Black body radiation

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Thermal Radiation Basics

All bodies (objects), no matter what temperature, emit a spectrum of thermal radiation in the form of electromagnetic waves.
These electromagnetic waves usually lie in the infrared region of the spectrum but could be emitted in the form of visible light or other wavelengths, depending on the temperature.
The hotter the object, the more infrared radiation it radiates in a given time.
The infrared radiation emitted from a hot object can be detected using a special camera.
The intensity and wavelength distribution of any emitted waves depends on the temperature of the body.
This can be represented on a thermal radiation curve (or black body radiation curve).

Hotter objects emit more thermal radiation

Black Body Radiation

Black body radiation is the name given to the thermal radiation emitted by all bodies (objects).
A perfect black body is defined as an object that absorbs all of the radiation incident on it and does not reflect or transmit any radiation.
Since a good absorber is also a good emitter, a perfect black body would be the best possible emitter too.
As a result, an object which perfectly absorbs all radiation will be black.
This is because the colour black is what is seen when all colours from the visible light spectrum are absorbed.

Temperature Effects on Emission

As the temperature increases, the peak of the thermal radiation curve moves to a lower wavelength and a higher intensity.
The peak of a thermal radiation curve moves to the left with increasing temperature.
From the electromagnetic spectrum, waves with a smaller wavelength have higher energy (e.g. UV rays, X-rays).
When an object gets hotter, the amount of thermal radiation it emits increases.
This increases the energy emitted and therefore the wavelength of the emitted radiation decreases.
At room temperature objects emit thermal radiation in the infrared region of the spectrum.
At around 1000 °C an object will emit a significant amount of red light.
At 6000 °C an object will mainly emit white or blue light (and some ultraviolet).
At even higher temperatures objects will emit ultraviolet or even X-rays.

Thermal Equilibrium

As an object absorbs thermal radiation it will become hotter.
As it gets hotter it will also emit more thermal radiation.
The temperature of a body increases when the body absorbs radiation faster than it emits radiation.
Eventually, an object will reach a point of constant temperature where it is absorbing radiation at the same rate as it is emitting radiation.
At this point, the object will be in thermal equilibrium.
An object will remain at a constant temperature if it absorbs heat at the same rate as it loses heat.

Radiation & Temperature Change

The temperature of a body can be regulated by balancing how much incoming radiation is absorbed and emitted (or reflected).
If an object starts to absorb radiation at a higher rate than it radiates it, then the object will heat up.
Likewise, if it loses radiation at a greater rate than it absorbs it, then the object will cool down.
This is how an emergency blanket works, to keep a trauma victim warm.
Rescue teams use light-coloured, shiny emergency blankets to keep accident survivors warm.
A light, shiny outer surface emits a lot less radiation than a dark, matt (non-glossy) surface.
This keeps the patient warm, as less infrared radiation is emitted than if an ordinary blanket had been used.

Temperature of the Earth

If the Earth had no atmosphere, the temperature on the surface would drop to about −180 °C at night, the same as the Moon’s surface at night.
This would happen because the surface would be emitting all the radiation from the Sun into space.
The Earth receives the majority of its heat in the form of thermal radiation from the Sun.
At the same time, the Earth emits its own thermal radiation, with a slightly longer wavelength than the thermal radiation it receives (the surface temperature of the Earth is significantly smaller than the surface temperature of the Sun).
Some gases in the atmosphere, such as water vapour, methane, and carbon dioxide (greenhouse gases) absorb and reflect back longer-wavelength infrared radiation from the Earth and prevent it from escaping into space.
These gases absorb the radiation and then emit it back to the surface.
This process makes the Earth warmer than it would be if these gases were not in its atmosphere.
The temperature of the Earth depends on several factors, such as the rate that light and infrared radiation from the Sun are reflected back into space, absorbed by the Earth’s atmosphere or surface, and emitted from the Earth’s surface and atmosphere into space.

The Greenhouse Effect

The rate of absorption and emission of radiation on Earth contributes to the Greenhouse Effect.
This is the natural process that warms the Earth's surface from the Sun.
The Sun's thermal radiation reaches the Earth's atmosphere where some radiation is reflected back to space.
Any radiation not reflected is absorbed and re-radiated by greenhouse gases.
The absorbed radiation then warms the atmosphere and the surface of the Earth.
This is similar to what happens in a greenhouse to keep a humid, and warm temperature to grow plants.

