About this experiment
A shadow forms where an object blocks light. Because light travels in straight lines, you can predict exactly where a shadow will be and how big it is. Move a torch and a cut-out figure in a room and watch the shadow on the floor and the wall grow and shrink. Then go outside: plant a 1 m stick in the playground and watch its shadow turn and change length as the Sun crosses the sky, in summer and in winter.
Learning goals
- Explain that a shadow is the dark area where light is blocked by an object
- Use straight light rays to predict the size and position of a shadow
- Describe how a shadow gets bigger as the object moves closer to the light
- Sort materials into opaque, translucent and transparent by the shadows they make
- Describe how the length and direction of a shadow from the Sun change during the day and through the year
How to use it
- 1Start with the torch low and the figure close to it. Look at the shadow on the wall. Slowly drag the figure towards the wall and describe what happens to the shadow.
- 2Put the figure back at 1.2 m. Raise the torch and watch the shadow move down the wall and onto the floor. When does the shadow stop reaching the wall?
- 3Switch on the light rays. Follow the ray that just touches the top of the head: the edge of the shadow is where that ray lands.
- 4Change the material to translucent and then to transparent. Compare how dark the shadow is each time.
- 5Switch to Sun and stick. Press Play the day and watch the shadow of the stick. At what time is it shortest? Which way does it point at noon?
- 6Compare June and December at 9:00 and at 12:00. In which month are the noon shadows longer?
The model
Assumptions
- The torch is a point of light, so shadows have sharp edges. A real torch has a wide bulb and gives a soft, blurry edge called the penumbra.
- The figure is a flat cut-out facing the wall, and the wall is 3 m from the torch.
- The Sun view is for a place at 40° north (for example Madrid, Beijing or New York) using local solar time, so noon is when the Sun is highest. South of the equator the Sun is in the north at noon.
- Translucent and transparent materials are shown with fixed amounts of light passing through. Real materials vary.
Try this in class
- Q1You want a huge shadow of your hand on the wall for a shadow puppet show. Where should your hand be?
- Q2Why does the shadow of a tree point the other way in the afternoon than in the morning?
- Q3A friend says the shadow is bigger because the torch is brighter. Is that right? How could you test it?
- Q4Why is a shadow at noon in June shorter than a shadow at noon in December?
- Q5Which would make the best curtains for a bedroom: an opaque, translucent or transparent material? Why?
Frequently asked questions
Why is a shadow the same shape as the object?
Light travels in straight lines, so the light that would have passed the edges of the object carries on in straight lines to the wall. The edge of the shadow traces the outline of the object, stretched or shrunk depending on the distances.
Why does the shadow get bigger when the object is closer to the torch?
Close to the torch the object blocks a wider fan of light rays. Those rays keep spreading out on their way to the wall, so the dark patch they leave is bigger.
Is my shadow shortest at 12 o’clock?
It is shortest when the Sun is highest, called solar noon. Because of time zones and summer time, that can be anywhere from about 11:30 to 13:30 on the clock.
Why is a shadow not completely black?
Light bounces off the walls, the floor and the sky and some of it reaches the shadow from other directions. That is why you can still see things inside a shadow.