Demonstration

This experiment uses a commercially available apparatus to compare the thermal conductivity of different metals.

Apparatus and Materials

• Ingenhousz's apparatus, or similar (see diagram below)
• Paraffin wax

Health & Safety and Technical Notes

Read our standard health & safety guidance

The apparatus has a number of rods, each made of a different metal. Accompanying information will identify them.

The rods must be coated with wax. This can be done by one of the following methods:

• Take the rods out of the water box and lay them in a chilled tin tray containing molten paraffin-wax. Remove quickly, hold vertically to allow the excess wax to drain off, and push them back into the water box.
• Keep the rods in the water box. Paint each rod with a paint brush dipped in very hot molten wax. This produces an uneven, thick coating of wax, which must then be thinned by blowing a Bunsen flame up and down the rod. (This is a poor method, only successful in very skilful hands.)

A simple and easy-to-use set of conductivity bars is now available from:

Timstar

Four bars made of different metals are mounted on the same plastic support. Each bar has a liquid crystal strip showing temperature changes along the bar.

Timstar Laboratory Suppliers, Marshfield Bank, Crewe, Cheshire. CW2 8UY Tel: 01270 250459 Email: [email protected]

Procedure

1. Fill the water bath with hot water. Note how far along the rods the wax has melted when the apparatus reaches a steady state.

Different designs are available. Generally they use a bath of hot water to heat the rods.

Teaching Notes

• Note that the speed at which a particular temperature (such as the melting point of wax) travels along the bar when one end is heated is essentially the speed of the temperature waves. This involves specific heat capacity and density as well as conductivity. Thus a rod of lead makes a quick start in the race although it is a poor conductor, but the wax-melting will not have travelled far when a steady state is attained.
• Energy is dissipated (stored thermally in the surroundings) from the surface of each rod. If, for a steady state, the distance from the heated end to the melting point of the wax is twice as great for rod A as for rod B, then rod A has only half the temperature gradient but twice the surface area for heat losses. So rod A must have four times the conductivity of rod B.
• The versions of this apparatus currently available are of the static warm water type. The preferred form has steam passing continuously through the apparatus. This is left to attain a steady state. Slider rings indicate the progress of the melting wax.

This experiment was safety-tested in January 2007

Class practical

Observing convection currents in a beaker of water.

Apparatus and Materials

For each group of students

• Bunsen burner
• Tripod
• Glass tube, approximately 150 mm long x 3 mm internal diameter
• Pyrex beaker, 600 ml
• Potassium manganate VII crystals

Health & Safety and Technical Notes

Read our standard health & safety guidance

You need to get the potassium manganate VII crystals into the beaker of water without colouring it unduly. Put a glass tube into the beaker so that it rests on the base of the beaker. Then drop the crystal through the tube.

Procedure

1. Fill the beaker with water. Put individual crystals of potassium manganate VII on the bottom of the beaker, using the method suggested above.
2. Heat the water gently over the Bunsen burner and observe the motion of the coloured water. For a clear effect, use a small flame and no gauze between Bunsen and beaker (Pyrex beakers do stand this).
3. Note the path that the coloured water takes from the heater to the top of the water and back down again.
4. When repeating the experiment you should always start with a new batch of cold water.

Teaching Notes

This experiment was safety-tested in August 2007

Class practical

Watching convection currents in water.

Apparatus and Materials

For each student or group of students

• Bunsen burner
• Test-tube (hard glass)
• Potassium manganate VII crystals
• Glass tube, approximately 150 mm long x 3 mm internal diameter

Health & Safety and Technical Notes

This experiment is quite safe so long as the Bunsen flame is small.

Read our standard health & safety guidance

Procedure

1. Fill the hard glass test-tube with cold water. When the water is still, put the glass tube into the test-tube so that it rests on the base of the test-tube. Drop a single, very small, crystal of potassium manganate VII through the tube so that it falls to the bottom, leaving little colour.
2. Hold the test-tube near the top but not above water level. Heat it gently with a Bunsen flame at the bottom of the tube to see the streamers of dye begin to circulate. As soon as the tube is felt to be warmed by the hot water, stop heating.
3. Empty the test-tube, allow it to cool, wash it and fill again with cold water. When the water is at rest, add another very small crystal without stirring.
4. This time hold the tube at the bottom and heat with the Bunsen flame near the top of the tube, just below the water surface.
5. Continue until the water at the top of the tube just begins to boil, but the bottom remains cool. No coloured streamers are observed.

