Light, Shadows and Reflections
Chapter Notes — Class 7 Science (Curiosity)
1. Introduction
Keshav visits his friend Jatin's village in the Western Ghats and is amazed by the forests, streams and especially the glowing dance of fireflies at night.
He learns from Jatin's grandparents that fireflies use light to communicate, but their numbers are falling because of light pollution and deforestation.
On the way back to the city, Keshav watches the moonlit hills and wonders: does the Moon produce its own light? Or is it just reflected sunlight, as he read in his science book? As he thinks, he notices something interesting: light travels in a straight line through a uniform medium.
These thoughts lead him to explore sources of light, how light behaves when it meets different materials, and how images and shadows are formed.
2. Sources of Light
Light enables us to see objects and is produced by various sources, both natural and artificial.
Natural Sources of Light
Natural sources of light occur in nature. They include the Sun, stars, fire, lightning and certain animals, such as fireflies, that emit their own light.
Artificial Sources of Light
Humans have developed several artificial sources of light over time. Early artificial sources included fire made by burning fuels such as oil and wax.
With technological advancement, electric lights such as bulbs, tube lights and LED lamps have become common.
Some objects produce their own light, while others are seen because they reflect light. On this basis, objects can be divided into two types: luminous and non-luminous.
Luminous Objects
These are objects that emit their own light.
Examples include the Sun, stars, lightning, natural fire and certain animals such as fireflies.
These objects do not produce their own light but reflect light from luminous sources. The Moon is a non-luminous object that shines by reflecting sunlight.
Other examples include planets such as Mars and Venus, and everyday objects such as mirrors.
Science and Society: LED lamps are energy-efficient, brighter and longer-lasting than traditional bulbs. They help reduce electricity bills and are environmentally friendly. Recognising these benefits, the Indian government actively promotes their use. However, used LED lamps must be properly recycled and not discarded with regular waste.
3. Does Light Travel in a Straight Line?
Light travels in a straight line under normal conditions. This property is known as rectilinear propagation of light.
When light passes through holes that are aligned in a straight line, it creates a bright spot on a screen. If the holes are misaligned, no spot appears. This shows that light normally travels in a straight path and does not bend around an obstacle.
Similarly, a candle flame is visible through a straight pipe but not through a bent pipe. This confirms that light follows a straight path.
Dive Deeper: Light generally travels in a straight line through a uniform medium. However, under special conditions, such as when light passes from one transparent medium to another, its direction can change. This fascinating concept is explored in higher grades.
Caution: Lasers should always be used under teacher supervision. Only low-power laser pointers are suitable for basic use. Never point a laser at anyone's eyes because it can cause serious injury.
4. Light Through Transparent, Translucent and Opaque Materials
When light encounters different materials, its behaviour depends on the properties of the material.
Transparent Materials
Transparent materials allow light to pass through almost completely, enabling us to see objects clearly on the other side.
Examples include glass and clear plastic.
Translucent materials allow some light to pass through, but they scatter the light. Therefore, objects seen through them appear blurred.
Examples include tracing paper and frosted glass.
Opaque materials do not allow light to pass through. They block light completely.
Examples include cardboard, thick cloth and metal sheets.
A shadow is a dark region formed when an object blocks light from reaching a surface.
Conditions for Shadow Formation
Shadow formation requires three components:
A light source, such as the Sun or a torch.
An opaque object to block the light. A translucent object may form a faint shadow.
A screen, such as a wall, floor or the ground, on which the shadow appears.
Opaque objects create the darkest and most clearly defined shadows.
Translucent objects produce lighter and less distinct shadows.
Transparent objects may produce very faint shadows or no noticeable shadow at all.
From these observations, we can conclude:
Shadows are formed when an opaque object blocks light from reaching a surface, which acts as a screen.
To observe a shadow, we need a light source, an opaque object and a screen.
The size, shape and clarity of a shadow depend on the positions of the object, light source and screen.
Changing the colour of an object does not affect the colour of its shadow.
Shadows help us understand the presence and shape of objects, but they do not reveal the colour of the objects.
Fascinating Facts: Shadow play, or shadow puppetry, is an ancient art in which flat cut-out puppets are placed between a light source and a screen to create lifelike movements. Different Indian regions have unique styles, such as Charma Bahuli Natya in Maharashtra, Tholu Bommalata in Andhra Pradesh, Togalu Gombeyaata in Karnataka, Ravana Chhaya in Odisha, and Tholpavakoothu in Kerala. These performances entertain people and convey important messages to the community.
