Light is all around us and plays a very imp role in our daily life. Have you ever seen fireflies glowing in the dark? These tiny insects use light to talk to each other at night. But sadly, because of light pollution in cities, we see fewer fireflies these days.
When we look at the Moon at night, it seems to glow. But did you know that Moon doesn’t make its own light? It just reflects the light coming from Sun. This is why we can see Moon shining in the sky.
In this chapter, we will study about different things related to light. We will learn about:
- How light travels
- What makes shadows
- How we see our reflection in mirrors
- Different types of materials and how light passes through them
Objects around us can be divided into two types – those that make their own light (luminous objects) and those that don’t make light but reflect it (non-luminous objects). Light always travels in a straight line, and because of this property, we get shadows and reflections.
Sources of Light
Natural Sources of Light
Sun is the most imp source of light for us. Without Sun, there would be no life on Earth. Sun gives us light and heat during day time. Apart from Sun, there are many other natural sources of light:
- Stars – They are like Sun but very far away from us
- Lightning – During thunderstorms, we see bright flashes of light
- Fire – When things burn, they give light
- Some animals – Fireflies, some fish in deep ocean, and some other animals can make their own light
Artificial Sources of Light
Long time ago, when there was no electricity, people used fire to get light. They would burn animal fat or oil in lamps. This was the only way to get light after sunset.
Today we have many artificial sources of light:
- Electric bulbs – Most common in our homes
- Tube lights – Used in offices and schools
- LED lamps – These are becoming very popular now
- Candles – Still used during power cuts or for decoration
Why LED lamps are better?
- They use less electricity, so they save money
- They are good for environment
- They give brighter light
- They last much longer than normal bulbs
- They don’t get very hot
But remember, when LED bulbs stop working, we should throw them in proper waste bins because they have some chemicals that can harm environment.
Luminous and Non-Luminous Objects
Luminous Objects: These objects make their own light. Examples are:
- Sun
- Stars
- Electric bulb
- Candle flame
- Firefly
Non-Luminous Objects: These objects don’t make light. They only reflect light that falls on them. Examples are:
- Moon
- Planets like Mars, Venus
- Mirror
- Books
- Trees
- Our body
Moon looks bright at night but it’s actually a non-luminous object. It just reflects sunlight that falls on it.
Does Light Travel in a Straight Line?
Yes, light always travels in a straight line. We can see this in many ways:
- When you use a torch, the light beam is straight
- When sunlight comes through a window, you can see straight rays
- Laser light in water shows a straight path
- Car headlights make straight beams
Light cannot bend or turn by itself. It always goes straight until something stops it or changes its direction.
Simple Activity: Take a torch and switch it on in a dark room. You will see that light makes a straight beam. Even if you want light to go around a corner, it won’t go by itself.
In some special cases, light can bend, but normally it travels straight. This straight line movement of light is very imp for understanding shadows and reflections.
Light through Transparent, Translucent, and Opaque Materials
All materials around us behave differently when light falls on them. Based on how much light can pass through them, we divide materials into three types:
Transparent Materials
These materials let light pass through them completely. We can see clearly through transparent materials.
Examples:
- Clear glass
- Clear plastic
- Clean water
- Air
- Some crystals
When you look through a glass window, you can see everything clearly on the other side. This is because glass is transparent.
Translucent Materials
These materials let some light pass through them, but not all. We cannot see clearly through translucent materials. Things look blurry through them.
Examples:
- Tracing paper
- Frosted glass
- Thin cloth
- Butter paper
- Some plastics
If you put tracing paper on a book, you can see that there is something behind it, but you cannot read the words clearly.
Opaque Materials
These materials don’t let any light pass through them. We cannot see anything through opaque materials.
Examples:
- Wood
- Metal
- Cardboard
- Thick cloth
- Our body
- Books
When you hold a book in front of a light, no light passes through it. The book is opaque.
Shadow Formation
Shadows are dark areas that form when something blocks light. We see shadows everywhere – under trees, behind buildings, of our own body.
How Shadows Form
For shadow to form, we need three things:
- Light source – Like Sun, bulb, torch
- Opaque object – Something that blocks light
- Screen – A surface where shadow falls (like wall, ground)
When light from a source falls on an opaque object, the object blocks the light. The area behind the object becomes dark. This dark area is called shadow.
