Black Body Basics

Black Body Basics 🔥

Point a thermal camera at anything — a kettle, a wall, a person — and it glows. Every object is constantly giving off **infrared (IR) radiation**. How much it radiates depends on two things: how **hot** it is, and what its **surface** is like.

Everything radiates 🌡️

**All bodies** both **emit** and **absorb** infrared radiation, all the time. The **hotter** a body is, the **more infrared it radiates each second**. A body also radiates across a range of wavelengths — as it gets hotter, the radiation shifts to shorter wavelengths (which is why metal glows red, then white, as it heats).

Infrared facts

Select ALL THREE statements about infrared radiation that are TRUE.

• Every body both emits and absorbs infrared radiation
• The hotter a body is, the more infrared it radiates each second
• A good absorber of infrared is also a good emitter
• Only hot objects emit infrared; cold ones do not
• Shiny surfaces are the best absorbers of infrared
• A body's temperature has no effect on how much it radiates

Surfaces matter (RP10) ⬛

In **Required Practical 10** you compare surfaces. The result: • **Matt black** surfaces are the **best absorbers AND best emitters** of infrared. • **Light, shiny (silvery)** surfaces are **poor** at both — they mostly **reflect** infrared. A key rule: a **good absorber is a good emitter**.

Match each idea to what it means

Keeping tea hot

An interactive activity.

The hottest roof

Four identical cars are left in bright sunshine. Which roof colour absorbs infrared fastest and gets hottest?

• Matt black
• Shiny silver
• Glossy white
• They all absorb exactly the same

The perfect black body ⚫

Physicists imagine an ideal object called a **perfect black body**: one that **absorbs all** the radiation that hits it (reflecting none) and is also a **perfect emitter**. Real objects only approach this, but it is a useful model — especially for understanding stars and planets.

A balancing act ⚖️

A body warms up if it **absorbs more radiation than it emits**, and cools down if it **emits more than it absorbs**. So a body stays at a **constant temperature** only when the radiation it **absorbs equals** the radiation it **emits** — the two are in balance.

Constant temperature

A planet's temperature stays constant over time. What must be true about its radiation?

• It absorbs radiation at the same rate as it emits radiation
• It absorbs much more than it emits
• It emits much more than it absorbs
• It neither absorbs nor emits any radiation

Why a planet warms

An interactive activity.

Radiation summary

Every object both emits and _____ infrared radiation; the hotter it is, the more it radiates. Matt _____ surfaces are the best absorbers and emitters. A body stays at a constant temperature when the radiation it absorbs equals the radiation it _____. If it absorbs more than it emits, its temperature _____.