đĄ Chapter Introduction
Light is an important form of energy that allows us to see the world around us. We see objects when light from a source reaches our eyes, either directly or after reflecting from an object.
In this chapter, we explore how light travels, how shadows are formed, and how light behaves when it falls on different surfaces. We also learn about reflection and the images formed by mirrors.
âī¸ 1. What is Light?
Light is a form of energy that enables us to see objects. Objects can be classified according to whether they produce their own light or merely reflect light.
Luminous Objects
Objects that produce their own light are called luminous objects.
Examples: Sun, stars and a burning candle.
Non-Luminous Objects
Objects that do not produce their own light but can be seen when light falls on them are called non-luminous objects.
Examples: Book, chair and Moon.
đī¸ 2. How Do We See Objects?
We see an object when light from the object enters our eyes. For most objects around us, the light reaching our eyes has first been reflected from their surfaces.
đ 3. Materials and Light
Different materials interact with light in different ways. Based on how much light can pass through them, materials can be classified into different categories.
Transparent
Materials through which most light can pass are called transparent materials.
Example: Clear glass.
Translucent
Materials that allow some light to pass through but scatter it are called translucent materials.
Example: Frosted glass.
Opaque
Materials that do not allow light to pass through them are called opaque materials.
Example: Wood.
đ 4. What is a Shadow?
A shadow is a dark region formed when an opaque object blocks the path of light.
A shadow generally requires a source of light, an opaque object and a screen or surface on which the shadow can form.
đĻ 5. Conditions Required for a Shadow
A simple shadow experiment can be performed using a torch, an opaque object and a wall.
- There must be a source of light.
- There must be an opaque object that blocks the light.
- There should be a surface or screen on which the shadow falls.
đ 6. Size of a Shadow
The size of a shadow can change depending on the relative positions of the light source, object and screen.
Object closer to the light source
When an opaque object is moved closer to a light source, its shadow can become larger.
Object closer to the screen
When the object is moved closer to the screen, its shadow can become smaller and sharper, depending on the arrangement.
đ 7. Shape of a Shadow
The shape of a shadow depends on the shape of the object and the positions of the light source, object and screen.
Changing the position of an object or light source can change the appearance and size of its shadow.
đˇ 8. Pinhole Camera
A simple pinhole camera can be made using a box with a tiny hole on one side and a screen on the opposite side.
Light from an object travels through the tiny hole and produces an image on the screen.
How does it work?
- Light travels in straight lines.
- Light from different points of an object travels through the tiny hole.
- The rays cross at the hole.
- An image is formed on the opposite screen.
đ 9. Light Travels in Straight Lines
In a uniform medium, light generally travels in straight lines. This is called the rectilinear propagation of light.
The formation of shadows and the working of a pinhole camera provide simple demonstrations of this property of light.
đĒ 10. Reflection of Light
When light falls on a surface and bounces back, the phenomenon is called reflection of light.
Mirrors provide a familiar example of a surface that reflects a large amount of light in a regular manner.
đ 11. Important Terms in Reflection
Incident Ray
The ray of light that falls on a reflecting surface is called the incident ray.
Reflected Ray
The ray that bounces back from the reflecting surface is called the reflected ray.
Normal
An imaginary line drawn perpendicular to the reflecting surface at the point of incidence.
Angle of Incidence
The angle between the incident ray and the normal.
Angle of Reflection
The angle between the reflected ray and the normal.
âī¸ 12. Laws of Reflection
Reflection follows certain rules called the laws of reflection.
First Law
The incident ray, reflected ray and normal at the point of incidence lie in the same plane.
Second Law
These laws help us understand how light behaves when it strikes a reflecting surface.
đĒ 13. Plane Mirror
A plane mirror is a flat reflecting surface. The mirror in many household dressing mirrors is an example of a plane mirror.
Characteristics of an image in a plane mirror
- The image is upright.
- The image is virtual.
- The image is of the same size as the object.
- The image appears as far behind the mirror as the object is in front.
- The image shows lateral inversion.
âī¸ 14. Lateral Inversion
The apparent left-right reversal produced by a plane mirror is called lateral inversion.
This is why some letters or words may appear reversed when viewed in a mirror.
