Hot Stuff – Lesson 3 – Light and heat from the Sun
Students move into direct sunlight and put their hands out to face the Sun (weather permitting). They then melt various solids on black and white tiles. They learn that the Sun produces energy, giving light and heat to Earth. Finally they learn that the Sun’s light and heat comes as photons. Some of these photons change into phonons and warm things on Earth.
Introduction: Review Lesson 2 and introduce the Lesson 3 learning intentions using the Hot Stuff PowerPoint.
Activity – Feeling the Sun’s warmth: Students put their hands out in the sunlight and discuss what they feel.
Activity – Heating of different coloured objects: Students place Copha, butter, and ice on black and white tiles out in the sun and see how the colour affects what happens to the substances.
Activity – Where did the water go?: Examine the two jars that had equal amounts of water in each that were set up the previous week. Observe what happened and explain why this happened in terms of heat and water molecules.
Discussion: With the aid of a video, explain that huge temperatures and pressures deep inside the Sun cause hydrogen atoms to join together to make helium atoms, releasing massive amounts of energy. Students are reminded about phonons and introduced to photons in sunlight.
Activity – Revisit the Nerf gun bullet photon analogy: Students review the previous lesson’s activity to reinforce their understanding of photons and how sunlight has ‘bulletiness’ by using a Nerf gun; the Nerf gun bullets represent photons in this analogy.
Review and introduce the next lesson: Review all new words and continue the Word Wall. Inform students that in the next lesson they will learn about how heat travels in solids.
The learning intentions for this lesson are that students will:
- know that the Sun produces energy as light and heat
- explain that light from the Sun comes to Earth as photons
- explain that some photons change into phonons to warm things on Earth
- Have access to a very bright light that can be used inside if the day is very cloudy. The school may have a floodlight that is used to light outside areas or your school covered area may have a spotlight for assembly drama events that could be borrowed. Otherwise, relatively cheap smaller flood lights can be bought at hardware stores.
- Display the two jars of water that were set up at the end of the last lesson in a prominent place to discuss with the class.
- Try out the small solar cars, see the short instructional video (optional)
- Black and white tiles (or card)
- Crushed ice, Copha, and butter or similar
- Toy solar cars (optional)
- Nerf guns with Nerf gun bullets
- 2 identical wide-mouthed jars, one with a tight-fitting lid (from previous lesson)
Review lesson 2, including how heat energy is transferred by vibrating atoms and molecules and that these ‘bundles’ of vibrations are called phonons.
Introduce the Lesson 3 learning intentions using the Hot Stuff PowerPoint.
Students move outside into a sunny spot, turn towards the Sun, being very careful not to look directly at it, and place the palm of their hands towards the Sun. (If it is cold outside you might find a window that receives direct sunlight.)
Danger: Never look directly at the Sun and wear sun protection, particularly if the UV rating for the day is above 3.
In their groups, students should discuss the following:
- What do you feel? The palms of your hands feel warm.
- Yes, you feel your palms get warm. Where does the warmth come from? The photons (visible light and other types of photons from the Sun) strike the palms of your hands, they turn into phonons which are bundles of vibrational heat energy. These cause the molecules that make up the skin to vibrate more.
The teacher explains that our senses allow us to see, hear, smell, taste and feel things in our environment using special nerve cells in our eyes, ears, nose, tongue and skin. In the case where we feel something warm (or cold) nerve cells absorb the infrared photons and send signals to our brain telling us how warm or cold it is.
Sunlight has three main parts to it: the light that helps us see in the daytime, the warming infrared that we can feel when we are in the sunshine, and the dangerous, burning UV which causes sunburn.
On the right you will find a demonstration video for this activity. We recommend viewing this video before proceeding with the text description below.
You can now follow the ‘melting solids in the sun’ activity that may have been completed as an alternative in Lesson 1, but this activity will answer the question: do all coloured surfaces absorb heat energy at the same rate. (If it is not sunny enough, an inexpensive floodlight or heat lamp should be suitable as an indoor replacement.)
To carry out this investigation students will set out small, roughly equal sized pieces of ice, Copha (coconut oil based shortening), and butter on a white tile and a black tile, or a piece of black and a piece of white cardboard/ paper.
Students make an initial observation and watch the three solids on the black and white surface for about 10 minutes (depending on the weather).
Students will return later during the lesson to record changes they observe in their science journal. They may use class devices to take photos of their set-ups (paper/tiles) at the start and at the end of the lesson.
If you have access to infrared thermometers, you may encourage students to take measurements of the two different surfaces.
- Why are white objects white and why are black objects black? White objects reflect most of the photons that land on them, so they look bright, but black objects absorb most of the photons so they look dark because the photons aren’t able to reach our eyes.
- Why does the black surface get hotter than the white surface? When an object absorbs a photon, the energy from the photon turns into jiggling energy in the atoms and molecules called phonons, which make up heat. Because black surfaces absorb more photons, they get hotter faster.
Danger:
- Never look directly at the sun and wear sun protection, particularly if the UV rating for the day is above 3.
- Be careful the tiles are not too hot to handle, they may burn students’ hands.
At the conclusion of the last lesson the class set up two identical clear glass jars of water.
- Who can remind me what we did? We placed a mark about ¾ the way up the outside of the container with a fine permanent marker or piece of tape, being sure they are at identical heights so the volume in each container will be the same.
- Why did we put a lid firmly on one jar and leave the other open to the air? To find out if there is a difference in the water level in the two jars after they are left for some time.
