Educational Galaxy

Chapter 11: Light: Shadows and Reflections — EG's own notes, in simple words

Book: Curiosity (CBSE, Class 7)  |  Chapter 11  |  Medium: English

Diagram (labels are printed on the image): A lamp sending out a straight beam of light that falls on a surface. The picture shows light travelling in straight lines rather than a mirror; it is a general diagram, not a scene from the book.

A child seeing a reflection in a mirror. A general picture about light, not a book scene.

What the chapter is about

Light travels in straight lines. This simple fact explains shadows. When an opaque object blocks light, a dark region forms behind it where light cannot reach. This is the shadow. The dark central part is the umbra, and the lighter outer part, where only some light is blocked, is the penumbra. Transparent objects let light pass and cast almost no shadow, while translucent objects let some light through and cast a faint shadow.

When light falls on a smooth, shiny surface like a mirror, it bounces back. This is reflection. The ray that falls on the mirror is the incident ray, and the ray that bounces off is the reflected ray. The two laws of reflection say: the angle of incidence equals the angle of reflection, and the incident ray, the reflected ray and the normal all lie in the same plane.

A plane mirror forms an image that is upright, the same size as the object, and as far behind the mirror as the object is in front. The image is laterally inverted, which means left and right appear swapped — that is why the word on an ambulance is written backwards, so drivers see it correctly in their mirrors.

11.1 Sources of light

The chapter opens in a Maharashtra village where Keshav watches hundreds of fireflies flashing at night. Fireflies are natural sources of light, like the Sun and the stars; electric lights, lamps and torches are artificial sources. Objects that emit their own light are called luminous objects. Objects that do not emit their own light are called non-luminous objects: the Moon does not make its own light, it only reflects sunlight, which is why it shines in our sky. Sadly, fireflies are becoming fewer because of light pollution, loss of forest cover and excessive tourism — the chapter asks you to ask elders whether fireflies were seen in your region earlier.

11.2 Does light travel in a straight line?

Yes: light travels in a straight line. That is why a torch beam looks straight, why shadows have the shape of the object that blocks the light, and why light cannot simply bend around a corner to reach a place behind an obstacle. (Bending around corners in a fine way is a higher-grade topic, the chapter notes — for this class, straight lines explain shadows, mirrors and pinhole images.)

11.3 Light through transparent, translucent and opaque materials

Clear glass and clean water are transparent: light passes through them almost fully, so things behind them are seen clearly. Frosted glass and butter paper are translucent: they let some light through, but the view is blurred. Wood, metal and cardboard are opaque: they block light completely, and that is why opaque objects cast shadows while transparent ones do not.

11.4 Shadow formation

When an opaque object comes in the path of light, it blocks the light and a dark patch — its shadow — appears on the screen or ground behind it. The shadow's size depends on distances: move the torch closer to the ball and the shadow grows larger; move the torch farther away and the shadow shrinks. Move the ball closer to the screen and the shadow gets smaller and sharper; move it away and it grows. Your own shadow through the day follows the same rule: it is longest in the morning and evening and shortest around noon, when the Sun is highest.

11.6 Images formed in a plane mirror

A plane mirror forms an image that is erect (upright), the same size as the object, and laterally inverted — your left hand appears as the right hand in the image, and your right as the left. That is why the word on an ambulance is written in reverse: in the rear-view mirror of the vehicle ahead, the reversed letters read correctly as AMBULANCE. The image cannot be caught on a screen placed behind the mirror, unlike a pinhole-camera image. Letters that look the same in a mirror are the symmetrical ones: T and O, for example, appear identical to themselves.

11.7–11.8 Pinhole camera, periscope and kaleidoscope

A pinhole camera is a box with a tiny hole on one side and a screen on the other. Light from an object passes through the pinhole in straight lines and forms an image on the screen — upside down, that is, inverted. A tree viewed through a pinhole camera therefore appears with its top at the bottom. Two useful devices use plane mirrors: a periscope places two plane mirrors in a Z-shaped box so that reflections from the two mirrors let you see over an obstacle, and a kaleidoscope uses three rectangular mirror strips to make repeating coloured patterns.

What you should learn

Learn why shadows form, the two laws of reflection, and the properties of an image in a plane mirror, including lateral inversion.

• Classify sources as natural or artificial, and objects as luminous or non-luminous.
• Predict how shadow size changes when the torch, object and screen move.
• Describe plane-mirror images (erect, same size, laterally inverted) and pinhole images (inverted).

Chart / Table: Shadows and mirror images

Table: Shadows and mirror images

Steps: making a pinhole camera

1. Take a box and make a tiny hole (pinhole) in the middle of one side.
2. Fix a translucent screen (such as butter paper) on the opposite side inside the box.
3. Point the pinhole at a bright object in daylight and look at the screen: the image is inverted.