The greenhouse effect: radiation from the Sun and Earth

Core Practical: Investigating Thermal Radiation

Aim: to investigate how the amount of infrared radiation absorbed or radiated by a surface depends on the nature of that surface.
Independent variable = colour; dependent variable = temperature.
Control variables: identical flasks (except for their colour), same amounts of hot water, same starting temperature of the water, same time interval.
Method: set up four identical flasks painted black, grey, white and silver; fill with hot water ensuring the same initial temperature; measure temperatures at regular intervals (e.g. every 30 seconds for 10 minutes).
All warm objects emit thermal radiation in the form of infrared waves.
The intensity (and wavelength) of the emitted radiation depends on the temperature of the body, the surface area of the body, and the colour of the surface.
Most of the heat lost from the beakers will be due to conduction and convection, which is the same for each beaker as colour does not affect heat loss in this way.
Any difference in heat loss between the beakers must, therefore, be due to infrared (thermal) radiation.
To compare the rate of heat loss of each flask, plot a graph of temperature on the y-axis against time on the x-axis and draw curves of best fit.

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Übungsfragen

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1.Which type of electromagnetic radiation do all objects emit as thermal radiation?
Easy
- AInfrared
- BUltraviolet
- CGamma
- DRadio
2.All objects, no matter what temperature, emit infrared radiation.
Easy
True or false?
3.What is a perfect black body?
Easy
- AAn object that absorbs all radiation incident on it and reflects or transmits none
- BAn object that only absorbs visible light and reflects all other wavelengths
- CAn object that emits no radiation at all
- DAn object that is always black in colour
4.Which of the following statements about a perfect black body are correct? (select all that apply)
Medium
- AIt absorbs all radiation incident on it.
- BIt reflects all radiation incident on it.
- CIt is the best possible emitter of radiation.
- DIt does not emit any radiation.
- EIt does not transmit any radiation.
5.A perfect black body must always appear black in colour.
Easy
True or false?
6.As the temperature of a black body increases, what happens to the peak of its radiation curve?
Medium
- AIt moves to a lower wavelength and a higher intensity.
- BIt moves to a higher wavelength and a lower intensity.
- CIt moves to a higher wavelength and a higher intensity.
- DIt moves to a lower wavelength and a lower intensity.
7.Match each surface colour with its correct property regarding infrared radiation.
Medium
- Matt black
- Shiny silver
- White
- Best absorber and emitter
- Poor absorber and emitter, reflects well
- Poor absorber and emitter, reflects some
8.An object at a constant temperature is absorbing radiation at the same rate as it is emitting radiation.
Easy
True or false?

