1. What Is Radiation?
Radiation is the transfer of heat through electromagnetic waves. Unlike conduction and convection, radiation does not need any medium—heat can travel through empty space. This is how the Sun’s heat reaches Earth.
All objects emit thermal radiation, and hotter objects emit more of it.
2. How Radiation Works
Every object gives off electromagnetic waves depending on its temperature. These waves carry energy. When they reach another object, the energy is absorbed and increases the object's temperature.
The higher the temperature, the greater the energy emitted.
2.1. Emission of Radiation
Hotter objects emit more radiation and at shorter wavelengths. This is why a heating coil glows red—it emits both light and heat.
2.2. Absorption of Radiation
When radiation falls on another object, some of it is absorbed, some reflected, and some transmitted. The absorbed part increases the object's temperature.
3. Surface and Colour Effects
The nature of a surface greatly affects how much radiation it absorbs or emits.
3.1. Good Absorbers and Emitters
- Dark, dull, rough surfaces absorb and emit radiation very well.
- This is why dark clothes feel hotter in sunlight.
3.2. Poor Absorbers and Reflectors
- Bright, shiny, and smooth surfaces reflect most of the radiation.
- They absorb very little heat and remain cooler in sunlight.
3.3. Practical Uses
- Thermos flasks use shiny surfaces to reduce heat loss by reflection.
- Solar cookers are painted black to absorb maximum radiation.
4. Thermal Radiation and Temperature
The intensity of radiation depends strongly on temperature. Even objects at room temperature emit radiation, but hotter objects emit much more.
4.1. Relationship With Temperature
As temperature increases, the amount of radiation emitted rises quickly.
4.2. Examples Based on Temperature
- Human bodies emit infrared radiation.
- Electric heaters emit visible red light when hot.
- The Sun emits radiation across a wide range of wavelengths.
5. Radiation Through Vacuum
Radiation is the only mode of heat transfer that works without particles. It can travel through completely empty space.
5.1. Key Point
Conduction and convection need particles. Radiation does not. This allows heat from the Sun, which is millions of kilometres away, to reach Earth.
5.2. Examples
- Heat felt from a distant bonfire.
- Warmth from the Sun reaching Earth.
- Heat lamps used in kitchens and bathrooms.
6. Greenhouse Effect and Radiation
Radiation plays a major role in the greenhouse effect. The Sun’s radiation passes through the atmosphere easily, but the Earth emits infrared radiation back. Some of this infrared radiation is trapped by greenhouse gases, leading to warming.
6.1. Why Infrared Gets Trapped
Greenhouse gases like carbon dioxide and water vapour absorb infrared radiation, preventing it from escaping fully into space.
7. Everyday Examples of Radiation
- Feeling heat from sunlight even on a cold day.
- An electric heater warming a room without blowing air.
- Shiny car sunshades reflecting heat to keep interiors cool.
- Infrared cookers heating food without physical contact.