What do you notice about the shapes of lenses?
They have curved surfaces.
Systems for looking into Space
- The stars we see at night - are those all the stars there are?
- How do scientists find out what stars are like?
- How come you can't see other planets when looking up at the sky, but we have some beautiful close-up photographs?
Telescopes
The telescope was first discovered by the Dutch. In Holland, a lot of the people were sailors and sea-explorers so they used their telescopes at sea to see if ships far away on the horizon were friends or maybe pirates.
A telescope makes faraway things look bigger and closer.
Here is an introduction you can use with your learners. From the earliest times that people could look and wonder, people have looked up into the night sky and wondered what the bright things up there are. They could see how these things moved but they did not know how far away they were or what they were. They knew that the planets are different to the stars. The stars always stay in their same patterns, but the planets move closer to a star each night, and then further away.
Galileo Galilei was a professor of mathematics at the University of Padua, Italy. In 1609 he heard that somebody in Holland had made a telescope, and he worked out how to make one himself.
Galileo used his telescope to look at the planets in the night sky, and he made careful notes of what he saw there. He was the first person to see that Jupiter had moons. He saw that Saturn has rings and he saw that Venus has phases like the Moon has. He also used his telescope to show people that the Milky Way was really made of billions of stars. He wrote books that taught people about telescopes and what they could show us in the night sky.
Nowadays there are big telescopes in many parts of the world, and these telescopes have cameras to photograph the sky.
If you are based in Cape Town, you can take your learners to visit the South African Astronomical Observatory (http://www.saao.ac.za/) and if you are based in Johannesburg you can go on a tour of the Johannesburg Observatory http://www.saasta.ac.za/index.php?option=com_content&view=article&id=73&Itemid=63.
Telescopes have shown us that there are thousands of millions of stars that we could not see with our naked eyes. Some of those stars are so far away that their light has been travelling for millions of years to reach us.
The Southern African Large Telescope
One of the biggest telescopes in the world is here in South Africa, near the town of Sutherland. The telescope is called the Southern African Large Telescope or SALT. The telescope uses lenses and a very big mirror to see the stars and take photographs of them.
Point out to learners in this photograph of the SALT mirrors that each mirror is a hexagon shape (like a beehive) and each mirror focuses the light onto a single spot. The learners will see how to do this in the next activity. When doing this in the activity with all the mirrors outside, bring back their attention to this image of the mirrors and the similarity.
Work in groups for the next activity as it may be hard to source enough materials (lenses and mirrors) or each learner to be able to do the activity by themselves.
How lenses and mirrors make telescopes work
MATERIALS:
- Each group needs a lens. You can use hand-lenses, or round empty bottles with water in them. Or you can use the glass part of a light bulb, full of water. Your teacher will show you how to remove the inside parts of an old light bulb.
- Each group needs a mirror. It can be a small mirror, or you can make a mirror. You can glue the shiny foil from a potato-chip packet onto cardboard.
To make a lens from a light bulb, heat the metal cap of a dead light bulb in a flame. Then carefully lower the hot part into cold water so that the cold water just reaches the glass. The glass will crack around the metal cap and the insides of the bulb will fall out into the water. This will leave you with the ball-shaped glass part. If you fill it with water, it makes a good lens with a short focal length.
Put masking tape or Elastoplast on the broken edge of the glass to prevent children cutting themselves. Of course, we are talking about bulbs with a filament, not the new fluorescent energy-saver bulbs.
INSTRUCTIONS (Part One):
- Hold up the lens and look at something on the wall. You can look at a poster, for example.
- If you are far away from the wall, the poster will seem to be upside down.
- When you come closer to the wall, the poster will be right way up and bigger. The lens is magnifying the poster (to magnify means to make bigger).
QUESTIONS:
Why do you think this shape is necessary for the lens?
The light must pass through a curved shape so that the light bends; when the light from an object has come through the curved glass we may see the object enlarged (looking bigger). The curve makes the object look bigger as the light is bent.
INSTRUCTIONS (Part Two):
- Now the class must go outside into the sunshine. Take the lens and your mirrors with you. This will work best on a hot, clear day.
- Your teacher can point out a place on the wall that is in the shade.
Make a chalk circle on the wall, about 15 cm in diameter. The learners must aim at that circle.
- Use your mirrors to reflect sunlight onto that spot.
- The class can spread out; it does not matter where you stand. Just make sure you move your mirror so that it helps to light up the spot your teacher showed you.
- If everyone reflects sunlight onto that one spot, it will become very bright there.
QUESTIONS:
How could you make the spot brighter?
Ask them to make a prediction - they must say what will work to make the spot brighter. Answer: The class can use even more mirrors.
Will the spot feel hot? Make a prediction. (To "predict" means you say what is going to happen.)
How will you find out whether your prediction was correct?
For example, get a report from one person who goes to put her hand there.
The Sun does not give us only light. It also gives us heat. You know that if you stand facing the Sun with your eyes closed, you can still feel heat from the Sun on your face.
How could the class make the spot hotter?