Mehr Fragen

9.Which of the following describes the relationship between radiation and temperature that would cause a body to decrease in temperature?
Medium
- AAbsorption rate: higher, Emission rate: lower
- BAbsorption rate: lower, Emission rate: higher
- CAbsorption rate: equal, Emission rate: equal
- DAbsorption rate: slightly higher, Emission rate: slightly lower
10.If the Earth had no atmosphere, why would its surface temperature drop drastically at night?
Medium
- AThe surface would emit all its radiation into space without any atmosphere to absorb and re-emit it back.
- BThe surface would stop absorbing radiation from the Sun.
- CThe surface would reflect all radiation from the Sun.
- DThe surface would become a perfect black body.
11.Which factors affect the temperature of the Earth's surface? (select all that apply)
Medium
- AThe rate at which solar radiation is reflected back into space
- BThe rate at which solar radiation is absorbed by the atmosphere and surface
- CThe rate at which radiation is emitted from the Earth's surface and atmosphere
- DThe colour of the Earth's surface
- EThe distance of the Earth from the Sun
12.Increasing the amount of greenhouse gases in the atmosphere causes global warming because they absorb and re-emit more infrared radiation back to Earth.
Medium
True or false?
13.Why is a light-coloured, shiny emergency blanket used to keep trauma victims warm?
Medium
- AIt reflects body heat back and emits very little infrared radiation.
- BIt absorbs body heat and re-emits it as infrared radiation.
- CIt conducts heat away from the body slowly.
- DIt is a perfect black body and emits radiation efficiently.
14.Place the following surfaces in order from best emitter of infrared radiation to worst emitter.
Medium
- Matt black
- White
- Shiny silver
15.A student investigates how the amount of infrared radiation emitted by different surfaces depends on their colour. They fill four identical flasks painted black, grey, white, and silver with hot water and record the temperature every 30 seconds. Which variable is the independent variable?
Medium
- AColour of the flask
- BTemperature of the water
- CTime interval
- DVolume of water
16.In the same investigation, which of the following is a control variable?
Medium
- AThe starting temperature of the water
- BThe colour of the flask
- CThe temperature of the water after 10 minutes
- DThe amount of infrared radiation emitted
17.A good absorber of infrared radiation is also a good emitter.
Easy
True or false?
18.Which statement about the emission of thermal radiation is correct?
Medium
- AThe intensity and wavelength distribution of emitted radiation depend on the temperature of the body.
- BThe intensity of emitted radiation is independent of temperature.
- CThe wavelength distribution of emitted radiation is the same for all bodies.
- DOnly hot objects emit thermal radiation.
19.Draw one line between each temperature and the type of radiation an object mainly emits at that temperature.
Easy
- Room temperature
- About 1000 °C
- About 6000 °C
- Mainly infrared
- Significant amount of red light
- Mainly white or blue light, plus some ultraviolet
20.An object is at a constant temperature. Which statement about its absorption and emission of radiation is correct?
Easy
- AIt absorbs radiation at the same rate as it emits radiation.
- BIt absorbs radiation faster than it emits radiation.
- CIt emits radiation faster than it absorbs radiation.
- DIt does not absorb or emit any radiation.
21.A body absorbs radiation at a lower rate than it emits radiation. What happens to its temperature?
Medium
- AIt increases.
- BIt decreases.
- CIt stays the same.
- DIt first increases then stays the same.
22.Which of the following factors affect the temperature of the Earth's surface? (select all that apply)
Medium
- AThe rate at which radiation from the Sun is reflected back into space.
- BThe rate at which radiation is absorbed by the Earth's atmosphere and surface.
- CThe rate at which radiation is emitted from the Earth's surface and atmosphere into space.
- DThe colour of the Earth's surface.
- EThe distance between the Earth and the Moon.
23.All objects emit infrared radiation, regardless of their temperature.
Easy
True or false?
24.A black body radiation curve is plotted for an object. The temperature of the object is then increased. What happens to the peak of the curve?
Medium
- AIt moves to a longer wavelength and a lower intensity
- BIt moves to a shorter wavelength and a higher intensity
- CIt moves to a shorter wavelength and a lower intensity
- DIt stays at the same wavelength but the intensity increases
25.An object at room temperature emits thermal radiation mainly in which region of the electromagnetic spectrum?
Easy
- AUltraviolet
- BVisible light
- CInfrared
- DX-rays
26.An object is emitting radiation at a greater rate than it absorbs radiation. What happens to its temperature?
Medium
- AIt increases
- BIt decreases
- CIt stays the same
- DIt increases then stays constant
27.Match each surface with the correct description of its emission and absorption of infrared radiation.
Medium
- Black, matt surface
- Shiny silver surface
- White surface
- Good absorber and good emitter
- Poor absorber and poor emitter
- Reflects most radiation and absorbs little
28.In the core practical investigating thermal radiation, four identical flasks are painted black, grey, white and silver. They are filled with hot water and the temperature is recorded every 30 seconds. Which of the following is the independent variable?
Medium
- AThe colour of the flask
- BThe temperature of the water
- CThe volume of water in each flask
- DThe time interval between readings
29.In the core practical investigating thermal radiation, which of the following is the dependent variable?
Medium
- AThe colour of the flask
- BThe temperature of the water
- CThe starting temperature of the water
- DThe time interval between readings
30.In the core practical investigating thermal radiation, which of the following are control variables? (select all that apply)
Medium
- AThe colour of the flask
- BThe volume of water in each flask
- CThe starting temperature of the water
- DThe time interval between readings
- EThe shape of the flask
31.In the core practical investigating thermal radiation, why is it important that the flasks are identical apart from their colour?
Medium
- ATo ensure that any difference in heat loss is due to the colour of the surface
- BTo make the experiment quicker to carry out
- CTo ensure that the flasks all cool down at the same rate
- DTo make sure the flasks are all the same colour
32.In the core practical investigating thermal radiation, most of the heat lost from the flasks is due to conduction and convection. Why is this not a problem when investigating radiation?
Medium
- ABecause conduction and convection are the same for each flask
- BBecause conduction and convection do not occur in liquids
- CBecause radiation is the only method of heat transfer in the experiment
- DBecause the flasks are painted different colours