Practice set — try it yourself

1. Why does an opaque object cast a shadow?
   Hint: Think about light travelling straight.
2. State the first law of reflection.
   Hint: Think about the two angles.
3. What is the normal in reflection?
   Hint: Think about a line at right angles to the mirror.
4. Why is AMBULANCE written backwards on the vehicle?
   Hint: Think about mirror images.
5. How is a shadow from a translucent object different?
   Hint: Think about some light passing through.

Questions with Answers

1. Why do shadows form?
   Light travels in straight lines. When an opaque object comes in its path, light cannot pass through, so a dark region, the shadow, forms behind the object where light does not reach.
2. State the laws of reflection.
   First, the angle of incidence equals the angle of reflection. Second, the incident ray, the reflected ray and the normal to the surface at the point of incidence all lie in the same plane.
3. Describe the image formed by a plane mirror.
   The image is upright, the same size as the object, and as far behind the mirror as the object is in front of it. It is a virtual image and it is laterally inverted, so left and right appear swapped.
4. What is lateral inversion? Give one daily example.
   Lateral inversion is the swapping of left and right in a mirror image. For example, the word AMBULANCE is written in reverse on ambulances so that drivers ahead see it correctly in their rear-view mirrors.
5. Why does a clear glass slab cast almost no shadow?
   Glass is transparent, so light passes through it almost fully. With no light blocked, there is almost no dark region behind it, unlike an opaque object.

More questions with answers — extension

1. Match the torch position with the shadow: torch close to the ball / torch far away / ball removed / two torches on the left — with larger shadow / smaller shadow / bright spot / two shadows.
   Torch close to the ball → larger shadow. Torch far away → smaller shadow. Ball removed → a bright spot (no shadow at all). Two torches on the left → two shadows, one from each torch.
2. You measure your shadow at 9 AM, 12 noon and 4 PM. At which time is it shortest, and why?
   Shortest at 12 noon. At noon the Sun is highest in the sky, so the light falls most steeply and the shadow is shortest; in the morning and evening the Sun is lower, so shadows stretch long.
3. Write your name on paper and hold it parallel to a plane mirror. Sketch the image in words. What difference do you notice, and why?
   The image is laterally inverted: each letter that is not symmetrical appears reversed left-to-right, so the whole name reads backwards. This happens because the mirror swaps left and right while keeping up and down the same.
4. Statements: A — the image in a plane mirror is laterally inverted. B — the images of the letters T and O appear identical to themselves in a plane mirror. Choose: both true, both false, only A true, or only B true.
   Both statements are true. Lateral inversion is a real property of plane-mirror images, and T and O are symmetrical letters, so reversing them left-to-right leaves them looking unchanged.
5. You are given a tube and two small plane mirrors. Can it be made into a periscope? If yes, where would you fix the mirrors?
   Yes. Fix one mirror at the top end of the tube and one at the bottom end, each tilted at 45 degrees, facing each other in a Z arrangement. Light from above reflects off the top mirror down the tube, then off the bottom mirror into your eye, letting you see over an obstacle.
6. A bird flying high in the sky casts no visible shadow on the ground, but when it swoops low, its shadow appears. Explain.
   High up, the bird is far from the ground, so its shadow is spread over a wide area and blurred — the Sun is not a point source, so with distance the shadow's dark core shrinks and its faint outer part spreads until it is too faint to notice. Near the ground, the shadow is compact, dark and sharp enough to see.
7. Suppose you view a tree through a pinhole camera. Describe the image you would sketch.
   An inverted image: the tree appears upside down on the screen, with its top at the bottom and its roots at the top, because light from the top of the tree travels in a straight line through the pinhole to the bottom of the screen, and light from the bottom travels to the top.
8. Can a small piece of plane mirror form an image of a large tree? Predict, then explain how you would test it.
   Yes — size does not limit a mirror's image. Hold the small mirror so it faces the tree and look into it: you will see the whole tree, or a part of it depending on distance and angle. The image is the same size as the object in a plane mirror, but only the part within the mirror's view reaches your eye.
9. Why is the Moon called non-luminous even though it shines brightly at night?
   Because the Moon produces no light of its own: its brightness is reflected sunlight. An object is luminous only if it emits its own light, like the Sun or a firefly; the Moon merely reflects light made elsewhere.
10. A frosted glass window lets light in but you cannot see clearly through it. Is it transparent, translucent or opaque — and would it cast a sharp shadow?
    Translucent: it lets some light through but scatters it, so the view is blurred. It would not cast a sharp, dark shadow like an opaque object; at most a faint, diffuse patch, because light still passes through it.

How to use this page: Read the summary, say the Key Terms in your own words, then try the practice set without looking. Use the Questions with Answers to check yourself afterwards. Ask your teacher or parent to check your answers.