CLASS 7 SCIENCE · CHAPTER 11 · STUDY NOTES
Light: Shadows and Reflections
Trace light paths to explain shadows, mirror images and simple optical devices.
01 / EXPLORE & UNDERSTAND
Making light and reflecting light are different
A bright-looking object is not always a source of its own light.
1. Identify the source
The Sun, a glowing lamp and a firefly produce light. A mirror, the Moon and an ordinary book are seen using light from elsewhere. A switched-off lamp is not luminous just because it can shine when switched on.
2. Follow light into the eye
To see a book, light from a source reaches it and some is reflected into your eyes. Moonlight is reflected sunlight. The object must send light towards the eye, either by emitting it or reflecting it.
A little deeper: Use lighting thoughtfully
LED lamps generally need less electrical energy than incandescent lamps for similar light output. Unnecessary night lighting can disturb wildlife. Use suitable lighting where needed and follow local e-waste arrangements for old lamps.
Your turn
Question
Is a shining mirror luminous?
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02 / EXPLORE & UNDERSTAND
Aligned holes let a beam pass
A ray is a way to represent light’s direction.
Keep the holes in one line
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1. Compare aligned and shifted holes
In the matchbox experiment, a torch sends light through three aligned holes to a screen. Moving one box sideways blocks the straight path and removes the bright spot. The light does not choose to turn towards the shifted hole.
2. State the model’s conditions
In a uniform medium, light is modelled as travelling straight. Mirrors redirect it and a change of material can bend its path. The school activity tests a straight path through air, not every possible light effect.
Your turn
Question
Why does one shifted hole remove the bright spot?
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03 / EXPLORE & UNDERSTAND
How much light passes—and how clearly?
Transparency describes the material and its condition.
1. Compare three cases
Clear glass is usually transparent: light passes with little scattering. Tracing paper is translucent: light passes but detail becomes blurred. Thick cardboard is opaque to visible light: it blocks transmission.
2. Avoid absolute shortcuts
Transparent does not mean every ray passes unchanged: some light can reflect or be absorbed. Thickness, tint and surface roughness matter. Even a clear object can make a faint pattern or shadow by redirecting or reducing light.
Your turn
Question
A window admits daylight but hides clear outlines. Which description fits?
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04 / EXPLORE & UNDERSTAND
A shadow marks blocked light
The screen shows where less light reaches.
Shape tells the story
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1. Place the three parts
Put an opaque object between a lamp and a wall. Rays blocked by the object cannot reach part of the wall, creating a dark patch. The wall is the screen; the ground or floor can also serve as one.
2. Separate shadow from picture
A shadow usually shows an outline, not the object’s surface pattern or colour. A red card and a blue card of the same shape can give the same shadow under the same light. Rotating either card can change the outline.
Your turn
Question
Does a striped object necessarily make a striped shadow?
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05 / EXPLORE & UNDERSTAND
Position changes a shadow’s size
Keep the source and screen fixed while moving the object.
Move the object. Watch the shadow.
Nearer the lamp → a bigger shadow.
1. Move towards the source
With a small lamp and fixed screen, an object closer to the lamp blocks a wider bundle of diverging rays. Its shadow becomes larger. Moving it towards the screen makes the shadow smaller and nearer the object’s size.
2. Explain blurred edges
An extended light source sends light from many points. Some screen regions receive light from only part of it, producing a partial shadow. This is why real shadows may have soft edges rather than perfect outlines.
A little deeper: Sun shadows and time
As the Sun’s apparent position changes, a fixed object’s shadow changes direction and length. A higher Sun usually gives a shorter shadow on level ground. Clock noon and local solar noon need not be the same.
Your turn
Question
Why should you not move both lamp and object when testing object position?
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06 / EXPLORE & UNDERSTAND
A mirror redirects light
The beam follows one straight path before reflection and another afterwards.