33.In the core practical investigating thermal radiation, a student plots a graph of temperature against time for each flask. What is the correct way to compare the rate of heat loss of each flask?
Medium
- ADraw curves of best fit and compare their gradients
- BCompare the starting temperatures of each flask
- CCompare the final temperatures of each flask
- DCompare the colours of the flasks
34.In the core practical investigating thermal radiation, which of the following is a safety consideration?
Medium
- AKeep water away from all electrical equipment
- BUse a data logger instead of a thermometer
- CTake repeated readings for each flask
- DRead the thermometer at eye level
35.In the core practical investigating thermal radiation, a student wants to reduce random errors. Which of the following would help?
Medium
- ATake repeated readings for each coloured flask
- BUse the same starting temperature for each flask
- CUse a larger hole for the thermometer
- DUse a stopwatch with a resolution of 0.01 s
36.In the core practical investigating thermal radiation, a student uses a data logger connected to a digital thermometer. What is the main advantage of this?
Medium
- AIt gives more accurate readings
- BIt makes the experiment safer
- CIt allows the flasks to be painted different colours
- DIt reduces the starting temperature of the water
37.In the core practical investigating thermal radiation, a student finds that the black flask cools down the fastest. What can be concluded about the black surface?
Hard
- AIt is the best emitter of infrared radiation
- BIt is the best absorber of infrared radiation
- CIt is the worst emitter of infrared radiation
- DIt reflects the most infrared radiation
38.In the core practical investigating thermal radiation, a student finds that the silver flask cools down the slowest. What can be concluded about the silver surface?
Hard
- AIt is the best emitter of infrared radiation
- BIt is the worst emitter of infrared radiation
- CIt is the best absorber of infrared radiation
- DIt emits more infrared radiation than the black surface
39.Which of the following are true about the core practical investigating thermal radiation? (select all that apply)
Medium
- AThe flasks should be filled with the same volume of water
- BThe starting temperature of the water should be the same for each flask
- CThe temperature should be recorded at regular intervals
- DThe flasks should be painted the same colour
- EThe experiment should be carried out in pairs to coordinate timing
40.Which statement describes what happens to the peak of a thermal radiation curve as the temperature of a body increases?
Easy
- AThe peak moves to a lower wavelength and a higher intensity
- BThe peak moves to a higher wavelength and a lower intensity
- CThe peak moves to a higher wavelength and a higher intensity
- DThe peak moves to a lower wavelength and a lower intensity
41.What is the name of the state reached when an object absorbs radiation at the same rate as it emits it?
Easy
42.Complete the sentences about the greenhouse effect using the words from the box.
Easy
Radiation from the Sun enters the Earth's atmosphere. Some of this radiation is ____ back to space and some of the radiation is ____ by the surface of the Earth. The Earth re-emits the ____ radiation at longer wavelengths. Some radiation is ____ by greenhouse gases, which re-emit radiation back into Earth's atmosphere.
43.A body is at thermal equilibrium and its temperature is 25 °C. It absorbs radiation at a rate of 50 W. At what rate does it emit radiation? Give your answer in watts.
Medium
44.Complete the sentence about black body radiation.
Easy
A perfect black body is an object that ____ all of the radiation incident on it.
45.A body is in thermal equilibrium and absorbs radiation at a rate of 80 W. At what rate does it emit radiation? Give your answer in watts.
Medium
46.A body is in thermal equilibrium and absorbs radiation at a rate of 120 W. At what rate does it emit radiation? Give your answer in watts.
Medium
47.Complete the sentence about the Earth's temperature balance.
Medium
The Earth's temperature depends on the balance between incoming radiation that is ____ back into space and radiation that is ____ by the surface.
48.A body is in thermal equilibrium and absorbs radiation at a rate of 75 W. At what rate does it emit radiation? Give your answer in watts.
Medium
49.A body is in thermal equilibrium and absorbs radiation at a rate of 200 W. At what rate does it emit radiation? Give your answer in watts.
Medium
50.A body absorbs radiation at a rate of 150 W and emits radiation at a rate of 120 W. By how much does its temperature change per second? Assume the specific heat capacity is 500 J/kg°C and the mass is 2 kg. Give your answer in °C/s.
Medium
51.Complete the sentences about the greenhouse effect using the words from the box: absorbed, reflected, emitted, re-emitted.
Medium
Radiation from the Sun enters the Earth's atmosphere. Some of this radiation is ____ back to space and some is ____ by the surface. The Earth re-emits radiation at longer wavelengths, which is ____ by greenhouse gases and then ____ back to the surface.
52.What name is given to the state reached when an object absorbs radiation at the same rate as it emits it?
Medium
53.Explain what is meant by the term thermal equilibrium.
Hard
54.Explain how the temperature of a body is related to the balance between incoming radiation absorbed and radiation emitted.
Hard
55.Explain how the greenhouse effect warms the Earth's surface.
Hard
56.Explain how the temperature of the Earth's surface is affected by the balance between incoming radiation absorbed and radiation emitted.
Hard
57.Explain why the Earth's surface temperature would decrease so drastically at night if the Earth had no atmosphere.
Hard
58.A body is at thermal equilibrium and its temperature is 25 °C. It absorbs radiation at a rate of 50 W. At what rate does it emit radiation? Give your answer in watts.
Medium
- A50 W
- B45 W
- C55 W
- D60 W
59.A body is in thermal equilibrium and absorbs radiation at a rate of 80 W. At what rate does it emit radiation? Give your answer in watts.
Medium
- A88 W
- B80 W
- C96 W
- D72 W
60.A body is in thermal equilibrium and absorbs radiation at a rate of 120 W. At what rate does it emit radiation? Give your answer in watts.
Medium
- A108 W
- B120 W
- C132 W
- D144 W
61.A body is in thermal equilibrium and absorbs radiation at a rate of 75 W. At what rate does it emit radiation? Give your answer in watts.
Medium
- A83 W
- B91 W
- C75 W
- D67 W
62.A body is in thermal equilibrium and absorbs radiation at a rate of 200 W. At what rate does it emit radiation? Give your answer in watts.
Medium
- A200 W
- B220 W
- C180 W
- D240 W
63.A body absorbs radiation at a rate of 150 W and emits radiation at a rate of 120 W. By how much does its temperature change per second? Assume the specific heat capacity is 500 J/kg°C and the mass is 2 kg. Give your answer in °C/s.
Medium
- A-0.97 °C/s
- B0.03 °C/s
- C1.03 °C/s
- D2.03 °C/s

