sunny conditions. They are also proficient at emitting thermal radiation, which explains why chargers for laptops and car radiators are often black - aiding in cooling processes.
• Shiny objects, in contrast, reflect thermal radiation, absorbing minimal heat. Although they emit low levels of radiation, they take longer to cool down compared to black objects.

An image of a hot object taken in both Infrared and visible light. The black surface emits more thermal radiation (infrared) than the shiny surface