Light is the most wonderful form of energy around us. It helps us see the world. Without light, we cannot see anything. The Sun is our main source of light, and other sources like lamps, candles, and electric bulbs also give us light. In this chapter, we learn about light, how it travels, and how shadows and reflections are formed.
We begin with the sources of light and the classification of objects into luminous and non-luminous. Then we learn that light travels in a straight line. This property of light helps us understand how shadows are formed. A shadow is formed when an opaque object blocks the path of light.
We also study reflection of light. When light falls on a shiny surface, it bounces back, and this is called reflection. We can see ourselves in a mirror because of reflection. In this chapter, we also learn to make a simple pinhole camera, which is based on the straight-line travel of light.
Objects that give out light are called sources of light. Sources of light can be divided into two types: - Natural sources of light: These sources of light are present in nature. The Sun is the most important natural source of light. Other natural sources are stars, fireflies, and lightning. - Artificial sources of light: These are man-made sources of light. Electric bulbs, tube lights, lamps, candles, and torches are artificial sources of light.
Objects like the Moon do not give out their own light. The Moon reflects the light of the Sun. This is why we can see the Moon shining at night. Similarly, planets reflect sunlight.
Objects can be classified into two groups based on whether they give out light: - Luminous objects: Objects that give out their own light are called luminous objects. The Sun, stars, burning candles, and electric bulbs are luminous objects. - Non-luminous objects: Objects that do not give out their own light are called non-luminous objects. We see them only when light from a luminous object falls on them and is reflected to our eyes. The Moon, the Earth, tables, and chairs are non-luminous objects.
We see non-luminous objects because they reflect light. When sunlight falls on a tree, the light reflects from the tree and reaches our eyes, and we see the tree.
Light travels in a straight line. This is called the rectilinear propagation of light. We can prove that light travels in a straight line with a simple experiment using three cardboard sheets with holes in a straight line. When a candle is placed behind the sheets, we can see its flame only when all three holes are in a straight line. If we shift any one sheet, the light is blocked and we cannot see the flame.
The straight-line travel of light explains many things: - We can see an object only when light from it travels straight to our eyes. - Shadows are formed because light travels in straight lines. - A pinhole camera works because light travels in straight lines.
A shadow is a dark region formed when an opaque object blocks the path of light. When light falls on an opaque object, the light cannot pass through it. A dark area is formed behind the object, and this dark area is called the shadow of the object.
For a shadow to form, three things are needed: 1. A source of light. 2. An opaque object that blocks the light. 3. A screen or surface on which the shadow falls.
Shadows have some interesting features: - The shadow of an object is always black or dark, regardless of the colour of the object. - The shadow is formed on the side of the object opposite to the light source. - A shadow can tell us about the shape of the object, but it does not show the details or colours of the object. - The size of a shadow changes when the distance between the object and the light source changes. When the object is closer to the light source, the shadow is bigger, and when the object is farther from the light source, the shadow is smaller.
Objects are classified based on how much light can pass through them: - Transparent objects: Objects that allow all light to pass through them are called transparent. We can see through them clearly. Glass, clear water, and clean air are transparent. - Opaque objects: Objects that do not allow any light to pass through them are called opaque. We cannot see through them. Wood, iron, and stone are opaque. - Translucent objects: Objects that allow some light to pass through them are called translucent. We can see through them but not clearly. Butter paper and frosted glass are translucent.
Shadows are formed only by opaque objects. Transparent objects do not form shadows, and translucent objects form very faint or light shadows.
When light falls on a shiny or smooth surface, it bounces back. This bouncing back of light from a surface is called reflection. We can see ourselves in a mirror because light reflects from the mirror back to our eyes.
The best reflecting surface is a polished or shiny surface like a mirror. A plane mirror gives a clear reflection. Other objects also reflect light, but not as clearly. A polished steel spoon, a shiny bucket, and calm water also reflect light and act like mirrors.
Reflection has many uses: - We use mirrors to see ourselves and to look at objects behind us. - Mirrors are used in cars and motorcycles for the driver to see vehicles behind. - Periscopes, used in submarines, use mirrors to see objects above water.
A pinhole camera is a simple device that works on the principle that light travels in a straight line. It is made from a closed box or a dark container with a tiny hole on one side. On the opposite side, there is a translucent screen made of butter paper or tracing paper.
When we keep a pinhole camera near a candle or a well-lit object, an image is formed on the screen. The image has some important features: - The image is upside down (inverted). - The image is real because it can be seen on a screen. - The image is smaller than the object.
The image is inverted because light travels in straight lines. Light from the top of the candle goes through the hole and falls at the bottom of the screen, and light from the bottom of the candle falls at the top of the screen. This is why the image appears upside down.
A plane mirror reflects light in a regular way and gives a clear image. When we look into a plane mirror, we see our own image. The image we see in a plane mirror is: - Upright (right side up). - The same size as the object. - Laterally inverted, which means the left side of the object appears on the right side of the image. This is why the word AMBULANCE is written in reverse on ambulances, so drivers ahead can read it correctly in their mirrors.
Reflections are all around us. We see our face in the mirror, the reflection of trees in a pond, and our shadow on a sunny day. Understanding light and reflection helps us explain all these everyday observations.
An interesting fact about shadows is that the colour of the object does not affect the colour of its shadow. Whether we hold a red ball or a blue ball in front of a light, the shadow is always dark. This is because a shadow is simply the absence of light in that region.
However, the shape of the shadow is related to the shape of the object. If the object is held in different positions, the shape of the shadow changes. This is why we can make different shadow shapes with our hands, like a bird or a dog, in front of a wall.
| Object Type | Light Passing | Example |
|---|---|---|
| Transparent | All light passes | Glass, clear water |
| Translucent | Some light passes | Butter paper, frosted glass |
| Opaque | No light passes | Wood, iron, stone |
| Feature | Shadow | Image in Pinhole Camera |
|---|---|---|
| Type | Dark region | Real image |
| Orientation | Depends on position | Inverted (upside down) |
| Colour | Always dark/black | Shows colours of object |
| Formation | Opaque object blocks light | Light travels in straight line |
In this chapter, we discovered the fascinating world of light. We learned that light comes from natural and artificial sources, and that luminous objects give out their own light while non-luminous objects reflect it. We understood that light travels in a straight line, which explains the formation of shadows and the working of the pinhole camera. We studied how opaque objects block light to form shadows, which are always dark regardless of the object's colour. We also explored reflection and how mirrors give us our image. Every time we see our shadow on a sunny day, our reflection in a mirror, or an upside-down image in a pinhole camera, we are observing the beautiful principles of light.