Karteikarten

What is black body radiation?

The thermal radiation emitted by all bodies (objects) in the form of electromagnetic waves.

In which region of the electromagnetic spectrum does thermal radiation usually lie?

Infrared, but it can also be visible light or other wavelengths depending on the temperature.

How does the amount of infrared radiation emitted by an object change with temperature?

The hotter the object, the more infrared radiation it radiates in a given time.

What is a perfect black body?

An object that absorbs all of the radiation incident on it and does not reflect or transmit any radiation.

Why is a perfect black body the best possible emitter?

Because a good absorber is also a good emitter, so an object that perfectly absorbs all radiation will also be the best emitter.

What happens to the peak of a thermal radiation curve as temperature increases?

The peak moves to a lower wavelength and a higher intensity.

At room temperature, in which part of the spectrum do objects emit thermal radiation?

In the infrared region of the spectrum.

At around 1000 °C, what visible light does an object emit?

A significant amount of red light.

At 6000 °C, what does an object mainly emit?

White or blue light, and some ultraviolet.

What is thermal equilibrium?

The point at which an object absorbs radiation at the same rate as it emits radiation, so its temperature remains constant.

When does the temperature of a body increase?

When the body absorbs radiation faster than it emits radiation.

When does a body cool down?

When it emits radiation at a greater rate than it absorbs it.

Why are emergency blankets light-coloured and shiny?

A light, shiny surface emits a lot less radiation than a dark, matt surface, so less infrared radiation is emitted and the patient stays warm.

What would happen to Earth's surface temperature at night if it had no atmosphere?

It would drop to about −180 °C, the same as the Moon's surface at night, because the surface would emit all the radiation from the Sun into space.

How does the wavelength of thermal radiation emitted by Earth compare to that received from the Sun?

Earth emits thermal radiation with a slightly longer wavelength than the thermal radiation it receives from the Sun.

Name three greenhouse gases.

Water vapour, methane, and carbon dioxide.

How do greenhouse gases warm the Earth?

They absorb longer-wavelength infrared radiation emitted by Earth and re-emit it back to the surface, making Earth warmer than it would be without them.

What factors determine the temperature of the Earth?

The rates at which light and infrared radiation from the Sun are reflected back into space, absorbed by the atmosphere or surface, and emitted from the surface and atmosphere into space.

What is the Greenhouse Effect?

The natural process that warms the Earth's surface: some solar radiation is reflected back to space, the rest is absorbed and re-radiated by greenhouse gases, warming the atmosphere and surface.

In the core practical investigating thermal radiation, what is the independent variable?

The colour of the surface.

In the core practical investigating thermal radiation, what is the dependent variable?

The temperature.

State two control variables in the core practical investigating thermal radiation.

Identical flasks (except for colour), same amounts of hot water, same starting temperature of the water, and same time interval.

Why must any difference in heat loss between coloured flasks in the core practical be due to infrared radiation?

Because most heat lost is due to conduction and convection, which are the same for each beaker as colour does not affect these processes.

What is the expected result for the cooling of black, grey, white, and silver flasks?

The black flask cools the fastest, then grey, then white, and the silver flask cools the slowest.

How does the intensity and wavelength distribution of emitted radiation depend on temperature?

As temperature increases, the intensity of emitted radiation increases and the wavelength of peak intensity decreases.

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