Now that we have the class outside with mirrors, let's use the opportunity to teach about radiation and solar cookers. The Sun radiates electromagnetic waves with many frequencies. Our eyes respond to some of those frequencies and we see light. If the frequency is a little lower, our eyes cannot respond (we don't see it) but our skin responds and we can feel the radiation. These lower frequencies are called infrared radiation.
Answer: The class can use even more mirrors.
Can you make it even hotter by passing all the sunlight through a lens, onto the wall?
You should try this with the class. You will need a large hand lens about the size of a saucer. Move the lens until the spot of light on the wall is as small as you can get it. Let one of the learners put his hand in the spot and feel it. (It's unlikely to burn him.)
INSTRUCTIONS (Part Three):
- Put a candle inside a glass jar and put on the lid.
Put the candle inside the jar to ensure that the warm air is not blown away from the candle and that it does not escape by convection. Here we are extending the activity beyond telescopes to let the learners think about solar cookers. In Gr. 5 they will have seen solar cookers - you can show them a picture of one and let them note how large the reflector is. The larger the reflector, the more energy the cooker can put into the food per second.
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Can you melt the candle by using your mirrors?
The class should be able to do this if they have enough mirrors. They must hold the mirrors steady and not wobble them around.
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Can you melt chocolate by using your mirrors?
Try this if they cannot melt the candle. Just decide who will get he melted chocolate afterwards!
All the mirrors work together like one big mirror, even though they are far apart. They all collect a little bit of energy from the Sun and send it to the bright spot.
The mirror of a telescope works like that. The light from some stars is very faint because the stars are very far away. But the big mirror collects all the faint light and focuses it to one lens. Then the telescope can gather (put together) enough light from the star to get a photograph of the star.
The Square Kilometre Array (SKA)
The SKA is a different kind of telescope for looking at the stars. Stars send out energy in light but also in radio waves. The SKA will receive radio waves that our eyes cannot see.
The SKA website has many resources to access to facilitate your class discussion of this project (http://www.ska.ac.za/learn/index.php). Do you remember the notes above that the Sun radiates at many frequencies? Some frequencies we can see, others we can only feel. There are many other frequencies that we can't detect, unless we have a radio receiver.
An array means a large number of the same items. For example, when the desks in your classroom are all lined up neatly, we can call that an array of desks.
The SKA will have an array of several thousand dishes like those in the picture. When you add together the area of all the dishes, the total area will be the same as one square kilometre. A square kilometer is an area in the shape of a square and the sides are each 1 km long. The area of the square will be 1 km2. That is why the telescope is called the Square Kilometre Array.
There was actually a contest between South Africa and Australia to see who would host the SKA. Both countries really wanted it and the bid and voting went on for 9 years. Then at the beginning of 2012, it was announced that it would rather be hosted in both countries, but the larger portion of the dishes is to be in South Africa and Africa.
Good question, Sophie! Let's find out.
The dishes in the picture look like the satellite TV dishes that you see on some people's houses. Those dishes collect the weak TV signal that comes from a satellite high up in Space.
In the SKA, each dish collects a little of the radio signals that come from the stars, and sends it to a computer. The computer puts together all the signals to make a new picture of that star.
Scientists from many countries are working together to build the SKA in the Northern Cape. Most of the telescopes will be near the town of Carnarvon.
Some of the dishes will be very far away from Carnarvon. They will also collect signals from stars and send them to the computer in Carnarvon. This helps to improve the quality of the image that gets generated by the computer. Some dishes will be in Ghana, Zambia, Mozambique and Madagascar.
The SKA
INSTRUCTIONS:
- Look at the pictures of SKA in the text before.
- Answer these questions.
QUESTIONS:
How is the picture of the SKA dishes like the picture of the class using mirrors to make a bright spot on the wall?
It has many dishes, like the many mirrors that the class is using.
Why does the SKA need so many collecting dishes?
The signal from some stars is very weak.
Why does it help the SKA to have dishes far away in Ghana, Kenya, and Mozambique?
How many dishes will be in Madagascar?
Madagascar will have two dishes, but assess whether the learners can find Madagascar on the map.
The SKA will be able to pick up signals that stars sent out thousands of millions of years ago. The signals have been travelling through Space for all that time. When the SKA picks up those signals, we will learn something about that time when the universe began, thousands of millions of years ago.
- The objects we see in the sky are very far away.
- We can use telescopes to see them more clearly, and to measure how far away they are.
- Lenses can focus light to make a clear image.
- Mirrors help to collect more light when stars are very faint (not bright).
What does a telescope do?
It makes faraway objects look closer.
Before Galileo, nobody knew that the planet Jupiter had moons. What was the reason?
The moons were too small to see with the naked eye, but Galileo's telescope made them big enough to see.
What does SKA stand for?
Square Kilometre Array
Why are the words "square kilometre" in the name?
The areas of all the dishes will add up to an area of one square kilometre.
With a telescope we can see very, very many stars. Why did we not know about all those stars before there were telescopes?
Those stars are too faint to see with the naked eye.
Astronomers build their telescopes far away from cities. Think of a reason why they do this and write it down.
The bright lights of the city make the sky bright and so it is harder to see the stars.