Teaching Notes

• If the test-tube is held at the top of the water and heated from the bottom, convection trails can be seen in the water. If the test-tube is held at the bottom and the water heated at the top, there are no convection trails. The water at the bottom stays cool. A piece of ice fixed (with gauze) at the bottom of the test-tube will not melt even when the water is boiling at the top. Melting has to await conduction of energy through the water, which is a slow process in comparison with convection.
• The hot water transfers energy with it when it moves. That is the process of convection. It happens in a fluid whether it is a liquid or a gas. On a large scale it happens in the oceans and in the atmosphere, in hot water heating systems and in ventilation. Winds are just convection currents.

This experiment was safety-tested in March 2006

Class practical

Visualizing convection currents in air.

Apparatus and Materials

• Compact light source (slide projector or other high intensity lamp can be used but needs dim-out), 100 W
• Bunsen burner
• Translucent screen
• LT variable voltage supply (capable of 8 A at 12 V)

Health & Safety and Technical Notes

Read our standard health & safety guidance

Procedure

1. Light the Bunsen and put it about 1/2 metre in front of the compact light source. Cast the shadow either on to the wall or on to a translucent screen.
2. Students should look at the shadow and discuss what they see.

Teaching Notes

The convection currents from the Bunsen flame can be clearly seen. The warm air rises up above the flame and falls down away from it.

This experiment was safety-tested in March 2006

Class practical

Simple observations on transmitting and absorbing radiation.

Apparatus and Materials

• Radiant heater, connected to the mains
• Insulating sheet approximately 250 x 250 mm with hole
• Glass plate approximately 250 x 250 mm
• Retort stand, boss, and clamp
• Slab of rock salt, approximately 250 x 250 mm

Health & Safety and Technical Notes

Supervise this experiment closely. Watch that students do not get too close to the heater.

Read our standard health & safety guidance

Procedure

1. Set up the heater on the bench and connect it to the a.c. mains.
Fix the insulating sheet in front of the heater with a retort stand, boss, and clamp. Adjust it so that the hole is level with the glowing element.
2. Ask students to look at the element through the hole in the insulating sheet. They should hold the back of their hand near the hole to feel the radiation.
3. Students should now use their cheek as a detector, about 25 cm away from the hole.
Insert a book between cheek and hole to see what happens; move the book in and out several times.
4. Move farther away from hole until the heating effect can only just be detected on the cheek.
Repeat the experiment of inserting a book. The book can be put in by a partner either near the cheek or near the source. See if there is any difference due to the time taken for the radiation to travel.
5. Insert the glass plate between the cheek and the hole. Move the plate in and out several times.
This can be repeated using two sheets of the glass plate held together to give a thicker sheet. (Be ready to move out of the way quickly if it is too hot when the glass plate is removed.)
6. If a slab of rock salt is available, replace the glass plates with the rock salt.

Teaching Notes

• In these experiments, special detecting instruments such as thermopiles are deliberately avoided. Students use their cheeks or the backs of their hands as the detecting device.
• Students should notice that the infra-red radiation appears to travel in straight lines through the hole. As they move their head from side to side, the heating effect is cut off and so is the light. Both heating effect and the light appear to travel together.
• Also the intensity of the infra-red radiation varies with distance from the source, and the infra-red radiation does not pass through thick materials such as a book. The infra-red radiation appears to travel instantaneously when the heater is aligned with the hole because it has a high speed (the same as that of light).
• Although visible light (also radiation) gets through the glass plate, the more abundant longer wavelength infra-red radiation does not appear to pass through the glass. Rock salt is translucent to visible light and transparent to infra-red.
• Links can be made to greenhouses and sealed-up cars on a hot day. Radiation from the Sun reaches us across empty Space.

This experiment was safety-tested in April 2006

Class practical

Using the hand as a detector to compare the absorption of infra-red radiation.

Apparatus and Materials

• Radiant heater
• Insulating sheet with hole
• Retort stand, boss, and clamp
• Crystallizing dishAluminium leaf (not kitchen foil)
• Vegetable black
• Paint brush
• Methylated spirit

Health & Safety and Technical Notes

Only aluminium leaf should be used. Do not use the thicker aluminium kitchen foil, which has a higher thermal capacity.

Some pupils may burn their hands without realizing the danger, unless an appropriate warning is given. Appropriate words are given in the "Procedure" below.