Types of Shadows
Dark Shadows:
- Made by opaque objects
- Very little or no light passes through
- Examples: Shadow of a person, tree, building
Light Shadows:
- Made by translucent objects
- Some light passes through, so shadow is not completely dark
- Examples: Shadow of tracing paper, thin cloth
Very Faint Shadows:
- Even some transparent objects can make very light shadows
- This happens because they reflect or absorb tiny amount of light
- Examples: Thick glass, clear plastic bottle
Imp Points about Shadows
- Shadow size changes: If object is close to light source, shadow is big. If object is far from light source, shadow is small.
- Shadow shape: Usually shadow has same shape as object, but sometimes it can look different depending on angle of light.
- Shadow color: Shadow is always dark (black or grey). Even if object is red or blue, shadow will be dark. Object’s color doesn’t change shadow’s color.
- Multiple shadows: If there are two light sources, object will make two shadows.
Shadow Puppetry
In India, shadow puppetry is a very old art form. Artists use cut-out figures made of leather or cardboard. They hold these figures behind a white cloth screen and shine light on them. This makes shadows on the screen that tell stories.
Different states in India have their own style of shadow puppetry:
- Karnataka has “Togalu Gombeyaata”
- Andhra Pradesh has “Tholu Bommalata”
- Kerala has “Tolpava Koothu”
Reflection of Light
When light falls on a shiny surface, it bounces back. This bouncing back of light is called reflection.
Surfaces that Reflect Light
Good Reflectors:
- Mirrors
- Polished metal
- Still water
- Shiny spoons
- Chrome surfaces
Poor Reflectors:
- Rough surfaces
- Paper
- Cloth
- Unpolished wood
How Reflection Works
When you shine a torch on a mirror, the light bounces back from mirror surface. The light that bounces back is called reflected light. This reflected light also travels in straight line.
Simple Activity: Take a small mirror and place it in sunlight. Tilt the mirror slowly and watch how the reflected light spot moves on the wall. This shows that when we change mirror’s position, direction of reflected light also changes.
Images Formed in a Plane Mirror
A plane mirror is a flat mirror like the one we use in our bathroom. When we stand in front of a plane mirror, we see our image.
Properties of Images in Plane Mirror
The image we see in a plane mirror has special properties:
- Same size: Image is exactly same size as the object
- Upright: Image is not upside down, it stands straight
- Virtual: We cannot catch this image on a screen. It only appears to be behind the mirror
- Same distance: If you stand 2 feet away from mirror, your image also appears 2 feet behind the mirror
- Laterally inverted: Left and right sides are swapped
Lateral Inversion
This is the most interesting thing about mirrors. When you raise your right hand in front of mirror, it looks like the image is raising left hand. This swapping of left and right is called lateral inversion.
Real Life Example: You must have seen ambulance vehicles on road. The word “AMBULANCE” is written in reverse on front of ambulance. This is because when other drivers see ambulance in their rear-view mirror, the reversed letters look normal due to lateral inversion. This helps them read “AMBULANCE” properly and give way.
History of Mirrors
Long time ago, people didn’t have glass mirrors. They used:
- Polished stones
- Polished metals like bronze or silver
- Still water in ponds
In Kerala, there is a special type of metal mirror called “Aranmula Kannadi”. It is made of special metal alloy and is very famous. These mirrors are still made by hand using old techniques.
Pinhole Camera
A pinhole camera is a simple device that can form images without using any lens. It works on the principle that light travels in straight line.
How Pinhole Camera Works
A pinhole camera is made of:
- A box or tube
- A tiny hole on one side
- A screen (white paper or cloth) on the other side
When light from an object passes through the tiny hole, it forms an image on the screen inside the box.
Properties of Pinhole Camera Images
- Inverted: Image is upside down
- Real: Image can be seen on screen
- Same colors: Image has same colors as the object
- Smaller: Usually image is smaller than the object
Why is Image Inverted?
Light from top of object goes straight through pinhole and hits bottom of screen. Light from bottom of object goes straight through pinhole and hits top of screen. This makes the image upside down.
Difference between Mirror and Pinhole Camera
| Mirror | Pinhole Camera |
|---|---|
| Image is upright | Image is inverted |
| Image is virtual | Image is real |
| Image is same size | Image is usually smaller |
| No screen needed | Screen needed to see image |
Making Some Useful Items
Using properties of light, we can make some useful devices:
Periscope
Periscope is a device that helps us see things that are hidden behind obstacles.
