Light fills our world every single day. It helps us see colors, read books, and enjoy nature. Have you ever watched tiny fireflies blink in the dark? These little insects produce their own glow to send signals to one another. Sadly, bright city lights have made it harder to spot them today.
When you gaze at the Moon on a clear night, it looks bright and glowing. However, the Moon does not create light by itself. It simply bounces sunlight toward Earth. That reflected sunlight is what makes the Moon visible in the night sky.
In this lesson, we will explore many interesting ideas about light. You will learn how light moves, why shadows appear, how mirrors create images, and how different materials allow different amounts of light to pass through.
Sources of Light
Natural Sources
The Sun is the most powerful natural source of light for our planet. Without it, life would not exist. It gives us brightness and warmth during daytime. Besides the Sun, nature offers other light sources too:
- Stars — They are distant suns burning far away in space.
- Lightning — During storms, huge electrical sparks flash across the sky.
- Fire — When materials burn, they release light and heat together.
- Certain animals — Fireflies, some deep-sea fish, and a few other creatures can make their own light through special body processes.
Artificial Sources
Long before electricity was discovered, people burned oils and animal fats in lamps to light up their homes after sunset. Today we have many modern options:
- Electric bulbs — Found in almost every home.
- Tube lights — Common in classrooms and offices.
- LED lamps — Becoming popular because they are efficient and durable.
- Candles — Still used during power failures or for decoration.
They consume less electricity, save money, stay cool, shine brighter, last longer, and are gentler on the environment. When an LED bulb stops working, place it in the correct waste bin because it contains small amounts of chemicals that should not enter regular trash.
Luminous and Non-Luminous Objects
Luminous objects generate their own light. You can see them even in complete darkness:
- The Sun
- Stars
- An electric bulb when switched on
- A burning candle
- A firefly
Non-luminous objects cannot produce light. They become visible only when light from another source falls on them and bounces toward our eyes:
- The Moon
- Planets such as Mars and Venus
- Mirrors
- Books, trees, and our own bodies
Does Light Travel in a Straight Line?
Yes. Under normal conditions, light always travels in a straight path. You can notice this in many daily situations:
- The beam from a torch in a dark room is perfectly straight.
- Sunlight entering through a window shows straight rays.
- A laser pointer through water reveals a straight line.
- Car headlights create straight beams on the road.
Light does not bend or turn corners by itself. It continues straight until something blocks it or changes its direction.
Figure 1: A torch sends out straight beams of light that do not curve.
Light Through Different Materials
Every material behaves differently when light falls on it. Based on how much light passes through, we divide materials into three groups.
Transparent Materials
These allow almost all light to pass through. You can see objects clearly on the other side.
- Clear glass
- Clean water
- Air
- Clear plastic
- Some crystals
Translucent Materials
These allow only some light to pass through. Objects on the other side look blurry or foggy.
- Tracing paper
- Frosted glass
- Thin cloth
- Butter paper
- Certain thin plastics
Opaque Materials
These block light completely. You cannot see anything through them.
- Wood
- Metal
- Cardboard
- Thick cloth
- Books and the human body
Figure 2: Transparent materials let all light through, translucent materials let some through, and opaque materials block light fully.
Shadow Formation
A shadow is a dark patch that forms when an object blocks light. You see shadows under trees, behind buildings, and when you stand in sunlight.
What Is Needed for a Shadow?
Three things must come together:
<- Light source — such as the Sun, a bulb, or a torch.
- Opaque object — something that stops light.
- Screen — a surface where the dark patch appears, like a wall or the ground.
When light rays hit an opaque object, the object stops them. The region behind the object receives no light, so it looks dark. That dark region is the shadow.
Figure 3: A shadow forms when an opaque object blocks light between a source and a screen.
Types of Shadows
- Dark shadows — Created by opaque objects. Almost no light reaches behind them. Examples include the shadow of a person, a tree, or a building.
- Light shadows — Created by translucent objects. Some light slips through, so the shadow is not completely black. Examples include the shadow of tracing paper or thin fabric.
- Faint shadows — Even transparent objects can cast a very weak shadow because they absorb or reflect a tiny bit of light. A thick glass sheet or a clear plastic bottle may do this.
• Size: If the object is close to the light source, the shadow grows larger. If the object is far from the source, the shadow shrinks.
• Shape: A shadow usually matches the object’s shape, though the angle of light can distort it slightly.
• Color: A shadow is always dark, usually black or grey. The color of the object does not change the shadow’s color.
• Multiple shadows: Two light sources will produce two shadows from the same object.
Shadow Puppetry
Shadow puppetry is an ancient art in India. Artists hold cut-out leather or cardboard figures behind a white cloth screen and shine a light on them. The shadows on the screen act out stories.
Different states have their own styles:
- Karnataka — Togalu Gombeyaata
- Andhra Pradesh — Tholu Bommalata
- Kerala — Tolpava Koothu
Reflection of Light
When light strikes a smooth, shiny surface, it bounces back. This bouncing is called reflection.
