In this activity students use a lamp and piece of paper to show how the light from the Sun spreads out more when it strikes the Earth at an angle.

Apparatus and Materials

Each student will need
• Large piece of card
• Board to which card can be fixed
• Lamp with cardboard cylinder
• Two different coloured marker pens

Procedure

1. Put a cardboard cylinder around the end of the lamp to provides a rougly circular area of light.
2. Draw a line around the area where the light falls when the card is perpendicular to the light source,
3. Repaet for when the board is at an angle.

Teaching Notes

The demonstration shows that when light hits a surface at an angle it is spread out over a greater area than if it strikes the surface perpendicularly. Any energy transfer is therefore spread over a greater area.

In place of the card, a photocopied map could be used to make the demonstration look more like a part of the Earth's surface.

Attach thermochromic plastic to a globe to show that temperature in the UK depends on whether our hemisphere is tilted towards or away from the Sun.

Apparatus and Materials

• World globe
• Filament lamp (or electric heater)
• Self adhesive thermochromic plastic

Preparation

Cut the thermochromic plastic into a strip and place it vertically on the globe next to the UK. Set the lamp-globe distance to ensure the thermochromic plastic strip shows a range of colours.

Procedure

1. Rotate the base of the globe so that the northern hemisphere is tilted directly towards the lamp (summer in the UK)
2. Switch on the lamp and highlight the changing colours of the thermochromic plastic (counter-intuitively lower temperature is indicated by red and higher by blue).
3. Switch off the lamp and rotate the base of the globe so the nothern hemisphere is tilted directly away from the lamp (winter in the UK). Emphasise that you have not changed the lamp-globe distance.
4. Switch the lamp back on.

Discussion prompts

• When the northern hemisphere is tilted away from the Sun is it summer or winter in the UK?
• What season is it south of the equator?

Teaching Notes

This demonstration tackles the common misconception that winter happens because the Sun is further away. Compare the UK (55°N) to a similar latitude south of the equator (eg Bouvet Island in the South Atlantic at 54°S) to emphasise that summer in the northern hemisphere corresponds to winter in southern, and vice versa,

Explain that you are turning the globe around for convenience. The direction in which the Earth’s rotation axis points doesn't really swap between summer and winter. Which hemisphere is leaning towards the Sun changes because of the Earth’s annual journey around the Sun.

Learning outcome

Students identify corresponding seasons for northern and southern hemispheres.

A small white ball and light source are used to show how the phases of the Moon are created from the point of view of the Earth.

Apparatus and Materials

• Small polystyrene or ping pong ball
• A stick to which the ball can be attached
• 3 lamps with card cylinders attached

Health & Safety and Technical Notes

Read our standard health & safety guidance

Teaching Notes

• Although you can try this demonstration with a single lamp, the phases appear more defined if 3 lamps are used with their light directed towards the “Moon” using cardboard cylinders.
• The lamps should be placed at least 2 metres from where the Earth-Moon system will be.
• In this demonstration a student (Earth) sat on a computer chair that allowed them to rotate their position so that they always faced the Moon (white ball) as the Moon orbited the Earth.
• The phases are quite convincing, and could be photographed or shown to the rest of the class using a webcam connected to a computer and projector.
• You can use a similar set up to demonstrate Solar Eclipses.

A ball fixed to a hula hoop is used to model how the Moon’s inclined orbit to the Earth means we do not get a solar eclipse every month.

Apparatus and Materials

• A plastic ball
• A hula hoop
• 3 lamps with card cylinders attached

Health & Safety and Technical Notes

Read our standard health & safety guidance

Teaching Notes

• You will need to attach your ball to the hula hoop. You could do this in many ways, such as velcro, glue, sticky tape etc... or you could cut the ball in half and fix it either side of the hoop more permanently.
• The Moonʼs orbit is inclined to the ecliptic plane (the plane of the solar system that includes the Earthʼs orbit and the Sun) by about 5 degrees. This means that only at certain times
does the Earth, Moon and Sun achieve a line of sight effect that can cause the Moon to pass between the Earth and Sun.
• The Sun is much further away from the Earth than the Moon, so although it is much larger than the Moon, it only appears as big as the Moon in the sky. This allows the Moon to
completely cover the Sun during a total eclipse.
• Not all eclipses are total, sometimes the Moon does not pass directly in front of the Sun because the alignment of the various orbits is not quite exact enough. This causes a partial
eclipse. On other occasions slight variations in the various orbits mean the Moon is too far away from the Earth to appear big enough to cover the Sun. This is called an annular eclipse.
• The eclipse effect in this demonstration is quite easy to see, and members of a class could take it in turns to see this effect, or you could photograph it (or use a webcam) and show it on a screen.