How to Make Periscope:
- Take a Z-shaped tube or box
- Fix two plane mirrors inside at both ends
- Make sure mirrors are parallel to each other
- Both mirrors should be at 45-degree angle
How Periscope Works: Light from the object hits first mirror and reflects to second mirror. Second mirror reflects this light to our eyes. This way we can see objects that are above or behind obstacles.
Uses of Periscope:
- Submarines use periscopes to see above water surface while staying underwater
- Tanks use periscopes to see outside while staying protected inside
- People use periscopes to see over crowds in festivals or events
- Scientists use periscopes to observe dangerous things from safe distance
Kaleidoscope
Kaleidoscope is a device that makes beautiful patterns using reflections.
How to Make Kaleidoscope:
- Take three plane mirrors of same size
- Join them to make a triangular tube
- Put some colored beads, glass pieces, or small objects at one end
- Close the other end with a hole to look through
How Kaleidoscope Works: When you look through the hole and turn the kaleidoscope, the colored objects inside move. The three mirrors reflect these objects many times, creating beautiful symmetric patterns.
Uses of Kaleidoscope:
- Artists and designers use kaleidoscopes to get ideas for patterns
- Children enjoy playing with kaleidoscopes
- Some people use kaleidoscopes for relaxation and meditation
- Textile designers use kaleidoscope patterns for clothing designs
In a Nutshell
Here are the imp points we studied in this chapter:
About Light Sources:
- Luminous objects make their own light (like Sun, stars, bulb)
- Non-luminous objects only reflect light (like Moon, planets, mirror)
- Sun is the main natural source of light
- We have many artificial sources like LED lamps, tube lights
About Light Travel:
- Light always travels in straight line
- Light cannot bend by itself around corners
- We can see straight beams of light from torch or laser
About Materials:
- Transparent materials let all light pass through (like clear glass)
- Translucent materials let some light pass through (like tracing paper)
- Opaque materials don’t let any light pass through (like wood, metal)
About Shadows:
- Shadows form when opaque objects block light
- We need light source, object, and screen for shadow formation
- Shadow size depends on distance between object and light source
- Shadow color is always dark, no matter what color the object is
About Reflections:
- Shiny surfaces reflect light
- Mirrors form images that are same size, upright, and laterally inverted
- Image distance from mirror equals object distance from mirror
- We cannot catch mirror images on screen
About Pinhole Camera:
- It forms real images that can be seen on screen
- Images are inverted (upside down)
- Images have same colors as objects
- It works because light travels in straight line
Questions and Answers
1. Which of the following are luminous objects? Mars, Moon, Pole Star, Sun, Venus, Mirror
To answer this question, we need to remember what luminous objects are. Luminous objects are those that make their own light.
Let’s check each option:
- Mars: It’s a planet that reflects sunlight. Not luminous.
- Moon: It reflects sunlight, doesn’t make its own light. Not luminous.
- Pole Star: It’s a star that makes its own light. Luminous.
- Sun: It makes its own light and heat. Luminous.
- Venus: It’s a planet that reflects sunlight. Not luminous.
- Mirror: It only reflects light, doesn’t make light. Not luminous.
Answer: Sun and Pole Star are luminous objects.
2. Match 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 |
Answer:
- 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
3. What happens to shadow in these situations: if the torch is close to the ball, if the torch is far away, if the ball is removed from the set-up, if two torches are present on the left side of the ball.
Let’s think about each situation:
- Torch close to ball: When light source is close to object, shadow becomes larger. This is because light rays spread out more after being blocked by the object.
- Torch far from ball: When light source is far from object, shadow becomes smaller. Light rays are more parallel when coming from far distance.
- Ball removed: If there’s no object to block light, there will be no shadow. Instead, there will be a bright spot of light on the screen.
- Two torches: If there are two light sources, the ball will create two shadows on the screen. Each torch will create its own shadow of the ball.
Answer: Close torch – larger shadow, far torch – smaller shadow, no ball – bright spot, two torches – two shadows.
4. Suppose you view the tree through a pinhole camera. Sketch the outline of the image of the tree formed in the pinhole camera.
When we see a tree through pinhole camera, the image will be inverted (upside down). This happens because:
- Light from top of tree goes straight through pinhole and hits bottom of screen
- Light from bottom of tree goes straight through pinhole and hits top of screen
So the image will show:
- Tree’s top part at the bottom of image
- Tree’s trunk and bottom part at the top of image
- Tree’s left side will appear on right side of image
- Tree’s right side will appear on left side of image
The image will have same shape as tree but will be completely upside down and will show the tree’s natural colors.
