Master Chapter 11 Class 7 - Light: Shadows and Reflections (Curiosity) with comprehensive NCERT Solutions, Practice Questions, MCQs, Sample Papers, Case Based Questions, and Video lessons.
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Welcome to Chapter 11. Our journey begins in a village in the Western Ghats, where a boy from the city named Keshav is visiting his friend Jatin. Having always lived in a big city, Keshav is amazed by the dark, quiet world of the village, a world filled with new sounds and sights.
But the most fascinating sight of all, as he discovers, is the dance of hundreds of fireflies at night, flashing their tiny lights in the darkness. Later, on the bus ride home, he watches the headlights of the bus cut sharp beams through the night air and sees the entire landscape lit up by the soft glow of the Moon.
This experience fills Keshav with wonder and a series of deep, scientific questions:
Where does light come from?
Why do fireflies and the bus headlights produce their own light, while the Moon just glows?
Does the Moon make its own light, or is it just reflected sunlight?
And, as he watches the beams from the bus, he has a key insight: "light seems to move in a straight line!".
Is he right? This chapter is our investigation to answer these questions. We are about to embark on a journey into the fundamental nature of light. We will explore what it is, where it comes from, and the simple, unchangeable rules that govern its behavior. We will learn how these simple rules are responsible for everything from a simple shadow to the beautiful, complex images we see in a mirror.
Our first task is to sort the world into two new categories.
Luminous Objects: These are objects that emit their own light. They are sources of light. We will learn that the Sun is our main natural source. Other natural sources include stars, lightning, and even living things like fireflies. Artificial sources include a burning candle, a diya, and an electric lamp .
Non-Luminous Objects: These are objects that do not emit their own light. This is almost everything else you see—your book, your desk, the trees, and even the Moon. These objects are only visible because they reflect the light from a luminous source (like the Sun) into our eyes . This solves Keshav's first mystery: moonlight is just reflected sunlight.
Keshav's observation on the bus is the most important rule we will learn in this chapter. But how can we prove it? We will become experimenters.
The Aligned Holes Experiment: We will set up three matchboxes with a tiny hole in each, all in a perfect line. We will find that the light from a torch can pass straight through all three holes and hit a screen . But if we move just one of the matchboxes, the holes are no longer aligned, and the path is blocked. The light does not bend around the obstacle .
The Pipe Experiment: We will try to look at a candle flame. With a straight pipe, it is easy to see. With a bent pipe, it is impossible .
Both of these experiments prove the same, simple fact: Light travels in a straight line. This single rule is the key to understanding everything that comes next.
If light travels in a straight line, what happens when something gets in its way? This leads us to our next investigation: shadows.
First, we will need to remember our Grade 6 concepts of transparent, translucent, and opaque materials.
Transparent materials (like glass) let light pass through almost completely.
Translucent materials (like tracing paper) let light pass through partially.
Opaque materials (like cardboard or wood) block the light completely.
A shadow is nothing more than a dark patch formed on a screen where light has been blocked by an opaque object .
To see a shadow, we will discover that you always need three things:
A source of light (like a torch).
An opaque object (to block the light).
A screen (like a wall or the ground) for the shadow to be formed on .
We will also explore the properties of shadows. We will find that the colour of the object does not change the colour of the shadow (it is always dark). We will also see how the size and shape of the shadow can change depending on the position of the light and the object.
These core ideas—how light travels and how shadows are formed—are the foundation of optics. They seem simple, but they explain complex phenomena. It can be tricky to connect the straight-line path of light to the upside-down image in a pinhole camera. This is where Teachoo comes in. We are here to guide you step-by-step through each experiment, breaking down the why behind each observation. We help you trace the path of light, so you can see for yourself why a shadow forms, why a pinhole image is inverted, and why a mirror flips your reflection.
Our experiments with opaque objects lead to a new question. What happens if the opaque object is shiny, like a mirror or a polished steel plate?
It still creates a shadow, but it also does something else: it creates a bright spot of light, often on a completely different wall!. The shiny surface has changed the direction of the light.
This change in the direction of light by a mirror or shiny surface is called reflection of light.
We will investigate this by creating a single, thin beam of light (using a torch and a comb). When we place a plane mirror (a flat mirror) in its path, we will clearly see the beam of light bounce off the mirror and travel in a new, straight line .
This "bouncing" of light is what allows a mirror to do its most famous trick: create an image. When you look in a mirror, you are seeing your image, which is a reflection of you (the object) .
We will become detectives and study the image in a plane mirror, discovering its four unchanging rules:
The image is the same size as the object.
The image is erect (it is upright, not upside down) .
The image cannot be obtained on a screen. It only appears to be "behind" the mirror.
The image is laterally inverted. This is the strange left-right reversal. When you raise your left hand, your image raises its right hand . This is why the word AMBULANCE is written in reverse on the front of the vehicle, so drivers can read it correctly in their rear-view mirrors .
Once we have mastered these two great laws—that light travels in straight lines and that it reflects off mirrors—we can use them to build amazing things.
The Pinhole Camera: First, we will build a simple camera that has no mirror at all. A pinhole camera is just a box with a tiny hole (a pinhole) on one side and a translucent screen on the other. We will discover that this simple device creates a real image! But there is a surprise: the image is upside down (inverted). Why? Because light travels in straight lines! The light from the top of a tree travels straight through the pinhole and hits the bottom of the screen, while light from the bottom of the tree travels to the top of the screen.
The Periscope: We will learn how to use reflection to see around corners. A periscope is a Z-shaped box that uses two plane mirrors to bounce light around an obstacle. It is used in submarines and by soldiers in bunkers to see things that are not in their direct line of sight .
The Kaleidoscope: Finally, we will build one of the most beautiful toys ever created. A kaleidoscope uses three rectangular mirrors joined to form a triangle. When you put a few simple, broken pieces of coloured bangles inside and look through, the multiple reflections of reflections create an endless variety of stunning, symmetrical patterns .
From a simple firefly in a dark forest to the endless patterns in a kaleidoscope, this chapter will show you that the world is full of light, and by understanding its simple rules, you can understand—and create—true wonder.
To get started on this brilliant journey, click on any topic link to begin your exploration.