6.4 Black Body Radiation

An object which can be modelled as a black body is heated to different temperatures, represented by the four lines on Figure 1, and its spectrum is recorded on the graph. 

Figure 1

Identify the visible, ultraviolet and infrared wavelengths by correctly labelling the sections of the graph.

Using Figure 1, explain why metal turns from red to white and gets brighter as it gets hotter.

Higher only

The emission and absorption of radiation plays a large role in the Earth's environment.

Use the text in the box to complete the sentences below, describing the Earth's Surface-Atmosphere system.

The Earth's energy balance depends on how much energy is incoming from the _____________ and how much energy is returned to _____________.

If incoming and outgoing energy are in balance, the Earth’s temperature will remain ______________.

This system can be used to create models that help climate scientists predict temperature fluctuations based on concentrations of _________________   ____________.

Higher Only

The earth remains at a constant average temperature of ~15 °C, despite being in space which has an average temperature of -270 °C.

Explain how the Earth maintains a constant temperature.

Explain why a cloudy night is warmer than a clear one.

In 1800 the scientist William Hershel proposed that different colours of light have different temperatures associated with them.

To test his theory, he created a spectrum of light and placed thermometers in the different colours.

His experiment is shown in Figure 3 below.

Figure 3

In addition to this he also placed another thermometer nearby, outside the spectrum of light.

Explain why he placed a thermometer outside the spectrum.

In one of his experiments he placed a thermometer just to the side of the red end of the spectrum, as shown in Figure 4.

Figure 4

To his surprise, he found that it gave a higher temperature reading than all of the other thermometers.

Explain why.

The graph in Figure 7 below shows the spectrum of light emitted from hot objects and how the intensity of different frequencies varies with the temperature of the object.

Figure 7

The frequency range of visible light runs from around 4 10¹⁴ Hz (red) to 8 10¹⁴ Hz (violet). These frequencies are shown as dotted lines on the graph.

When objects are heated:

• As they get hotter, they emit greater amounts of infra-red radiation.
• When they get very hot, they start to glow red.
• Heated even further they start to glow yellow.
• Eventually an object can get so hot that it will glow white.

Use the graph in Figure 7 to explain each of the above observations.

Use the graph in Figure 7 to estimate the temperature at which an object will first start to glow red. Explain your answer.

Viewed high in the sky, the Sun appears white in colour.

Use the graph in Figure 7 to estimate the temperature of the Sun.

Explain your answer

Tick the correct boxes below to indicate which colour is the best absorber, and which is the best emitter, of thermal radiation.

Higher Only

On a hot sunny day, it is noticed that the black leather seats in a car start to get very hot.

The temperature, however, does not continue rising forever. It eventually settles at a high but steady temperature.

Explain why, referring to the thermal radiation absorbed and emitted by the seat.

Higher Only

The diagrams in Figure 8 below show the blackbody radiation received by the Earth during daytime and at night, along with the blackbody radiation emitted by the Earth.

Figure 8

The width of each arrow represents the amount of radiation received or emitted.

During the day the Earth receives radiation from the Sun, but at night this radiation is no longer received.

The Earth, on the other hand, emits radiation at a fairly constant rate.

Explain, using the above information, why it is usually warmer during the daytime than it is at night.

Higher Only

Clouds contain droplets of water which are very good at reflecting blackbody radiation.

Explain the effect clouds will have on the temperature during both daytime and night-time.

Make sure you refer to both the radiation received and emitted by the Earth.

Higher Only

Carbon dioxide is very good at reflecting the radiation emitted from the Earth, but is transparent to the radiation received from the Sun.

Explain why increased levels of carbon dioxide could lead to a rise in the Earth’s average temperature.