- Light is a form of energy that lets us see things. Objects that give out their own light are luminous (Sun, lamp); those that only reflect light are non-luminous (Moon, book).
- Materials are transparent (let almost all light through — glass), translucent (let some light through — butter paper) or opaque (block light — wood, metal).
- Light travels in a straight line (rectilinear propagation). This is why a shadow forms when an opaque object blocks light.
- A shadow shows the shape (outline) of the object but not its colour or details; it always forms on the side away from the source, on a screen.
- A pinhole camera uses straight-line travel of light to form a real, inverted image of a bright object.
- Reflection is the bouncing back of light from a surface. A smooth, shiny surface like a plane mirror reflects clearly and forms an image.
- A plane-mirror image is virtual, erect, same size, laterally inverted (left-right flipped) and as far behind the mirror as the object is in front.
- Board weightage: ~6 marks/year — 1-mark definitions, 2-mark property/difference questions, 3-mark shadow/reflection or pinhole-camera answers.
1. Light and How We See
Light is a form of energy that makes things visible to our eyes. In a completely dark room we cannot see anything, even with open eyes. As soon as we switch on a light, we can see. We see an object when light coming from it (either its own light or light reflected from it) enters our eyes.
1.1 Luminous and Non-luminous Objects
- Luminous objects give out their own light. Examples: the Sun, stars, a glowing bulb, a burning candle, a firefly.
- Non-luminous objects do not give out their own light; we see them because they reflect light falling on them. Examples: the Moon, a book, a table, a wall.
The Moon appears bright at night, but it is non-luminous — it merely reflects the light of the Sun.
2. How Materials Behave with Light
Objects differ in how much light they allow to pass through them.
- Transparent — allow almost all light to pass through; we can see clearly through them. Examples: clear glass, clean water, air.
- Translucent — allow only some light to pass; we can see only blurred or hazy outlines through them. Examples: butter paper, frosted glass, oily paper.
- Opaque — do not allow any light to pass through; we cannot see through them at all. Examples: wood, metal, stone, cardboard, a human body.
Take a torch and shine it on different objects: clear glass, butter paper, a wooden plank, frosted glass and a polythene sheet. Note how much light passes through and whether you can see objects on the other side. Glass lets light through fully (transparent), butter paper lets some through and looks hazy (translucent), and the wooden plank lets none through (opaque). This helps you classify materials as transparent, translucent or opaque.
3. Light Travels in a Straight Line
Light always travels along a straight path. This property is called the rectilinear propagation of light. The straight-line path of light is shown by a ray, and a bundle of rays is called a beam.
Take three identical cards and make a small hole at the same height in the centre of each. Stand them up in a line so the holes are aligned, and place a candle behind the first card. You can see the candle flame through all three holes only when the holes are in a perfectly straight line. If you shift the middle card sideways, the flame disappears. This proves that light travels in a straight line.
4. Shadows
When an opaque object comes in the path of light, it blocks the light and a dark patch forms on the screen behind it. This dark patch is called a shadow.
4.1 Properties of Shadows
- A shadow is always dark (black), whatever the colour of the object — it shows only the outline (shape), not the colour or details of the object.
- A shadow always forms on the side of the object away from the light source.
- The size of a shadow changes with the distance between the object, the light source and the screen. Moving the object closer to the light makes the shadow larger.
- The direction and length of an outdoor shadow change through the day as the Sun moves — shadows are long in the morning and evening and short at noon.
In a dark room, hold your hand between a torch and a wall. Move your hand closer to the torch — the shadow on the wall becomes bigger. Move your hand closer to the wall — the shadow becomes smaller and sharper. This shows that the size of a shadow depends on the positions of the object, the light source and the screen.
5. The Pinhole Camera
A pinhole camera is a simple device that works on the principle that light travels in a straight line. It is a closed box with a tiny hole on one side and a translucent screen (such as tracing paper) on the opposite side.
When the pinhole is pointed at a bright, well-lit object, light rays from the top of the object travel in straight lines through the hole and reach the bottom of the screen, while rays from the bottom reach the top. As a result, the image formed on the screen is upside down (inverted).