Demonstration

Teaching about comets? Bring them to life in class with this memorable demonstration!

Apparatus and Materials

• Large polythene sheet to protect floor
• Bin-liner bag, to line bowl and draw comet together
• Mallet, to crush some dry ice to powder
• Substantial plastic bag, in which to crush dry ice
• Gardening gloves (heavy duty type)
• Gardening gloves (heavy duty type)
• Balance, to find mass of comet (and hence calculate a hypothetical kinetic energy)

For two comets...

• Dry ice pellets, 10 kg
• Garden sand, 1 kg
• Water, 2 litres
• Soil, 1 handful (organic constituent)
• Worcestershire sauce (organic constituent)
• Smelling salts (organic constituent)

Health & Safety and Technical Notes

Dry ice sublimes at -78°C and will cause serious skin burns on contact, but momentary contact is unlikely to be a problem.

Do not confine in a sealed container as it will explode.

10 kg of dry ice will produce 5 m³ of gas, raising the level of CO₂ from 0.035% (natural) to safe-limit (USA) of 0.5% in a room 3 m high by 19 m on a side.

Make sure there is adequate ventilation, although if the dry ice is transported in a substantial expanded polystyrene box, little will sublime.

CO₂ is heavier than air therefore pools at ground level.

In theory trapped gas could fracture the comet or cause it to split, but this has never been recorded.

Read our standard health & safety guidance

The essential ingredients are dry ice, sand and water. The other items represent the organic molecules thought to be present in a comet. If it feels as if the comet will not bind into a snowball, it is because you have not used enough water. There is a natural tendency not to want to use too much water for fear of evaporating all the dry ice.

Do this in a well-ventilated area. Wear safety spectacles and gardening gloves.

Procedure

1. Line mixing bowl with bin liner.
2. Pour in half a litre of water and several handfuls of sand.
3. Stir and add crushed dry ice.
4. Stir and add Worcestershire sauce, soil and smelling salts.
5. Add more water. Make sure that there is a fairly violent release of CO₂, which indicates that you are cooling the mixture.
6. Draw the mixture together with the bin liner and squeeze between your gloved hands. You will feel the comet is binding into a solid mass. If it feels loose you require more water and may require more crushed dry ice. Uncrushed pellets on their own will not cool the water fast enough to form a solid mass.

Teaching Notes

• We gave a talk about comets before making our comet. We then found its mass and from this calculated the kinetic energy it would have if it struck the Earth at speeds of order 30 kilometres per second (the Earth orbits the Sun at 30 km s^-1) and compared this with the kinetic energy of an aeroplane.
• At the start of the class, we put a banana skin in with the dry ice, taking it out just before we made the comet. When the cold banana skin is dropped on the bench it shatters like china. This is a fun demonstration and also provides a vivid warning of the danger of dry ice.
• Comets, long thought to be fearful omens of trouble and doom, are popularly described as "dirty snowballs".
• Recent comet space missions reveal dry dusty or rocky surfaces, suggesting that ices are hidden beneath their crusts. It is now thought that short-period comets originate in the Kuiper Belt (beyond Neptune's orbit) whereas long-period comets originate much further from the Sun, in the Oort cloud.

Orbits are usually elliptical, this demonstrates an easy way to draw elliptical orbits that could represent orbits of planets, asteroids and comets in the solar system.

Apparatus and Materials

• Sheet of A3 (or bigger) paper
• Cork board or similar
• Map pins
• String loops
• Various coloured pens

Health & Safety and Technical Notes

Read our standard health & safety guidance

Teaching Notes

• Drawing elliptical orbits is quite easy, and you can use this method to show the shape of planet, asteroid and comet orbits.
• You draw the ellipse by placing the string around the two map pins, and putting a pen inside the string loop and moving it around the map pins keeping the string tight at all times.
• You will find that placing the map pins quite close together will produce an almost circular orbit. Placing them further apart may at first seem to make very little difference. You have
to move the map pins quite far apart before you achieve an ellipse that is more like the orbit of a comet.
• A simple activity for a class doing this, would be for students to produce orbits that could be representative of an inner planet, outer planet and a comet.
• It can take a little bit of effort before you get orbits that are neat and tidy, but most students will pick this up in a couple of minutes.
• You can use a diagram drawn using this method to show why the Earthʼs seasons are not caused by its distance from the Sun. The Earth is closer to the Sun in the northern
hemisphereʼs winter, and further away in the northern summer. This shows that the seasons are not caused by the Earth's distance from the Sun
• It is also obvious that when it is summer in the norther hemisphere, it is winter in the southern hemisphere. Therefore, distance from the Sun is not a factor that causes the seasons.