- Why did we place both jars near a window, preferably where they receive direct sunlight? To find out whether the sunlight has any effect on the water levels.
Now let’s look and see what has happened.
- Who can tell me what has happened? The water level in the jar with no lid dropped while the one with the lid on stayed the same.
In their groups, students discuss why this happened and come up with a brief sentence to explain why it happened. Their sentences must include ‘photons’ and ‘phonons’.
After a few minutes, check the sentences of some of the groups.
Watch the following video about the Sun with your students – The Sun: Happy Learning (3½ minutes):
In the Atom Frenzy lessons students learnt that two hydrogen atoms combine to make a hydrogen molecule.
- Who can remember how we wrote a ‘hydrogen molecule’? H2.
- In the Sun, hydrogen atoms combine together to make a different atom, helium. How do we write a helium atom? We write a helium atom as He.
- Who has heard about helium? Used to fill party balloons.
- Does anyone know why we use it to fill party balloons? It is safe and lighter than air – so it floats in air, like how air floats in water.
Reinforce the difference between hydrogen atoms joining and being held together by electrical forces to form H2 molecules (they are still hydrogen) and hydrogen atoms combining to form helium which is a different atom. When hydrogen atoms become helium a lot of energy is given out! This is how the energy is given off by the sun.
Remind students that sunshine has three different types of photons: the visible light photons that help us see in the daytime, the infrared photons that we can feel as heat when we are in the sunshine, and the ultra-violet photons which have high energy and cause sunburn and damage the outer layers of our skin.
All the parts of sunshine are made up of tiny ‘bundles’ of energy called photons.
Remind students what they learnt about phonons in the previous lesson. Some of the photons from the sun turn into phonons – tiny bundles of vibration that pass on heat energy – and this is how things in the sunshine warm up.
If you do the optional extension (below), discuss with students regarding the solar cars going faster in direct sunlight; this is because these special cars can use the energy from sunlight. The solar cells change the energy carried by photons from the Sun into electricity (which is electrons flowing) and electrical energy to power the cars.
- How do they do this? Photons from the sunlight create electricity in the solar cells which powers the cars.
- This is how solar cells or solar panels on roofs work too. Who has solar cells on their roof at home? Does the school have solar cells on its roof?
Briefly revisit the Nerf gun and bullets model to help students imagine what photons might be like and to help them appreciate that the light from the Sun is made of photons which take about 8 minutes and 20 seconds to reach Earth from the Sun. These photons provide the energy needed for living things to survive and thrive.
The Sun also makes high-energy ultraviolet or UV photons that can damage our skin and eyes. This is why we use sun protection including sunscreen. It protects us from the bulletiness of the ultraviolet photons.
Review all new words and write them on the class Word Wall. Inform students that in the next lesson they will learn about how heat travels in solids.
Optional Extension Task
Activity – Solar cars
See the activity demonstration video: https://youtu.be/w2K3WSObfos
Note: If it is cloudy, then this activity can be done indoors in varying amounts of artificial light; a floodlight is effective for producing sufficient electricity from the solar panel to drive the motor.
Working in groups with a toy solar car:
students should use their group’s solar car inside and then outside in shade and direct sunlight.
ask students where they found their solar car went fastest. (More light, fastest in direct sunlight).
Discuss in groups what they think is happening in the direct sunlight to make the car go faster. As a class, try to work out a theory for what is happening (the sunlight is used by the cars to make them go faster).
Setup – Growing healthy plants
Working in groups students set up two plastic containers per group with two bean seeds (mung, pinto, or broad beans) in each cup or cress seeds in cotton wool.
Water them until damp and leave both on the windowsill in light, warm conditions for the seeds to geminate and start growing.
Then students:
- place one of their containers of beans under a lightproof cardboard box in a corner of the classroom (alternative: place half the bean containers in a dark cupboard).
- place their other container in a position with good natural light, for example on a windowsill or on a table near a window in the classroom.
Be sure to water all plants regularly, but only enough to keep the cotton wool damp.
Observe what happens, every two days or so, to the beans in the light and beans in darkness as they germinate and grow.
Once the bean plants have sprouted: note the differences between the plants grown in light and those grown in darkness.
What can be concluded? (Photons in the light give their energy to plants to allow them to grow well).
The Sun: the star at the centre of our solar system. It is a gigantic ball of very hot plasma and nuclear fusion reactions in its core produce energy which the Sun gives off as photons that consist mainly as light, ultraviolet, and infrared radiation.
Hydrogen: element number 1, the simplest element, and is made of one proton and one electron. Teacher’s Note: there are two other forms of hydrogen, deuterium which has 1 proton and 1 neutron in its nucleus, and 1 electron in its electron cloud and tritium which has 1 proton and 2 neutrons in its nucleus, and 1 electron in its electron cloud.
Helium: element number 2 which has a nucleus of 2 protons and 2 neutrons in its nucleus, with 2 electrons in its electron cloud. Teacher’s Note: another form of Helium, He-3, can exist with 2 protons, 1 neutron, and 1 electron.
Solar cell or photovoltaic cell: an electronic device that changes the energy of photons directly into electricity.
Nuclear fusion: is a reaction in which the nucleus of two or more atoms combine to form different atomic nuclei and energy. In nuclear fusion in the Sun, hydrogen nuclei join together to make helium, and the energy that is released is given off by the Sun as photons.