Good and Poor Reflectors
Good reflectors send back most of the light that hits them:
- Mirrors
- Polished metal
- Still water
- Shiny spoons
- Chrome surfaces
Poor reflectors absorb most light and reflect very little:
- Rough walls
- Plain paper
- Cloth
- Unpolished wood
Figure 4: A light ray strikes a mirror and bounces back. The angle of incidence equals the angle of reflection.
Images Formed in a Plane Mirror
A plane mirror is a flat mirror, like the one in your bathroom. When you stand in front of it, you see an image of yourself. This image has special properties:
- Same size: The image is exactly as tall and wide as you are.
- Upright: The image stands straight; it is not upside down.
- Virtual: You cannot project the image onto a screen. It only seems to exist behind the mirror.
- Same distance: If you stand one meter in front of the mirror, your image appears one meter behind it.
- Laterally inverted: The left side of your body appears on the right side of the image, and vice versa.
Lateral Inversion
The most fascinating feature of a plane mirror is lateral inversion. If you raise your right hand, your image appears to raise its left hand. This left-right swap is called lateral inversion.
Figure 5: In a plane mirror, left and right sides are swapped. Your left hand appears as the image’s right hand.
History of Mirrors
Before glass mirrors were invented, people used polished stones, polished metals such as bronze and silver, and the still surface of pond water to see their reflections. In Kerala, a special handmade metal mirror called Aranmula Kannadi is still crafted using ancient techniques.
Pinhole Camera
A pinhole camera is the simplest device that can create an image without using any glass lens. It works because light travels in straight lines.
How It Is Made
A basic pinhole camera consists of:
- A closed box or tube.
- A tiny hole on one side.
- A screen made of white paper or cloth on the opposite inner side.
How It Works
Light from the object travels in straight lines through the tiny hole and lands on the screen inside the box. Because the rays cross at the hole, the image appears upside down.
Figure 6: Light rays from the top of the tree pass through the pinhole and hit the bottom of the screen, while rays from the bottom hit the top. This creates an upside-down image.
Properties of the Image
- Inverted: The image is upside down.
- Real: It can be seen on the screen inside the camera.
- Same colors: The image keeps the natural colors of the object.
- Smaller: The image is usually smaller than the actual object.
| Property | Plane Mirror | Pinhole Camera |
|---|---|---|
| Image position | Upright | Inverted (upside down) |
| Image type | Virtual (cannot be caught on screen) | Real (can be seen on screen) |
| Image size | Same size as object | Usually smaller than object |
| Need for screen | No screen needed | Screen is necessary |
Making Useful Items
Periscope
A periscope is a tool that lets you see objects hidden behind obstacles or above your line of sight.
How to Build One
You need a tube bent in a Z-shape and two small plane mirrors. Fix one mirror at the top bend and the second at the bottom bend. Both mirrors should be parallel and tilted at 45 degrees to the tube walls.
Figure 7: Light from the object reflects off the first mirror, travels down the tube, reflects off the second mirror, and reaches the viewer’s eye.
Uses of Periscopes
- Submarines use them to look at the surface while staying underwater.
- Tanks use them to observe the battlefield while remaining protected.
- People use them to see over crowds at festivals.
- Scientists use them to watch dangerous experiments from a safe place.
Kaleidoscope
A kaleidoscope is a toy that creates beautiful, colorful patterns using repeated reflections.
How to Build One
Take three identical rectangular mirrors and tape them together to form a triangular tube. Place small colored beads, glass pieces, or shiny objects at one end. Close the other end with a small hole to look through.
Figure 8: The three mirrors inside a kaleidoscope reflect colored pieces many times, creating symmetrical patterns.
Uses of Kaleidoscopes
- Artists and designers use them to find inspiration for new patterns.
- Children enjoy the ever-changing colors and shapes.
- Some people use them for relaxation and meditation.
- Textile designers borrow kaleidoscope patterns for fabric prints.
Questions and Answers
1Which of the following are luminous objects? Mars, Moon, Pole Star, Sun, Venus, Mirror
Luminous objects produce their own light. Let us check each item:
- Mars — A planet. It reflects sunlight. Non-luminous.
- Moon — Reflects sunlight. Non-luminous.
- Pole Star — A star that generates its own light. Luminous.
- Sun — Produces light and heat. Luminous.
- Venus — A planet that reflects sunlight. Non-luminous.
- Mirror — Only reflects light from other sources. Non-luminous.
Answer: The Sun and the Pole Star are luminous objects.