đ§ 15. Object and Image in a Plane Mirror
| Property | Image in a Plane Mirror |
|---|---|
| Orientation | Upright |
| Size | Same size as the object |
| Position | Appears behind the mirror |
| Distance | Image distance equals object distance |
| Left-right appearance | Lateral inversion |
⨠16. Regular and Diffuse Reflection
Reflection can occur from smooth as well as rough surfaces. The nature of the surface affects the way reflected light travels.
Regular Reflection
Reflection from a smooth surface where reflected rays remain orderly and a clear image can be formed.
Example: Plane mirror.
Diffuse Reflection
Reflection from an uneven surface where reflected rays scatter in different directions.
Example: Rough wall.
đ 17. Shadow vs Reflection
| Shadow | Reflection |
|---|---|
| Forms when an object blocks light. | Occurs when light bounces from a surface. |
| Usually appears as a dark region. | Can produce an image of an object. |
| Requires an opaque object. | Requires a reflecting surface. |
| Does not reproduce the object's colours. | Can show many visual features of the object. |
đ 18. Light in Everyday Life
We encounter reflection, shadows and other effects of light throughout everyday life.
- Mirrors allow us to see our reflection.
- Shadows help us understand the direction of light.
- Periscopes use reflection to help us see objects that are not directly in our line of sight.
- Rear-view mirrors help drivers observe vehicles behind them.
- Reflective surfaces are used in many optical devices.
- Pinhole cameras demonstrate the straight-line travel of light.
⥠Chapter 11 Quick Revision
- Light enables us to see objects.
- Luminous objects produce their own light.
- Non-luminous objects do not produce their own light.
- Transparent materials allow most light to pass through.
- Translucent materials allow some light to pass through.
- Opaque materials do not allow light to pass through.
- A shadow is formed when an opaque object blocks light.
- Light generally travels in straight lines in a uniform medium.
- Reflection is the bouncing back of light from a surface.
- The angle of incidence is equal to the angle of reflection.
- A plane mirror forms an upright, virtual image of the same size.
- Lateral inversion is the apparent left-right reversal in a mirror.
- Smooth surfaces can produce regular reflection.
- Rough surfaces produce diffuse reflection.
đ Important Terms
âī¸ Important Practice Questions
1. What is light?
Light is a form of energy that enables us to see objects.
2. What are luminous objects?
Objects that produce their own light are called luminous objects. The Sun and stars are examples.
3. What are transparent materials?
Materials that allow most light to pass through them are called transparent materials.
4. What is a shadow?
A shadow is a dark region formed when an opaque object blocks the path of light.
5. What are the conditions required to form a shadow?
A light source, an opaque object and a screen or surface are generally required for a clear shadow.
6. What is reflection?
Reflection is the bouncing back of light from a surface.
7. State the laws of reflection.
The incident ray, reflected ray and normal lie in the same plane, and the angle of incidence is equal to the angle of reflection.
8. What is lateral inversion?
Lateral inversion is the apparent left-right reversal produced by a plane mirror.
9. What is a plane mirror?
A plane mirror is a flat reflecting surface.
10. What is the difference between regular and diffuse reflection?
Regular reflection occurs from smooth surfaces and produces orderly reflected rays, while diffuse reflection occurs from rough surfaces and scatters reflected light in different directions.
đ¤ Think & Explore
â Frequently Asked Questions
What is Chapter 11 of Class 7 Science?
Chapter 11 of Class 7 Science is titled Light: Shadows and Reflections.
What are the main topics in this chapter?
The chapter covers light, luminous and non-luminous objects, transparent, translucent and opaque materials, shadows, straight-line travel of light, reflection and mirrors.
How is a shadow formed?
A shadow forms when an opaque object blocks light travelling from a light source.
What is reflection of light?
Reflection is the bouncing back of light when it falls on a surface.
What is the law of reflection?
One important law of reflection states that the angle of incidence is equal to the angle of reflection.
What is lateral inversion?
Lateral inversion is the apparent left-right reversal of an image formed by a plane mirror.
What type of image is formed by a plane mirror?
A plane mirror forms an upright, virtual image of the same size as the object, with lateral inversion.