Radiation

Radiation is the transfer of energy by electromagnetic waves.

All objects transfer energy to the surroundings by emitting infrared radiation. Hotter objects give off more infrared radiation. In this process, no particles are involved.

For example, look at this radiator:

If the radiator was hot, it would emit infrared waves in all directions, transferring thermal energy to the surroundings. When all the radiators are heating the house, they transfer thermal energy to the surroundings.

Let’s consider the example of a warm house:

Energy is transferred to the surroundings by radiation (from the walls, windows and roof) and conduction (from the windows, floor, roof and walls).

Since this energy transfer is common, there are several ways to reduce it, including:

  • Double glazing windows
  • Using curtains
  • Using carpets
  • Cavity wall insulation

The Sun’s Emission of Infrared Radiation

The sun emits infrared radiation, alongside many other waves, including visible light. Space is a vacuum, which means it contains no particles. Therefore, infrared radiation travels through space in the form of waves.

Detecting and Using Infrared Radiation

Detecting infrared radiation

All objects emit (give out) and absorb (take in) infrared radiation, which can be seen with an infrared camera.

The infrared camera uses different colours to indicate which areas of the surroundings are emitting or absorbing infrared radiation.

Using infrared radiation

Infrared radiation can be very useful. It transfers energy to the thermal energy stores of objects, which heats them up.

For example, when coal burns in a barbecue, radiation is emitted, which will transfer to the thermal energy stores of the food. As a result, the food heats up.

The colour of an object affects its ability to absorb or emit radiation.

Colours and Radiation

Consider the colours below:

When comparing the four colours…

Dull black:

  • Best emitter
  • Best absorber

Silver:

  • Worst emitter
  • Worst absorber

Although silver is a bad emitter and a bad absorber, the colour is useful because silver is the best reflector of radiation. This is why hot food is often wrapped in silver foil to keep it warm. The radiation emitted by the food is reflected back at the food, which keeps it warm.

Let’s look at an example:

Boiling water is poured into two different containers, and the temperature is measured at regular intervals.

The temperature of the water in the container with a black surface drops faster than in the container with a shiny surface. This shows how black surfaces are better emitters of radiation and worse reflectors of radiation, compared to shiny surfaces.

As black surfaces are great emitters, the water is cooler at all time intervals because it is the best at releasing the water’s heat radiation. Also, black surfaces are poor at reflecting heat radiation back into the water.

Shiny surfaces are poor emitters, meaning they release the water’s heat energy less effectively. As a result, the water remains warmer at all time intervals. Also, shiny surfaces are excellent at reflecting heat radiation back into the water.

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