Chapter Guide
Chapter Overview
1. Spherical Mirrors
The chapter introduces two types of spherical mirrors: concave mirrors, which curve inward, and convex mirrors, which curve outward. Using the inner and outer surfaces of a spoon, students observe how curved mirrors form different images.
2. Images Formed by Mirrors
The size and orientation of an image depend on the type of mirror and the distance of the object. A concave mirror can form enlarged or diminished images that may be erect or inverted. A convex mirror always forms an erect, diminished image and provides a wider field of view.
3. Uses of Spherical Mirrors
Concave mirrors are used in torch reflectors, vehicle headlights, dental mirrors and reflecting telescopes. Convex mirrors are used as vehicle side-view mirrors, road-safety mirrors and surveillance mirrors in shops.
4. Laws of Reflection
The chapter revises the two laws of reflection: the angle of incidence equals the angle of reflection, and the incident ray, reflected ray and normal at the point of incidence lie in the same plane. These laws apply to plane as well as spherical mirrors.
5. Converging and Diverging Mirrors
When parallel rays of light fall on a concave mirror, they come closer together, or converge. A convex mirror spreads the reflected rays apart, or diverges them. The chapter also demonstrates how a concave mirror can concentrate sunlight on a small area.
6. Lenses
A lens is a transparent material with curved surfaces that allows light to pass through it. A convex lens is thicker in the middle than at the edges, while a concave lens is thinner in the middle than at the edges. A curved water droplet can also act like a simple lens.
7. Images Formed by Lenses
A convex lens can form enlarged or diminished images that may be erect or inverted, depending on the object’s distance. A concave lens always forms an erect, diminished image.
8. Converging and Diverging Lenses
A convex lens converges parallel rays of light, while a concave lens diverges them. A convex lens can also concentrate sunlight on a small area, producing heat.
9. Uses of Lenses
Lenses are used in eyeglasses, cameras, telescopes and microscopes. The human eye also contains a convex lens that changes its shape to help us see objects at different distances.
Chapter at a Glance
- Identify concave and convex mirrors and lenses.
- Understand how image size and orientation change with object distance.
- Learn the two laws of reflection.
- Differentiate between converging and diverging mirrors and lenses.
- Explore the uses of mirrors and lenses in daily life.
- Understand how concave mirrors and convex lenses concentrate sunlight.
Learn Clearly
Important Concepts and notes
1. Mirrors
A mirror is a smooth, polished surface that reflects light and helps us see images of objects.
- Plane mirror: A flat reflecting surface. It forms an erect image of the same size as the object.
- Concave mirror: A spherical mirror whose reflecting surface curves inward, like the inner surface of a spoon.
- Convex mirror: A spherical mirror whose reflecting surface curves outward, like the outer surface of a spoon.
2. Images Formed by Mirrors
| Mirror |
Image formed |
| Plane mirror |
Erect and same size as the object. |
| Concave mirror |
When the object is close, the image is erect and enlarged. As the object moves farther away, the image becomes inverted; it is initially enlarged and then becomes smaller. |
| Convex mirror |
Always erect and diminished. The image becomes slightly smaller as the object moves away. |
Lateral inversion: In a mirror image, the left and right sides appear interchanged. This occurs with plane and spherical mirrors.
3. Uses of Mirrors
- Concave mirrors: Used in torch and headlight reflectors, by dentists to see enlarged images of teeth, and in reflecting telescopes.
- Convex mirrors: Used as vehicle side-view mirrors, at road bends and intersections, and for surveillance in shops.
Why are convex mirrors used in vehicles? They provide an erect, diminished image and a wider field of view, allowing drivers to see more of the area behind them. Objects may appear farther away than they actually are.
4. Laws of Reflection
- The angle of incidence is equal to the angle of reflection.
- The incident ray, the normal at the point of incidence, and the reflected ray all lie in the same plane.
Important terms:
- Incident ray: The ray of light that falls on a reflecting surface.
- Reflected ray: The ray of light that bounces off the surface.
- Normal: An imaginary line perpendicular to the surface at the point where light strikes it.
- Angle of incidence: The angle between the incident ray and the normal.
- Angle of reflection: The angle between the reflected ray and the normal.
5. Reflection of Parallel Rays
- Plane mirror: Parallel rays remain parallel after reflection.
- Concave mirror: Parallel rays converge after reflection.
- Convex mirror: Parallel rays diverge after reflection.
A concave mirror can concentrate sunlight onto a small area and produce intense heat. Never look directly at the Sun or at sunlight reflected by a mirror.
6. Lenses
A lens is a transparent object, usually made of glass or plastic, with curved surfaces. Unlike a mirror, a lens allows light to pass through it.
- Convex lens: Thicker in the middle and thinner at the edges.
- Concave lens: Thinner in the middle and thicker at the edges.
A water droplet can also act as a simple lens.
7. Images Formed by Lenses
| Lens |
Image formed |
| Convex lens |
When an object is close, the image appears erect and enlarged. As the object moves farther away, the image becomes inverted, initially enlarged and then smaller. |
| Concave lens |
Always forms an erect and diminished image. Its size varies with the distance of the object. |
8. Behaviour of Parallel Rays Through Lenses
- Convex lens: Converges parallel rays of light.
- Concave lens: Diverges parallel rays of light.
A convex lens can concentrate sunlight onto a small area and produce heat. Never look at the Sun through a lens.
9. Uses of Lenses
- Eyeglasses
- Cameras
- Telescopes
- Microscopes
The human eye contains a convex lens. Its shape changes to help us focus on nearby and distant objects.
10. Quick Revision Table
| Optical device |
Effect on parallel rays |
Image characteristic |
| Plane mirror |
Remain parallel |
Erect, same size |
| Concave mirror |
Converge |
Depends on object distance |
| Convex mirror |
Diverge |
Always erect, diminished |
| Convex lens |
Converge |
Depends on object distance |
| Concave lens |
Diverge |
Always erect, diminished |
Remember These Points
- Concave mirrors and convex lenses converge parallel rays.
- Convex mirrors and concave lenses diverge parallel rays.
- Convex mirrors provide a wider field of view.
- Image formation depends on the type of mirror or lens and the object's distance.
- Never look directly at the Sun through a lens or at sunlight concentrated by a mirror.
Examination Strategy
Exam Tips
- Learn the definitions: Revise plane, concave and convex mirrors, and convex and concave lenses.
- Remember image characteristics: A convex mirror and a concave lens always form erect and diminished images.
- Understand object distance: For concave mirrors and convex lenses, image characteristics change as the object moves closer or farther away.
- Memorise the laws of reflection: The angle of incidence equals the angle of reflection. The incident ray, normal and reflected ray lie in the same plane.
- Practise ray diagrams: Show how parallel rays behave with plane, concave and convex mirrors, and with convex and concave lenses.
- Prepare application-based questions: Know why convex mirrors are used in vehicles and how concave mirrors are used by dentists and in headlights.
- Revise lens uses: Be ready to explain how lenses are used in cameras, eyeglasses, telescopes and microscopes.
- Compare mirrors and lenses: A mirror reflects light, whereas a lens allows light to pass through it.
- Practise short-answer questions: Explain lateral inversion, the wider field of view of convex mirrors, and the converging or diverging action of optical devices.
- Remember the safety rule: Never look directly at the Sun through a lens or at sunlight concentrated by a mirror.
Quick Revision Before the Exam
| Device |
Action on parallel rays |
| Concave mirror |
Converges |
| Convex mirror |
Diverges |
| Convex lens |
Converges |
| Concave lens |
Diverges |
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