Read our standard health & safety guidance

Procedure

1. Set up the heating element with the insulating sheet in front of it. Ask a student to place the back of one hand near the hole for about 5 seconds. You might say:
"Hold your hand close to the hole in the screen, with the back of your hand towards the red-hot heater. Notice what you feel. Only do this for a short time to decide what you feel: one . . . two . . . three . . . four . . . five ... about that long. Do not hold your hand longer than this in case you burn it."
2. Cover the back of the student's hand with aluminium leaf and ask them to repeat this for a further 5 seconds. Now coat the aluminium leaf with a vegetable black paint. When the paint is dry, ask the student to place the back of the hand again near the hole for about 5 seconds.

Teaching Notes

• To put the leaf on the student's hand, get them to clench their fist and moisten the back of their hand with clean water until wet all over. Lay a sheet of aluminium leaf gently on the wet skin, blowing on it to push it on to the skin. Ask the student to unclench their hand a little to stop the leaf from cracking.

• To blacken the aluminium leaf, mix some vegetable black with methylated spirit to the consistency of thick soup. Apply with a soft paint brush on top of the leaf.

• To get rid of the paint and leaf, students should hold their hand under a running tap. Students should not try to rub off the paint or leaf. This will rub soot into the hands which will be difficult to remove.
• When infra-red radiation is absorbed by any surface, the surface is warmed up.
• The hand absorbs most infra-red radiation when it is painted black, a medium amount when it is not coated with anything, and the least when coated with a shiny surface. In the last case the infra-red radiation is being reflected away.
• White paper rather than aluminium foil stuck on the back of the hand and brought up to the heater may give a surprising result. The white paper and blackened foil absorb nearly the same amount of infra-red radiation, even though the white paper reflects most of the infra-red radiation. What is it about the respective properties of paper and aluminium foil?
• How Science Works extension: No measurements are taken in this experiment, but it is still possible to draw a valid conclusion about the nature of a surface and the absorption of infra-red radiation. You could:
• Use the experiment to illustrate the concept of valid evidence.
• Point out that, in a comparative analysis, numerical data is not always necessary.
• Ask students whether this experiment would still provide valid evidence if three or more colours were investigated. If they say it would not, ask them how the experiment could be adapted so that it did.
• Ask students to illustrate the findings of this data-free experiment in an appropriate way.

This experiment was safety-checked in January 2007

Class practical

A simple comparison of the infra-red radiation from two Bunsen flames.

Apparatus and Materials

For each group of students

• Bunsen burners, 2
• Insulating screens

Health & Safety and Technical Notes

Set up insulating screens with central apertures 1 metre from each flame. Then there is no danger of students putting their hands too close.

Read our standard health & safety guidance

Procedure

Light the two Bunsen burners and adjust them so that one flame is clear blue and the other yellow. Ask students to hold the back of their hand about 1 metre from the flames so that they can compare the energy radiating from each.

Teaching Notes

• The object of the experiment is to decide which flame is hotter.
• If the air-hole collars of the burners are easy to adjust, students may use a single burner and change the flame from clear to yellow.

This experiment was safety-tested in March 2006

Class practical

Comparing the infra-red radiation from a dull black and a shiny surface.

Apparatus and Materials

• Copper sheet, mounted, (or use Leslie's cube, with mains immersion heater)
• Thermopile
• Light-spot galvanometer
• Retort stand, boss, and clamp
• Bunsen burners, 4
• Methylated spirit
• Vegetable black

Health & Safety and Technical Notes

Supervise this experiment closely. Watch that students do not get too close to the hot plate.

Read our standard health & safety guidance

The mounted copper sheet is made from a sheet of 5-mm copper, blackened on one side, with 1-cm iron rod handle secured rigidly with two nuts and bolts.

Give one side of the copper sheet a coat of vegetable black mixed with methylated spirit. Allow it to dry so that it has a dull black surface. Polish the other side of the plate so that it is bright. (Tarnishing in the flame is inevitable.)

Procedure

1. Secure the copper sheet rigidly to the retort stand, using a boss. The sheet should be horizontal with the bright side downwards.
2. Heat it vigorously with four Bunsen burners underneath until it is as hot as possible.
3. Remove the Bunsen burners and turn the plate so that it is vertical. Avoid burning hand when doing this.
4. As quickly as possible ask students to hold the back of their hand first near but not touching the bright side of the plate, then the black side, and then back near the bright side. (Teacher supervision essential).
5. Alternatively, use a thermopile connected to a light-spot galvanometer as a detector of infra-red radiation.

