Thermal Radiation (Infrared)

Thermal radiation is heat transferred by infrared waves. It travels at the speed of light and does not need particles, so it can move through a vacuum. This is how the Sun warms the Earth.

Key ideas

• All objects emit infrared when they are above absolute zero. Hotter objects emit more energy each second.
• No medium needed: radiation can travel through empty space, unlike conduction and convection.
• Surfaces matter: dull, black surfaces are the best emitters and absorbers; shiny, white/silver surfaces are poor emitters and absorbers but good reflectors.
• Temperature difference and area also affect the rate of radiation: bigger temperature differences and larger surface areas increase energy transfer.
• Straight lines: infrared travels in straight lines and can be focused or blocked.

Energy balance and temperature

An object’s temperature changes depending on the balance between energy it absorbs and energy it emits by radiation:

• If energy in > energy out, it warms up.
• If energy in < energy out, it cools down.
• If energy in = energy out, its temperature stays constant.

Earth’s temperature depends on this balance: incoming radiation from the Sun versus infrared emitted by the ground and atmosphere.

Real-world connections

• A black car in sunlight heats up faster than a white car (better absorption).
• Vacuum flasks are shiny inside to reflect infrared back, keeping drinks hot or cold.
• “Space blankets” are thin, shiny sheets that reflect body heat back to you.
• Sitting by a campfire, you feel warmth on the side facing the flames (radiation).