Take two boxes that slide one inside the other. Make a small hole in the centre of one closed end, and replace the closed end of the other box with tracing paper. Slide the boxes together so the tracing paper faces the hole. Point the pinhole towards a bright object, such as a tree in sunlight or a candle flame, and look at the tracing paper from inside a dark cloth over your head. You see a small, inverted, coloured image of the object on the tracing paper. Sliding the inner box adjusts the image.
The natural pinhole camera: under a thick tree on a sunny day, small bright patches of light on the ground are actually pinhole images of the Sun, formed by the tiny gaps between leaves. During a partial solar eclipse, these patches become crescent-shaped.
6. Reflection of Light
When light falls on a smooth, shiny surface, it bounces back. This bouncing back of light from a surface is called reflection. A surface that reflects almost all the light falling on it, like a polished metal sheet or a mirror, gives a clear reflection.
6.1 The Plane Mirror
A plane mirror is a flat, smooth reflecting surface (ordinary mirrors used at home). When you stand in front of a plane mirror you see your image.
6.2 Properties of an Image in a Plane Mirror
- The image is virtual (it appears to be behind the mirror and cannot be caught on a screen).
- The image is erect (upright, the same way up as you).
- The image is the same size as the object.
- The image is as far behind the mirror as the object is in front of it.
- The image is laterally inverted — the left and right sides are interchanged. Your right hand appears as the left hand of your image. This is why the word AMBULANCE is written in mirror-image form on the front of an ambulance, so that drivers ahead can read it correctly in their rear-view mirror.
Write the word AMBULANCE on a card and hold it in front of a plane mirror. In the mirror the letters appear reversed left to right. Now write the word in mirror-image and hold it up — in the mirror it reads correctly. This is exactly why AMBULANCE is painted reversed on emergency vehicles, so the driver in front sees it the right way round in the rear-view mirror. This demonstrates lateral inversion.
7. Spherical Mirrors and Lenses (A Glimpse)
Besides flat (plane) mirrors, there are curved spherical mirrors, and transparent curved pieces of glass called lenses.
7.1 Concave and Convex Mirrors
- A concave mirror curves inward (like the inside of a spoon). It can form a magnified, erect image when an object is held close — used by dentists and barbers, and as shaving/make-up mirrors and in car headlights and torches.
- A convex mirror bulges outward (like the back of a spoon). It always forms a small, erect image and gives a wide view — used as rear-view mirrors in vehicles.
7.2 Concave and Convex Lenses
- A convex lens is thicker in the middle; it converges (brings together) light. It can magnify objects and is used as a magnifying glass.
- A concave lens is thinner in the middle; it diverges (spreads out) light and always forms a small, erect image.
Hold a bright steel spoon close to your face and look at your reflection in its inner (concave) side — you see a magnified, upright image. Now slowly move the spoon away — at a distance the image turns upside down. Look at the outer (convex) side — you always see a small, upright image. This shows how curved reflecting surfaces form different kinds of images.
8. Quick Comparison
| Feature | Shadow | Plane-mirror image |
|---|---|---|
| Formed by | Blocking of light by opaque object | Reflection of light from mirror |
| Colour | Always dark (black) | Same colours as object |
| Details shown | Only outline (shape) | Full details |
| Side | Away from light source | Behind the mirror |
- The Moon
- A book
- The Sun
- A mirror
- Transparent
- Translucent
- Opaque
- Luminous
- Bends around objects
- Travels in a straight line
- Travels in a curve
- Is absorbed by all surfaces
- A source of light
- An opaque object
- A screen
- A mirror
- Colour
- Outline (shape) only
- Inside details
- Brightness
- Erect and magnified
- Erect and same size
- Inverted (upside down)
- Virtual
- Shadows of leaves
- Pinhole images of the Sun
- Reflections of the Sun
- Rainbows
- Refraction
- Reflection
- Dispersion
- Absorption
- Real and inverted
- Virtual and erect
- Real and erect
- Virtual and inverted
- Magnification
- Lateral inversion
- Reflection
- Refraction
- It looks attractive
- Drivers ahead can read it correctly in the rear-view mirror
- It saves paint
- It glows at night
- Plane mirror
- Concave mirror
- Convex mirror
- Pinhole mirror
- Convex mirror
- Plane mirror
- Concave mirror
- Convex lens
- Edges
- Middle
- One side only
- It has uniform thickness
- Longest
- Shortest
- The same as in the morning
- Coloured
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