Light: Shadows and Reflections · Lesson 2 of 6
Materials, Light, and Shadows
“Test how materials transmit light, then explain the shape and size of shadows.”
• Classify materials by how much light passes through them. • Explain what is needed to observe a shadow on a surface. • Predict how changing the positions or tilt of an object changes its shadow. • Distinguish object colour from the appearance of the shadow. • Use observations to explain real shadows and shadow puppetry.
What Happens When Light Meets a Material?
A torch makes a spot on a wall when nothing blocks its path. Put different objects between the torch and wall and the spot changes. The amount of light that gets through depends on the material. Predicting first and then observing makes this a useful investigation rather than a list of material names to memorise.
A material through which light passes almost completely, allowing a clear view through it in ordinary conditions.
A material through which light passes only partly or is scattered, so a clear view of an object behind it is not obtained.
A material that does not allow light to pass through it in the observed setup.
| Material to test | Prediction to record | Typical observation | Likely classification |
|---|---|---|---|
| Clear window glass | Will most light pass? | Most of the torch light reaches the screen. | Transparent |
| Tracing paper | Will some light pass? | A weaker, spread-out patch reaches the screen. | Translucent |
| Cardboard | Will any light pass? | The central light is blocked. | Opaque |
| Thick cloth | Does its thickness or weave matter? | Usually little or no light passes. | Often opaque in this test |
For each material, hold the torch and screen fixed, place one test object in the same position, and record both the prediction and the observation. If your result differs from the table’s typical result, describe what you actually saw. Thickness and the particular material matter: the label belongs to the object as tested, not to every possible form of “paper” or “cloth.”
How a Shadow Forms
Put an opaque ball between a light source and a screen. Some light travelling toward the screen is blocked; the region that receives much less light looks dark. This is its shadow. In daily life the screen can be a wall, floor, or the ground. Removing the chosen screen removes the place where you were observing that shadow, although a shadow might fall on some other surface.
A darker region on a surface where an object blocks light that would otherwise reach that surface.
The investigation requires a shining source, an object in its path, and a surface on which the changed light can be observed. Turn off the torch and there is no directed beam to form the tested shadow. Remove the object and the light reaches the region again. An opaque object normally gives a darker shadow; a translucent one gives a lighter shadow. Some transparent objects may still leave a faint shadow because they do not send every bit of light along the original path.
The appearance of a shadow depends on the material, the light, and other light in the surroundings. “Transparent” also does not guarantee that no faint shadow can ever be seen.
Change One Part of the Setup
Hold a torch and screen fixed while moving a ball. When the ball moves closer to the torch, it intercepts a wider spread of the light that would reach the screen, so its shadow is generally larger. When it moves closer to the screen, the projected shadow is generally smaller and more like the ball’s outline. Try each move while keeping the other parts fixed so the cause of the change is clear.
Tilt a flat opaque card and its outline on the screen changes; the same object can cast different-looking shadows. Changing only the colour of an opaque card does not change the shadow to that colour, because the card is blocking light rather than projecting its own surface colour. Position, source size, and the distance to the screen can also affect how sharp the edge appears.
| Change in the investigation | What to look for | What it shows |
|---|---|---|
| Remove the ball | The dark patch disappears. | The object was blocking light. |
| Switch off the torch | The illuminated screen and its tested shadow disappear. | A source is required for this setup. |
| Move the ball toward the torch | The shadow generally grows. | The blocked region spreads toward the screen. |
| Move the ball toward the screen | The shadow generally shrinks. | Projection changes with position. |
| Tilt a flat object | The shadow outline changes. | A shadow is a projection, not a fixed copy. |
| Use another colour of opaque object | The shadow does not acquire that object colour. | Blocking light differs from showing surface colour. |
Problem
A torch and wall stay fixed. A ball is first close to the wall and then moved toward the torch. Predict the change.
- 1.Keep track of what stays fixed: torch and wall. Only the ball moves.
- 2.Nearer the torch, the ball blocks a wider set of paths leading to the wall.
- 3.The shadow on the wall is generally larger when the ball is closer to the torch.
Problem
A thin rectangular card casts a broad rectangular shadow. It is tilted almost edge-on. What changes?
- 1.The card is still opaque and light still meets it.
- 2.Tilting changes the outline presented to the incoming light.
- 3.The shadow can become much narrower; the object did not need to change shape.
Shadows in Art and Outdoors
Shadow puppeteers place cut-out figures between a lamp and a screen, then move the figures to change the projected shapes. Many Indian traditions use this technique for stories and community messages. Outdoors, the direction and height of sunlight change during the day, so a person’s shadow changes length and direction. Measure at morning, around noon, and late afternoon at the same place to investigate that pattern.
A bird high overhead may have a shadow too faint or spread out to notice on the ground; when it flies close, its shadow can become clearer. Two torches can make two shadows because each light source has its own blocked paths. For a further investigation, cover a torch with different transparent coloured sheets and observe the illuminated screen and shadow under controlled conditions; report the colours actually seen rather than assuming the object’s colour is projected.
Quiz
Which material usually allows almost all light to pass and a clear view through it?
Which three things are needed to observe a shadow in the torch investigation?
With torch and screen fixed, moving an opaque ball toward the torch usually makes its shadow what?
A red opaque card replaces a blue opaque card without changing the setup. What generally happens to the shadow’s colour?
Which action can change the outline of the shadow of a flat card?
Why might tracing paper form a lighter shadow than cardboard?
Practice Problems
- Describe the prediction and observation columns you would use to test glass, tracing paper, cardboard, and thick cloth.
- Explain why an opaque object can produce a dark region on a wall.
- Predict the result of removing the object, switching off the torch, and removing the screen, treating each as a separate test.
- A ball is moved first close to the screen and then close to the torch. Compare the likely shadow sizes and explain why.
- Explain why turning a flat card can change its shadow even when the card’s shape stays the same.
- Design a fair investigation of the changing length of your outdoor shadow across a day.
- Explain why a translucent object may form a lighter shadow and a transparent object a faint one.
- Describe how two torches or a coloured sheet over a torch would change what you investigate.
Key Takeaways
• Transparent, translucent, and opaque describe how much light passes through a tested material. • A shadow appears on a surface when an object prevents light from reaching part of it. • The positions of source, object, and screen affect shadow size, outline, and edge sharpness. • An opaque object’s colour is not projected as the colour of its shadow. • Use controlled changes to explain shadows in puppetry and everyday life.