Teaching Notes

• The plate will need to be re-heated after every 6 to 8 students have tried it.
• Radiation appears to come from hot surfaces. Copper is a good conductor and the two sides of the plate will be at the same temperature. You can bring your hand much closer to the shiny side than to the black side without burning it. More radiation is coming from the black side even though it is at the same temperature as the shiny side.
• An alternative experiment is the Leslie cube, made of copper, whose four vertical faces are finished differently. Boiling water is placed in the cube and the radiation detected with the cheek as the sensitive detector. The temperature is too low for this to be impressive using the back of the hand: a thermopile or other detector is better. (The water may be kept boiling with a mains immersion heater.)
• To make the distance comparison clear, both hands are brought up to either side of the plate. The hand on the shiny side can approach closer to the plate.
• How Science Works extension: No measurements are taken in this experiment, but it is still possible to draw a valid conclusion about the nature of a surface and infra-red radiation. You could:
• Use the experiment to illustrate the concept of valid evidence.
• Point out that, in a comparative analysis, numerical data is not always necessary.
• Ask students whether this experiment would still provide valid evidence if three or more colours were investigated. If they say it would not, ask them how the experiment could be adapted so that it did.
• Ask students to illustrate the findings of this data-free experiment in an appropriate way.

This experiment was safety-checked in August 2007

Class practical

Shows the properties of infra-red radiation.

Apparatus and Materials

For each group of students

• Mains lamps and holders, gas-filled and vacuum-filled (60 watt). Please note this piece of apparatus is very difficult to find.
• Copper calorimeter, large or a steam chest
• Immersion heater (mains powered)
• Vegetable black
• Thermometer (0°-100°C)
• Paper, white

Health & Safety and Technical Notes

If mains lamps are used, the holders should be the safety pattern where the contacts are isolated when the lamp is removed.

Read our standard health & safety guidance

Pre-focus (P13.5s) torch bulbs (2.37W) are available both vacuum and krypton filled. Distinguishing between these will require a sensitive detector.

Procedure

1. Keep some water boiling inside a copper box using an immersion heater. Alternatively the box can be kept at 100'C by passing steam through it. One face is shiny; one face is dull black having been coated with vegetable black, one face is covered with white paper. Use the back of the hand to compare the radiation.
2. Put a thermometer (0°-100°C) in a metal container filled with boiling water, and observe the rate of cooling. Do this first with a well-polished container. Then with a layer of vegetable black painted on the outside.
3. Switch on a 60 watt gas-filled mains lamp and a 60 watt vacuum-filled mains lamp near each other. Ask students to decide, as a detective problem, which of the two has gas inside.
4. Put a cheek near a mains lamp and switch it on and off to feel how promptly the radiation reaches the face.

Teaching Notes

• Step 1 is a version of Leslie's cube and demonstrates the differing amounts of radiation emitted from differently coloured surfaces.
• In step 2, the matt black can cools down most quickly because more radiation is emitted from it. Cooling curves could be plotted.
• In step 3 the surface of the gas-filled lamp will be hotter. This is because of the energy transferred through the gas by conduction, although the energy transferred by infra-red radiation will be similar.
• In step 4 the time lag is too short to distinguish because the radiation travels at a very high speed. (The speed of light.)

This experiment was safety-tested in April 2006

Class practical

To introduce ideas of energy transfer by heating and thermal capacity.

Apparatus and Materials

For each student group

• Datalogger with temperature sensor
• 1 litre beaker
• 250 ml beaker
• Insulating jacket
• Immersion heater
• 1 kg metal block (e.g. aluminium) with bores drilled for heater and temperature sensor
• Electric kettle or Bunsen burner to heat water rapidly
• Mug(s), ceramic OPTIONAL
• Cup(s), paper, polystyrene and plastic, with lids if possible OPTIONAL
• Different insulating materials (e.g. expanded polystyrene, newspaper, wool) OPTIONAL
• Instant coffee and tea bags OPTIONAL

Health & Safety and Technical Notes

An electric kettle is a much safer source of hot water than a Bunsen burner, tripod and gauze. However, immersion heaters also get hot and must be handled with care.

Read our standard health & safety guidance

Procedure

There are a number of things you can do with just temperature sensors.

1. Cooling curves. Fill a beaker with hot water from a kettle. Record its temperature once a second for a few minutes. If possible, produce a graph directly.
2. Compare cooling curves for beakers with different insulation, lids etc. Start each with water at the same temperature and record information from several sensors on the same graph.
3. Heating curves. Place sensors and heaters in beakers with 1 litre water and 250 ml water, and a 1 kg metal block. Start the heaters at the same time and with the same voltage and record the temperature-time graphs, all on the same display.

