Energy and Simple Technology
Primary 3 · Science and Elementary Technology
Energy and Simple Technology
Lesson 1 of 3
12 min
Let's begin
It is early morning. Uwase is standing outside for assembly, and the sun is low over the hills. She looks down and sees a tall, thin, dark Uwase stretching across the ground beside her.
She waves. The dark shape waves back. It is her shadow.
Today you will find out where light comes from, how a shadow is made, and why your shadow keeps changing all day long.
Go into a room at night and close the door. Everything disappears. You cannot see your own hand.
Now strike a match. Suddenly you can see the table, the chair, your hand. Nothing in the room changed shape. What changed is that light arrived.
A thing that makes its own light is called a source of light.
Some sources are made by nature. Nobody switched them on.
Other sources are made by people. Somebody lit them or switched them on.
The moon looks bright at night. So most people say the moon is a source of light. But it is not.
The moon makes no light of its own at all. The sun shines on the moon, and the moon bounces that light back down to us. It is like a mirror in the sky.
So the sun is the real source. The moon only borrows the sun's light.
Here is something you can see for yourself. Mugisha stands in a dark room and shines his torch. The beam goes straight out, like a long stick of light. It does not curve. It does not bend around the chair to find him.
Light travels in straight lines. Remember that sentence — it explains everything that comes next.
Mugisha shines his torch at the wall. The wall is bright. Now he holds his hand in the middle of the beam.
A dark hand appears on the wall.
Why? Because light travels straight, and his hand is in the way. The light that hits his hand stops there. It cannot bend around his fingers to reach the wall. So that piece of wall gets no light — and a place with no light looks dark.
That dark shape is the shadow.
A shadow is not a thing you can pick up. A shadow is simply a place where the light could not reach.
Take away any one of the three, and there is no shadow. Switch off the torch — the shadow is gone.
Hold each of these in front of a torch and watch what lands on the wall:
So the darkest shadows come from things that block light completely — your body, a stone, a wall, a door.
Which side? The shadow is always on the far side, away from the light. If the sun is in front of you, your shadow is behind you. Turn around, and your shadow moves to the other side. It can never be on the same side as the sun.
How big? Try this with a torch and a pencil. Hold the pencil right next to the torch — a huge shadow spreads over the wall. Now slide the pencil away from the torch, close to the wall — the shadow shrinks until it is almost the same size as the pencil.
Close to the light means a big shadow. Far from the light means a small shadow.
All day long. The sun rises in the east, climbs high, then goes down in the west. So:
That is why Uwase's shadow at assembly was so tall. The sun had only just come up.
Tip
Say it like this: light goes straight, so it cannot bend around you — and where it cannot reach, you get a shadow. Everything else follows. Sun low in the sky, light comes from the side, long shadow. Sun high overhead, short shadow.
Think about each one first. Say your answer out loud, then tap Show answer to check.
Now go outside in the morning and step on your own shadow's head. Then look for it again at midday. It will have run away and hidden under your feet!
What you learned today
Quick check
The moon makes its own light.