This demonstration is an easy way to show how gas molecules in the atmosphere scatter light to cause the sky to appear blue.

Apparatus and Materials

• Ray box and power supply
• Beaker of water
• Milk and pipette
• White card or screen

Health & Safety and Technical Notes

Read our standard health & safety guidance

Procedure

1. Set up the ray box so that it shines through a large beaker of water onto a screen. It helps in our demonstration to have a piece of card on top of the ray box so that light only escapes from it in the direction of the beaker.
2. You will need to adjust the distance between the ray box, beaker and screen to get the best focus of light on the screen. The beaker acts like a lens, and a good result is usually obtained if you produce a thin focused line of light on the screen.
3. It only needs a few drops of milk to begin to see the effect of the bluer light being scattered and a more orangey light colour apprearing on the screen. You can add more milk if it helps, and make sure the milk is stirred in to the water.

Teaching Notes

• Explain that white light is made up of a number of colours (more specifically red, green and blue) then it is easy to see that if the Earthʼs atmosphere scatters some blue light, then we are left with more red and green. If you mix red and green light you get yellow/orange light and this is what we see on the screen.
• You can also see the yellow/orange nature of the light if you look through the beaker towards the ray box.

Some digital cameras are sensitive to wavelengths of light that we cannot see with our eyes. This simple demonstration shows that an ordinary camera on a mobile phone can “see” the invisible infrared light emitted by a television (or other) remote control.

Apparatus and Materials

• Infared remote control
• Camera phone

Health & Safety and Technical Notes

Read our standard health & safety guidance

Teaching Notes

• Expensive cameras often contain an infrared blocking filter that stops infrared light from reaching the CCD (charge coupled device) that forms the light sensitive part of the camera. However, many cheap cameras and cameras on mobile phones do not contain these filters.
• Light from a remote control can be detected by many cameras, and it usually appears as white light rather than red.
• Colour CCDs that form the light sensitive part of a camera work by using an array of coloured filters over the pixels (picture elements) to allow some pixels to see red, some green and others blue. In the camera software the imaging system is then told which pixels saw which colour and a colour image can be produced. If the camera sees something that
is white, it sees equal amounts of red, green and blue. Infrared light can penetrate these coloured filters and so all the pixels (whether they are filtered red, green or blue) pick up the infrared. The software then interprets that information as being white.
• Apart from getting students to think about why you would want to block infrared light in a camera (blocking it improves picture quality), you could also ask why infrared light appears as white in the images.
• Many webcams can be easily modified to remove the infrared filter (if they have one) and you can carry out a range of experiments with these.

Using a variable resistor and a light bulb, it is possible to demonstrate how the colour of a star is related to its temperature.

Apparatus and Materials

• Power supply
• Lamp
• Variable resistor
• Connecting wires

Health & Safety and Technical Notes

Read our standard health & safety guidance

Teaching Notes

• Most students will have noticed that when a lamp has a small current passing through it the light emitted is very red or orange. However, most students will not have thought that this could be related to why cooler stars are redder, and hotter stars are whiter or even blue.
• As the current is increased by changing the resistance in series with the lamp, more current flows through the lamp, and more energy is transferred to the lamp each second. This causes the lamp filament to get hotter and glow brighter.
• As the current increase the light emitted changes from red, through yellow and on towards white. As the current increases the temperature is also increasing. This shows a direct link between temperature and colour of light emitted.
• It is not generally possible for the light emitted from this particular set up to reach a temperature where the lamp glows blue.
• Some discharge lamps that uses gases can reach temperatures that cause the lamps to emit a very distinctive blue/white light. You may have some of these types of lamp in theatrical lanterns in your school.
• A school hall or theatre can be a good place to demonstrate this change in colour as lights are dimmed.
• This link between colour and temperature is well known amongst photographers who take the colour-temperature of light into account when taking photographs

How to illustrate the life cycle of stars using a HertzsprungRussell diagram, a large sheet, and some students.