Teaching Notes

• These activities are excellent to emphasize the value of datalogging as the display is much easier to read than normal thermometers. Readings can be taken more often and with less chance of recording errors. Suitable software can produce an immediate graphical display to confirm that the data are being collected correctly.
Specific teaching points:
• This experiment can be used to calculate cooling rates in °C per second. The flattening curve shows that the rate of decrease of temperature is lower as the temperature falls.
• Without being quantitative, cooling curves which are produced live provide at-a-glance evidence for the effectiveness of different insulations.
• Comparing different masses of the same material (water is easiest) shows how the same amount of energy transferred causes different changes in temperature that depends on the mass. This is an introduction to thermal capacity and to the difference between energy transferred and temperature.
• Comparing the different materials (but same mass) is a further step on this road. The temperature of the aluminium will rise much more quickly than the 1 kg of water (1 litre). This is also partly because it will dissipate energy more slowly: it will take longer for energy to be transferred to the surface of the aluminium by conduction, and then be transferred to the surroundings by radiation, compared with time for convection currents to be set up in water. Hence the ratio of the rate of temperature rises is not the same as the ratio of the specific thermal capacities.
• If you want to use these methods to measure specific thermal capacities, then you need to ensure that you minimize energy dissipated to the surroundings with good thermal insulation.
• How Science Works extension: You could either set students a structured investigation and then follow with questions based on this or offer an open-ended investigation.
• Students could:
• identify and select the variables that they wish to measure and control
• produce their own experimental procedure, including the selection of appropriate time intervals.
• The amount of guidance given will very much depend on your students’ level of confidence and skills with designing their own experiments.
• Some groups could be set a very open-ended brief, ‘investigate cooling’. With others you might set the investigation in a real world context, suggesting some of the possible variables e.g. you could tell them that they are to investigate which is better to keep a cup of hot coffee warm for longest - a ceramic mug, a paper or a polystyrene cup? Most takeaway coffee cups have a lid, so this could be extended to investigating how effective the lid is at reducing energy dissipation. More advanced students could investigate whether tea and coffee behave in exactly the same way as water.
• Collecting data for cooling curves for cups of different materials is relatively straightforward, so students need only minimal guidance in the specifics of what they are to do.

Heating and cooling worksheet (Word, 58 KB)

Demonstration:

Students collect data and gain experience in using the inverse-square law for intensity of radiation. They use simple but ingenious apparatus to deduce a value that cannot be measured directly.

Apparatus and Materials

• Lamp, 240 V 150 W or 100 W
• Mains extension cable fed through earth-leakage circuit-breaker (ELCB)
• A4 paper, plain white, one sheet
• Optical pin or similar pointed object (e.g. drawing compass)
• Cooking oil, a few ml, in a small beaker or cup
• Tape measure or metre ruler

Health & Safety and Technical Notes

Make sure that participants do not look directly at the Sun. Ensure that the extension cable is safely positioned so as not to trip up passers-by, and that connections to the lamp and power supply are protected from moisture. Check that the ELCB is operating (by using its Test button) before use.

Read our standard health & safety guidance

The lamp should ideally be clear glass and held in a standard batten holder.

Procedure

This activity needs to be performed outdoors on a clear sunny day.

1. Use the pin to place a very small drop of oil on the paper – it should spread to form a translucent patch no more than 5 mm diameter and ideally smaller.
2. Hold the paper so that it is illuminated by the Sun on one side and by the lamp on the other, as shown in the diagram.

3. Viewing the paper from the ‘Sun side’, adjust its distance from the lamp so that the oil spot appears to merge with the surrounding paper.
4. Record the lamp distance d.
5. Assume that the light from the lamp and from the Sun varies in intensity according to an inverse-square law, and using the Earth-Sun distance of 1.50 x 10¹¹m, obtain a value for the Sun’s luminosity, L_sun.
6. Discuss factors that might affect the result.

