Radiation

Radiation is the transfer of energy by electromagnetic waves.

All objects transfer energy to the surroundings by emitting infrared radiation. Hotter objects emit more infrared radiation, and this process doesn’t involve particles.

For example, look at this radiator:

A radiator.

If this 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, heat energy is then transferred from the house to the surroundings.

Consider the example of a warm house:

A house with multiple red arrows coming out of it representing heat loss.

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

This tends to happen often, so there are ways to reduce the energy transferred to the surroundings:

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

The Sun Emitting Infrared Radiation

The sun amongst clouds.

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 displays different colours to indicate areas that are hotter or cooler. This reveals which parts 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 on a barbecue, it emits radiation that transfers to the food’s thermal energy stores. Therefore, the food heats up.

Food (hotdogs, bugers and asparagus) on a grill.

The colour of an object determines how good it is at absorbing or emitting radiation.

Colours and Radiation

Look at the colours below:

A spectrum containing dull black, shiny black, white and silver starting at the worst reflector and best absorber of radiation and the best emitter of radiation.

When comparing the four colours…

Dull black:

  • Best emitter
  • Best absorber

Silver:

  • Worst emitter
  • Worst absorber

Although silver is a poor emitter and absorber of radiation, its reflective properties make it the best choice for reflecting radiation.

This is why hot food is often wrapped in silver foil to keep them warm. The radiation the food is emitting is reflected back at the food, which keeps it warm.

Chicken within tin foil.

Let’s look at an example:

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

A volumetric cylinder with a silver surface and a volumetric cylinder with a black surface both with thermometers going through them. A graph shows that the black surface loses more heat than the silver over time.

As you can see, the water in the container with a black surface cooled faster than the water in the shiny-surfaced container. This shows how black surfaces are better emitters of radiation and worse reflectors of radiation, compared to shiny surfaces.

Since black surfaces are excellent emitters, they release the water’s heat radiation more effectively, making the water cooler at all time intervals. Also, black surfaces are the worst at reflecting heat radiation back into the water

Shiny surfaces are poor emitters, meaning they don’t release the water’s heat energy effectively. This keeps the water warmer at all time intervals. Also, shiny surfaces are the best at reflecting heat radiation back to the water.

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