2Match the items in Column A with those in Column B.
| Column A | Column B |
|---|---|
| Pinhole camera | Forms an inverted image |
| Opaque object | Blocks light completely |
| Transparent object | Light passes almost completely through it |
| Shadow | The dark region formed behind the object |
The correct pairs are:
- Pinhole camera → Forms an inverted image
- Opaque object → Blocks light completely
- Transparent object → Light passes almost completely through it
- Shadow → The dark region formed behind the object
3What happens to the shadow in these situations: (a) the torch is close to the ball, (b) the torch is far away, (c) the ball is removed, (d) two torches are present on the left side of the ball?
- Torch close to the ball: The shadow becomes larger. When the light source is near the object, the light rays spread out more after being blocked.
- Torch far from the ball: The shadow becomes smaller. Light rays from a distant source are more parallel, so the blocked area is narrower.
- Ball removed: There is no shadow. Without an object to block the light, the screen shows only a bright patch of light.
- Two torches present: The ball creates two shadows on the screen. Each torch produces its own separate shadow.
4Suppose you view a tree through a pinhole camera. Sketch the outline of the image of the tree formed in the pinhole camera.
The image inside a pinhole camera is always inverted. This happens because light travels in straight lines and crosses at the tiny hole:
- Light from the top of the tree passes through the pinhole and strikes the bottom of the screen.
- Light from the bottom of the tree passes through the pinhole and strikes the top of the screen.
Therefore, the sketch would show:
- The tree’s branches at the bottom of the image.
- The trunk and roots at the top of the image.
- The left side of the tree appearing on the right side of the image, and vice versa.
The image keeps the same shape and colors as the real tree, but it is completely upside down.
5Write your name on a piece of paper and hold it in front of a plane mirror so that the paper is parallel to the mirror. Sketch the image. What difference do you notice? Explain the reason.
When you hold your name in front of a plane mirror, the letters appear reversed from left to right. For example, if you write RAHUL, it will look like LUHAR in the mirror.
This happens because of lateral inversion. A plane mirror swaps the left and right sides of an object. The left side of the paper appears on the right side of the image, and the right side appears on the left.
Some letters such as A, H, I, M, O, T, U, V, W, X, Y look almost unchanged because they are symmetric. Their left and right halves are identical, so swapping them makes no visible difference.
6Measure the length of your shadow at 9 AM, 12 PM, and 4 PM with the help of a friend. Write down your observations. (a) At which time is your shadow shortest? (b) Why does this happen?
(a) Your shadow is the shortest at 12 PM (noon).
(b) This happens because of the Sun’s position in the sky:
- At 9 AM, the Sun is in the eastern sky and fairly low. Sunlight hits your body at a slanting angle, so the shadow stretches long across the ground.
- At 12 PM, the Sun is nearly straight overhead. The light falls almost vertically on you, so the shadow becomes very short and may even hide beneath your feet.
- At 4 PM, the Sun moves toward the western sky and is again low. The slanting light creates a long shadow, similar to the morning.
Figure 9: At noon the Sun is overhead, so shadows are shortest. In the morning and evening the Sun is low, creating long shadows.
7On the basis of the following statements, choose the correct option. Statement A: Image formed by a plane mirror is laterally inverted. Statement B: Images of alphabets T and O appear identical to themselves in a plane mirror.
Let us examine each statement:
- Statement A says that a plane mirror swaps left and right. This is true. We have learned that lateral inversion is a basic property of plane mirrors.
- Statement B says the letters T and O look the same in a mirror. This is also true. These letters are vertically symmetric, meaning their left and right sides are mirror images of each other. Other symmetric letters include A, H, I, M, U, V, W, X, and Y.
Answer: Both statements are correct.
8Suppose you are given a bent tube and two plane mirrors smaller than the tube’s diameter. Can this tube be used to make a periscope? If yes, mark where you will fix the plane mirrors.
Yes, the tube can be used to make a periscope if it has bends that allow light to be redirected.
Fix the two plane mirrors at the bends of the tube:
- Place the first mirror at the top bend, tilted at 45 degrees to the tube wall.
- Place the second mirror at the bottom bend, also tilted at 45 degrees.
- Both mirrors must be parallel to each other.
Light from the object enters the top, reflects off the first mirror down the tube, then reflects off the second mirror into the viewer’s eye. This arrangement lets you see over or around obstacles.
9We do not see the shadow on the ground of a bird flying high in the sky. However, the shadow is seen when the bird swoops near the ground. Explain why.
This happens because of the distance between the bird and the ground:
- When the bird flies high: The blocked sunlight has a long distance to travel before reaching the ground. Over that distance, the dark area spreads out and becomes very large and faint. At the same time, sunlight coming from many other directions fills in the dark area. The shadow becomes so weak and scattered that our eyes cannot detect it.
- When the bird flies low: The gap between the bird and the ground is small. The blocked light does not have room to spread out, so the shadow stays sharp, compact, and dark. We can easily see it on the ground.
Think of it like holding your hand near a wall under a torch: the shadow is sharp. Move your hand far from the wall, and the shadow grows bigger, lighter, and harder to see.
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