Teaching Notes

• This activity can be carried out in a few minutes as a quick demonstration, followed by calculation and discussion.
• Alternatively, the demonstration could be followed by setting a challenge to students: how can they design their own experimental set-up so as to reduce uncertainties in measurement?
• When the spot appears to merge into the surrounding paper, the intensity of illumination due to the lamp (seen through the translucent spot) is the same as that due to the Sun on the surrounding paper.
• With D the Earth-Sun distance, d the lamp distance and L _lamp the luminosity (power) of the lamp (150 W or 100 W):
• L_sun /(4π D ²⁾ = L_lamp /(4π d² ) which can be rearranged to obtain a value for L_sun.
• The calculated value generally lies within an order of magnitude of the accepted value for the Sun’s luminosity: L_sun = 3.9 x 10²⁶W.
• In addition to uncertainties in judging and measuring the correct position of the paper, the result is affected by two sources of systematic error.
• Solar radiation that reaches the Earth’s surface is absorbed by the atmosphere. The amount of absorption depends on the elevation of the Sun above the horizon, and atmospheric conditions. Near midday in the UK in summer, on a clear day, about 30% of the radiation may be absorbed.
• The judgement of the correct position for the paper depends on sampling only the fraction of radiation to which human eyes are sensitive. As the Sun and the lamp have very different temperatures, they do not emit the same fraction of visible radiation.

This experiment comes from University of York Science Education Group:

Salters Horners Advanced Physics

Diagrams are reproduced by permission of the copyright holders, Heinemann.

Demonstration

This experiment shows that electromagnetic radiation in the infrared region is emitted from warm objects such as the human body.

Apparatus and Materials

• Mirror galvanometer, with sensitivity of about 20 mm per μA
• X-band microwave detector with its horn

Health & Safety and Technical Notes

Do not use any source of power on the diode detector. Do not use a Gunn diode source.

Read our standard health & safety guidance

Do not use a transmitter. Do not apply any source of power.

Note: even under good conditions the galvanometer, with a sensitivity in μV, will have a deflection of only about 5% fsd.

David Sumner says: "I used a diode detector, Unilab 045.674, which comes complete with a horn. This detector has enormous bandwidth. Any similar X-band receiver can be used."

Procedure

1. Set up the apparatus
2. Cover the horn window with metal foil. Zero the galvanometer and carefully switch it to the most sensitive range.
3. Remove the foil and point the horn at the body, at a distance of a few centimetres. There will be a noticeable deflection.

Teaching Notes

• Students may be surprised to discover that they emit infra-red radiation. Thermal imaging systems used by the military and by emergency workers (e.g. seeking people trapped in burning or collapsed buildings) detect this infra-red radiation.
• You can show that the detector is responding to infra-red radiation by placing a simple aluminium reflector, painted black, between the radiation source (human body) and detector. The detector will show no effect. Infra-red photons are absorbed by the black coating; any microwaves noise will be reflected without any loss.
• The experiment can also be used when discussing radio telescopes. While gathering radio waves emitted from astronomical objects, radio telescopes also detect ‘noise’ in the form of infra-red radiation from Earth’s horizon, the atmosphere and the antenna itself.
• The operation of a radio telescope involves identifying noise power and improving the signal-to-noise ratio. Radio astronomers think of the various contributions to noise in terms of system noise ‘temperature’. Nobel prize-winners Wilson and Penzias were studying just such effects when they identified cosmic microwave background radiation, corresponding to a black body radiator at a temperature of 3 K.
• Electromagnetic radiation will be detected from the head, body, limbs, etc. and also from a plastic bucket of hot water. This will mainly be infra-red radiation but may also include some from the microwave region (depending on the detector used). Radiation will not be detected from a metal container, since reflective surfaces are poor radiators of infra-red radiation.
• The long wavelength portion of the electromagnetic spectrum gathered by a radio telescope is referred to as the Rayleigh-Jeans region. In this region, as wavelength increases, the solid angle of the beam that an antenna collects also increases, meaning it sees a greater surface emitting noise consisting of infra-red radiation. But as wavelength increases, the surface brightness decreases. These two effects counteract each other, so the noise power per bandwidth interval is uniform across the...

  Rayleigh-Jeans region

• Electromagnetic radiation gathered will warm the telescope’s detector, producing ‘Johnson noise’, random motions of electrons in a metal conductor. Johnson noise power, P , in watts, given by P = 4 kT Δ f , where k is Boltzmann's constant in joules per kelvin, T is the conductor temperature in kelvins, and Δ f is the bandwidth in hertz.
• Some astronomical detectors are cooled by liquid helium to reduce Johnson noise.

This experiment was originally submitted by David Sumner, a Science Technician at Glebelands School in Surrey. It now incorporates improvements suggested by microwave engineer Jiri Polivka, of Santa Barbara, California.