6. Reflection of Light
Reflection occurs when light bounces off a surface and changes its direction.
Reflection by Shiny Surfaces
Shiny surfaces, such as polished steel plates and plane mirrors, reflect light efficiently and can create bright spots on nearby surfaces.
For example, tilting a mirror can direct sunlight onto walls that are not directly lit.
When a beam of light strikes a plane mirror, its direction changes and it travels as a reflected beam. This is why mirrors are used to redirect light in many applications.
A plane mirror is a flat mirror that forms an image of an object placed in front of it. The image has several characteristic properties.
Image Characteristics
Same size: The image is the same size as the object.
Erect: The image is upright, with the top of the object appearing at the top.
Laterally inverted: The left side of the object appears as the right side in the image, and vice versa. This is called lateral inversion. For example, when you raise your left arm, the image appears to raise its right arm.
Ambulance Example: The word "AMBULANCE" is written in reverse letters on the front of ambulances. Drivers in vehicles ahead can see the word correctly in their rear-view mirrors. This helps them quickly recognise an approaching ambulance and give way.
Virtual: The image appears behind the mirror and cannot be obtained on a screen.
Distance Relationship: The image's distance behind the mirror is equal to the object's distance in front of the mirror. Moving closer to the mirror makes the image appear closer, while moving farther away makes it appear farther away.
Fascinating Fact: The exact time when mirrors were invented is unknown. Early mirrors were made by polishing stone or metal. With the invention of glass mirrors, the traditional art of making metal mirrors declined but still survives in places such as Kerala, where the unique Aranmula Kannadi metal mirror has been crafted for centuries.
7. Pinhole Camera
A pinhole camera is a simple device that uses a tiny hole to form an image on a screen.
Working Principle
Light rays from an object pass through a small hole, called a pinhole, and form an inverted, or upside-down, image on a screen such as tracing paper. The image shows the colours of the object but is reversed vertically.
Making a Sliding Pinhole Camera
Use two cardboard boxes, with one box slightly smaller so that it can slide inside the other. Remove one face of each box. Make a small hole in the face opposite the open side of the larger box.
Cut a square opening in the face opposite the open side of the smaller box and cover it with thin translucent paper, such as tracing paper, to act as a screen.
Slide the smaller box inside the larger one so that the tracing paper is inside.
Point the pinhole side towards a distant object in bright sunlight.
Look through the open side. Cover your head and the camera with a dark cloth so that the image can be seen clearly.
Adjust the smaller box forwards or backwards until the image appears on the tracing paper.
The image formed on the tracing paper is upside down, or inverted.
The image shows the colours of the object on the other side.
Dive Deeper: A pinhole camera forms an upside-down image, while a mirror produces a laterally inverted image that is not upside down. These concepts are explored further in higher grades.
8. Making Some Useful Items
The properties of light, such as its straight-line travel and reflection, are used to create practical devices.
Periscope
A simple periscope is made by placing two plane mirrors inside a Z-shaped box.
The reflections from these mirrors allow us to see objects that are not directly visible.
Periscopes are commonly used in submarines and tanks, and by soldiers to see outside their bunkers. You can also use one to see over taller people.
A kaleidoscope is made by joining three rectangular plane mirrors in a triangular shape and placing them inside a circular tube.
Coloured pieces, such as broken bangles or beads, are placed at one end. This end is covered with a transparent sheet and tracing paper.
Looking through the open end shows beautiful, ever-changing patterns formed by multiple reflections from the three mirrors.
Artists often use kaleidoscopes to find inspiration for new designs.
Luminous objects: Objects that emit their own light, such as the Sun, stars and fireflies.
Non-luminous objects: Objects that do not emit light but reflect light from other sources, such as the Moon and a mirror.
Rectilinear propagation: The property of light travelling in a straight line.
Transparent materials: Materials that allow light to pass through almost completely, such as glass.
Translucent materials: Materials that allow some light to pass through and scatter it, causing blurred visibility, such as tracing paper.
Opaque materials: Materials that block light completely, such as cardboard.
Shadow: A dark region formed when an object blocks light from reaching a surface.
Reflection: The change in direction of light when it bounces off a surface, such as a mirror.
Lateral inversion: The left-right reversal of an image in a plane mirror, in which the left side appears as the right side.
Pinhole camera: A device that uses a tiny hole to form an inverted image of an object on a screen.
Periscope: A device with two plane mirrors that allows us to view objects that are not directly visible. It is used in submarines and bunkers.
Kaleidoscope: A device with three mirrors that forms symmetrical patterns from reflected objects and is used for design inspiration.
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