Find the two straight parts of the path
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1. Trace the incoming and outgoing paths
A narrow torch beam reaches a plane mirror and changes direction. Tilting the mirror changes the outgoing direction. The arrow must lead from the source to the mirror and then away from it.
2. Connect reflection with seeing
A polished surface reflects light in an orderly way and can form a clear image. Rough surfaces also reflect light, but scatter it into many directions. We can see ordinary paper because it reflects light too.
Your turn
Question
Does a rough wall reflect no light?
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07 / EXPLORE & UNDERSTAND
An image appears behind a plane mirror
The object is in front; its image appears behind.
Compare the pen with its image
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1. Compare size and position
A plane mirror forms an upright image of the same size as the object. The image appears as far behind the mirror as the object is in front. Moving the object closer brings the image closer too.
2. Distinguish image from a screen projection
Light reflected from the mirror enters your eyes and appears to come from behind it. Placing a screen there will not catch this image directly. It is virtual, not an object hiding inside the mirror.
Your turn
Question
A toy is 30 cm in front of a plane mirror. How far behind does its image appear?
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08 / EXPLORE & UNDERSTAND
Why mirror writing looks different
A mirror image stays upright but familiar writing can look reversed.
1. Compare a marked hand or letter
Raise a hand while facing the mirror. The image’s raised hand appears opposite when you imagine a person facing you. An asymmetric letter such as F makes the reversal easier to see than a symmetric letter such as O.
2. Notice what does not flip
The top of your head remains at the top. The mirror reverses the direction perpendicular to its surface: front and back. Our usual comparison with a turned-around person produces the familiar left-right description.
Your turn
Question
Why is a symmetrical letter a poor test of mirror reversal?
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09 / EXPLORE & UNDERSTAND
A small hole produces an inverted image
Different parts of the object send light through the same small opening.
Trace one point at a time
1. Follow top and bottom rays
Light from the top of the object travels through the hole to the lower part of the screen. Light from the bottom reaches the upper part. Straight paths crossing at the opening explain the upside-down image.
2. Compare it with a shadow
A pinhole image can show the object’s colours and appears on a screen. A shadow mainly shows blocked light. The hole must be small enough to reduce overlapping rays, but an extremely small hole gives a dim image. Never look directly at the Sun.
Your turn
Question
Where does light from the top of a tree reach the pinhole-camera screen?
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10 / EXPLORE & UNDERSTAND
Two reflections let us see past an obstacle
Trace light from the object all the way to the eye.
Follow the two reflections
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1. Redirect at the upper mirror
In a simple periscope, light from an object reaches an upper sloping mirror and is sent down the tube. A lower mirror sends it towards the eye. The two mirrors must be aligned with the intended path.
2. Keep the distinction clear
The tube does not make light curve around a corner. Each mirror creates a change of direction between straight sections of path. Use safe mirror pieces or reflective material with adult help for construction.
Your turn
Question
What actually changes the ray’s direction in a periscope?
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11 / EXPLORE & UNDERSTAND
Repeated reflections create patterns
Many images can come from only a few coloured pieces.
Few pieces, many reflected images
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1. Look at the mirror arrangement
Three mirror strips form the sides of a triangular tube in a simple kaleidoscope. Reflections and reflections of reflections repeat the view of small coloured pieces into a pattern.
2. Explain a changing pattern
Turning the tube rearranges the pieces, changing the reflected pattern. The mirrors have not manufactured extra physical beads. Use beads or safe coloured pieces rather than broken glass for a model.
Your turn
Question
Why can a kaleidoscope show more shapes than the number of beads inside?
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Take these ideas with you
- Luminous objects emit light; others reflect it.
- Straight rays model travel through a uniform medium.
- A shadow is blocked light; its size depends on geometry.
- A plane-mirror image is upright, same-sized and virtual.
- A pinhole image is inverted and appears on a screen.
- Periscopes and kaleidoscopes use reflections.
Use your understanding in a new situation
Question
Give one difference between a mirror image and a pinhole image.
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Why is the Moon bright although it is non-luminous?
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