Answer: The tree’s image is upside down, with the top branches at the bottom and roots/trunk at the top.
5. Write your name on a piece of paper and hold it in front of a plane mirror such that the paper is parallel to the mirror. Sketch the image. What difference do you notice? Explain the reason for the difference.
When you write your name on paper and show it to mirror, you will notice that the letters appear reversed from left to right. For example:
- If you write “RAHUL”, it will appear as “LUHAR” in mirror
- If you write “PRIYA”, it will appear as “AYIRP” in mirror
This happens because of lateral inversion. In plane mirrors, left and right sides get swapped. However, some letters like “A”, “H”, “I”, “M”, “O”, “T”, “U”, “V”, “W”, “X”, “Y” look almost same even when reversed because they are symmetric.
The reason for this difference is the property of plane mirrors called lateral inversion. When light reflects from mirror surface, the left side of object appears on right side of image and right side of object appears on left side of image.
Answer: The name appears reversed due to lateral inversion in plane mirrors.
6. Measure the length of your shadow at 9 AM, 12 PM, and 4 PM with the help of your friend. Write down your observations:
(i) At which of the given times is your shadow the shortest?
Your shadow will be shortest at 12 PM (noon time).
(ii) Why do you think this happens?
This happens because of Sun’s position in sky during different times of day:
- At 9 AM: Sun is in eastern part of sky, not very high. Light falls on you at an angle, so shadow is longer.
- At 12 PM: Sun is almost directly overhead (especially during summer). Light falls on you from top, so shadow is very short.
- At 4 PM: Sun is in western part of sky, again not very high. Light falls on you at an angle, so shadow is longer again.
When Sun is high in sky, the angle of sunlight is more vertical, which creates shorter shadows. When Sun is low in sky, the angle of sunlight is more horizontal, which creates longer shadows.
Answer: Shortest at 12 PM; Sun’s high angle makes shadows smaller.
7. On the basis of 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’s check both statements:
Statement A: “Image formed by a plane mirror is laterally inverted.” This statement is TRUE. We have studied that plane mirrors show lateral inversion, where left and right sides are swapped.
Statement B: “Images of alphabets T and O appear identical to themselves in a plane mirror.” This statement is also TRUE. Letters like T and O are symmetric, which means they look the same even when left and right are swapped. Other symmetric letters include A, H, I, M, U, V, W, X, Y.
Since both statements are correct, the answer is that both statements are true.
Answer: Both statements are true.
8. Suppose you are given a tube of the shape shown and two plane mirrors smaller than the diameter of the tube. Can this tube be used to make a periscope? If yes, mark where you will fix the plane mirrors.
Yes, this tube can be used to make a periscope if it has the right shape (like Z-shape or has bends).
To make a periscope, you need to fix the two plane mirrors at the bends of the tube:
- Fix first mirror at the top bend at 45-degree angle
- Fix second mirror at the bottom bend at 45-degree angle
- Both mirrors should be parallel to each other
The working will be:
- Light from the object will hit the first mirror at top
- First mirror will reflect this light down the tube to second mirror
- Second mirror will reflect this light to your eye through the viewing hole
This way, you can see objects that are above or behind obstacles.
Answer: Yes, fix mirrors at tube bends to reflect light from top to viewer.
9. We do not see the shadow on the ground of a bird flying high in the sky. However, the shadow is seen on the ground when the bird swoops near the ground. Think and explain why it is so.
This happens because of how light spreads out over distance:
When bird is flying high:
- Bird is very far from ground
- Light blocked by bird has to travel a long distance to reach ground
- During this long travel, the blocked light area spreads out and becomes very large
- The shadow becomes so spread out and faint that we cannot see it clearly
- Also, other light coming from different angles fills up the shadow area
When bird is near ground:
- Bird is close to ground
- Light blocked by bird doesn’t have much distance to travel
- Shadow area doesn’t spread out much
- Shadow remains sharp and dark
- We can clearly see the bird’s shadow on ground
Think of it like this – if you hold your hand close to a wall with torch light, you get a sharp shadow. If you move your hand far from wall, shadow becomes bigger and lighter.
Answer: High altitude scatters shadow; near ground, it’s sharp due to closer light blocking.
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