Apparatus and Materials

• Large white sheet
• Felt pens to colour in the sheet
• Facts cards about stars
• Weights to hold down the sheet

Health & Safety and Technical Notes

Read our standard health & safety guidance

Teaching Notes

• The life cycle of a star can be a fairly dry subject, as there is no easy way to simulate it on the Earth. This interactive exercise allows students to think carefully about where a star might be on a Hertzsprung-Russell (H-R) diagram at different times of its life.
• By using a large sheet to make a H-R diagram, it is possible to create an interactive exercise that students can get physically involved with.
• Although the H-R diagram is quite an advanced idea for some students, it can be relatively easily explained as a diagram that shows the relationship between the temperature and luminosity (brightness) of a star. This allows for a number of activities to be carried out.
• As students walk on the H-R diagram, they are demonstrating an understanding of how a starʼs properties change throughout its life cycle. The use of fact cards allows one student to call out a pair of values relating to a star, while another student (or a group of students) decides where they should stand on the digram to represent that part of a stars life.
• The fact cards can either be in the right order so that students can clearly see how a starʼs life is mapped out, or they can be mixed up so that they first have to be sorted in to the right order.
• Allowing students to produce the diagram that they will use for this activity, or for another group who will do the activity, will also help them understand what happens to a star during its life cycle.

Using a group of students as light waves, it is easy to show how light can be red and blue shifted when astronomical objects are moving relative to one another.

Apparatus and Materials

• Students
• A stop watch
• A camera to record the demonstration

Health & Safety and Technical Notes

Read our standard health & safety guidance

Teaching Notes

• Red and blue shifting of light is a difficult idea to grasp without a visual aid. This demonstration allows you to use students in a practical way to demonstrate the idea of light changing wavelengths when the emitter is moving relative to the observer.
• This demonstration really needs to be done outdoors or in a large school hall. You might have some limited success in a classroom.
• The greatest difficulty in making this demonstration work is organising the students so they know what they are doing. Although the idea of walking backwards and forwards whilst waiting to be told you have been emitted as a wave of light sounds simple, it can be quite tricky to get right at first. However, it is worth the effort to see the result.
• You may need to check that your time keeper is calling the times accurately, it is easy to get distracted by having to start a stop watch and call out “go” when a light wave needs to be emitted at the same time.
• It is worth noting that we have made no distinction here between redshift caused by the Doppler effect and cosmological red shift caused by space expanding between different parts of the Universe.
• Using a video or stills camera to record the results is very worthwhile so that a record can be kept of what was observed. It is also possible to mark the students on the images as peaks or troughs in the wave, and then draw completed waves on to the images so the difference in wavelength can be seen more clearly

Class practical

Observations of the stars, planets and the Moon for students to make.

Apparatus and Materials

• Camera with B (open shutter) setting

Health & Safety and Technical Notes

Caution students about where and when (and with whom) they make their observations of the night sky, so that they do not put themselves at risk. If appropriate, inform parents/guardians.

Read our standard health & safety guidance

Procedure

1. Ask students to observe the sky at least twice in one evening, with an interval of about two hours between observations. (It will help if pictures of a few easy-to-identify constellations are available before the observing time, so that students will recognize them and can direct their observations towards them.)
2. Ask students to watch the Moon and to note its position relative to the stars. Then, one or two hours later, look again and note the new position of the Moon relative to the stars. The Moon appears to travel across the star pattern.
3. Extend the previous experiment to a month. Note the position of the Moon at the same hour on each possible night for a month. The observations should relate to the stars, and also to the position in the sky relative to the horizon. Ask students to draw the phases of the Moon throughout the monthly cycle. (There will be times when the Moon is invisible during the night and will only be seen during the day. The rising and setting of the Moon can often be found from diaries or the newspapers.)
4. Show students the brightest planets - Venus, Jupiter and, possibly, Saturn.

Teaching Notes

• Students will need to be prepared for this observation in anticipation that a clear starry night appears. Normally the best times are during winter when the skies are predicted to be clear and a frost is forecast. Viewing the sky away from the city lights is recommended. These observations will probably have to be done at home for many students.
• A compass is helpful so that students know in which direction they are looking.
• A record of observations should be made.
• For step 1 it will help if pictures of a few easy-to-identify constellations are available before the observing time, so that students will recognize them and can direct their observations towards them.
• In step 2 the Moon appears to travel across the star pattern.
• For step 3. there will be times when the Moon is invisible during the night and will only be seen during the day. The rising and setting of the Moon can often be found from diaries or the newspapers.

This experiment was safety-tested in April 2007