Light Travels in Straight Lines
Light comes from a light source, such as the sun, a torch, a lamp or a candle flame. It travels outwards in straight lines, very fast, until it hits something. We see an object when light from a source bounces off it and enters our eyes. Because light goes straight and cannot bend around corners, you cannot see someone hiding behind a wall, and a torch beam makes a straight-edged cone in a dusty room. This one idea, that light travels in straight lines, explains almost everything about shadows.
How a Shadow Is Formed
When an object that does not let light through stands in the path of the light, the rays that hit the object are stopped. The rays that pass just above, below and beside it carry on in straight lines. Behind the object there is now an area that the light cannot reach: that darker area is the shadow. In the lab, switch on the light rays and look at the ray that just grazes the top of the object. It marks exactly where the shadow ends.
A shadow has three simple properties that children can check for themselves:
- It has the same outline as the object, because the edges are made by the rays that just miss the object.
- It is dark, never coloured: a red toy still makes a dark grey shadow, because a shadow is simply less light.
- It is always on the opposite side of the object from the light.
Opaque, Translucent and Transparent
| Material | Light | Shadow | Examples |
|---|---|---|---|
| Opaque | Does not let light through | Dark shadow | Wood, metal, cardboard, stone, a book, your hand |
| Translucent | Lets some light through | Faint shadow | Frosted glass, tracing paper, baking paper, tissue paper |
| Transparent | Lets almost all light through | Hardly any shadow | Clear glass, clear plastic, cling film, clean water |
You can see clearly through transparent materials, you can see only blurry light and shapes through translucent ones, and you cannot see through opaque ones at all. That is why bathroom windows are often made of frosted glass: light comes in, but people outside cannot see in.
What Changes the Size of a Shadow?
With a torch shining on a toy in front of a screen, the shadow on the screen gets bigger when:
- the torch is moved closer to the toy, or
- the toy is moved closer to the torch (further from the screen).
The shadow gets smaller when the torch moves further away, or when the toy moves closer to the screen. When the toy is right against the screen, the shadow is almost the same size as the toy. The reason is the spreading light: close to the torch, the toy blocks a wide fan of rays, and that fan keeps spreading all the way to the screen. Older children can find a pattern: if the screen is three times as far from the torch as the toy is, the shadow is three times as tall as the toy.
A shadow on the floor behaves a little differently. When the torch is high, the light comes down steeply and the floor shadow is short; when the torch is low, the light skims along the floor and the shadow is long. Changing the colour of the toy or the brightness of the torch does not change the size of the shadow at all.
Shadows from the Sun
The sun is our biggest light source. It seems to move across the sky during the day, rising in the east and setting in the west, so shadows outside change through the day. In the morning the sun is low in the east and shadows are long and point west. Around the middle of the day the sun is high and shadows are short. In the evening the sun is low in the west and shadows are long again, pointing east. In Singapore, close to the equator, the sun is nearly overhead at around 1 pm, so at that time your shadow is a small patch close to your feet. Long ago, people used shadow sticks and sundials to tell the time this way.
Try It at Home: Shadow Puppets and a Torch Experiment
- In a dim room, shine a torch at a plain wall. Make animal shapes with your hands: a bird, a dog, a rabbit.
- Stand a small toy on a table between the torch and the wall. Measure the toy’s height.
- Put the torch 10 cm from the toy and measure the height of the shadow on the wall. Then try 15 cm, 20 cm and 30 cm, keeping the toy and the wall in the same place.
- Write the results in a table and draw a line graph. What happens to the shadow as the torch moves further away?
- Repeat with a cut-out shape made of cardboard, tracing paper and clear plastic, and compare how dark the shadows are.
The printable worksheet on this page has the results table, a graph grid and sentence starters for the conclusion, ready for this experiment.
Fair Tests
A good experiment is a fair test: you change one thing (the variable you are testing), you measure what happens, and you keep everything else the same. To find out whether the distance of the torch changes the size of the shadow, change only that distance, measure the height of the shadow each time, and keep the same toy, the same torch and the same screen in the same place. If you changed the toy too, you would not know which change made the difference.
How to Use This Page
- Explore the lab. Drag the torch and the object, or use the sliders. Read the shadow lengths from the boxes and the ruler.
- Switch the material between opaque, translucent and transparent and compare the shadows.
- Play the day in the sun section and watch where the flagpole’s shadow points.
- Choose a level and a challenge, then press Start. Every wrong answer comes with an explanation, and missed questions come back in a fix round.
- Print a worksheet: a shadow investigation with a results table and graph, a question sheet and an answer page.
Tips for Parents and Teachers
- Predict first. Before moving the torch, ask: will the shadow get bigger or smaller? Then test it.
- Use the right words. Light source, opaque, translucent, transparent, fair test: using them in conversation helps children use them in exams.
- Go outside. Mark the end of a stick’s shadow with chalk at 9 am, 11 am, 1 pm and 3 pm and talk about the pattern.
- Never look at the sun. Study the sun through its shadows only; looking at the sun, even briefly, can harm the eyes.
Nothing to Install, Nothing Uploaded
The shadow lab runs entirely in your browser on any phone, tablet or computer, with no account and no download. The last level and challenge you chose are remembered on this device; answers are never sent to us.
Frequently Asked Questions
Light travels in straight lines. When an object that does not let light through is in the way, the light cannot bend around it, so the area behind the object gets less light. That darker area is the shadow, and it has the same outline as the object.
Move the light closer to the object, or move the object closer to the light and further from the screen. The object then blocks a wider spread of light rays, so the shadow on the screen grows. Moving the light further away makes the shadow smaller.
Opaque materials such as wood, metal and cardboard do not let light through and make dark shadows. Translucent materials such as frosted glass and tracing paper let some light through, so they make faint shadows and you cannot see clearly through them. Transparent materials such as clear glass let almost all light through, so they make hardly any shadow.
When the sun is high in the sky, its light comes down steeply and the shadow is short. In the morning and evening the sun is low, its light comes in at a shallow angle and the shadow is long. In Singapore, which is close to the equator, the sun is nearly overhead around 1 pm, so shadows then are very short.