Chapter 10: Light: Mirrors and Lenses
Original notes — ready CBSE (NCERT, English Medium) • Class 8 • Science • Book: Curiosity
Chapter 10: Light: Mirrors and Lenses — EG's own notes, in simple words
Book: Curiosity (CBSE, Class 8) | Chapter 10 | Medium: English
Diagram (labels are printed on the image): Diagram (labels are printed on the image): a round glass lens in the centre with light beams, and small mirrors above it. A general picture about lenses and reflection, not a ray diagram from the book.
Looking through a hand lens at a magnified image. A general picture about lenses, not a book scene.
What the chapter is about
Light travels in straight lines, and when it meets a surface it can bounce back or bend. Bouncing back is called reflection, and bending when light passes from one medium to another is called refraction. Mirrors and lenses work with these two behaviours.
A plane mirror gives an upright image of the same size, but left and right appear swapped — this is lateral inversion. Curved mirrors behave differently: a concave mirror curves inwards and can focus light to a point, while a convex mirror curves outwards and gives a smaller, wider view. That is why convex mirrors are used as rear-view mirrors in vehicles.
Lenses are transparent pieces of glass or plastic that refract light. A convex lens is thicker in the middle and converges light rays to a focal point; it can magnify objects. A concave lens is thinner in the middle and diverges light rays, making objects look smaller. Lenses are used in spectacles, cameras and microscopes.
What you should learn
Notice the difference between reflection and refraction, what concave and convex mirrors and lenses do, and where each is used.
Key Terms
- Reflection — The bouncing back of light when it strikes a surface.
- Refraction — The bending of light when it passes from one medium to another.
- Concave mirror — A mirror curved inwards, like the inside of a spoon; it can focus light.
- Convex lens — A lens thicker in the middle than at the edges; it converges light rays.
- Focal point — The point where rays of light meet (or appear to meet) after reflection or refraction.
Remember
- Plane mirrors give upright, same-size images with lateral inversion.
- Concave mirrors focus light; convex mirrors give a wider, smaller view.
- Convex lenses converge light and magnify; concave lenses diverge light.
- Reflection is bouncing back; refraction is bending on entering a new medium.
Exam Tip / Common Mistake
Common mistake: mixing up concave and convex. Remember: a concave mirror curves in like a cave, and a convex lens bulges out in the middle — the shapes tell you the names.
Chart / Table: Mirrors and lenses
| Point | Detail |
|---|---|
| Plane mirror | Upright, same size, laterally inverted image |
| Concave mirror | Focuses light; used in torches, shaving mirrors |
| Convex mirror | Wide, smaller view; used as rear-view mirrors |
| Convex lens | Converges light; magnifies; used in cameras, spectacles |
| Concave lens | Diverges light; used in some spectacles |
Practice set — try it yourself
- What is reflection? What is refraction?
Hint: Think about bouncing and bending. - Why does a plane mirror swap left and right?
Hint: Think about lateral inversion. - Which mirror is used as a vehicle's rear-view mirror, and why?
Hint: Think about a wide view. - How does a convex lens differ from a concave lens?
Hint: Think about thick and thin middles. - Why does a spoon in a glass of water look bent?
Hint: Think about light changing medium.
Questions with Answers
- What is the difference between reflection and refraction?
Reflection is light bouncing back from a surface, like from a mirror. Refraction is light bending when it passes from one medium to another, like from air into water. - Why is the image in a plane mirror laterally inverted?
A plane mirror reverses the front-back direction. Our brain reads this as a left-right swap, so a person's right hand appears as the left hand in the mirror image. - Why are convex mirrors used as rear-view mirrors?
Convex mirrors diverge reflected light and give a smaller, upright image covering a wider area. The driver can therefore see more of the road behind the vehicle. - How does a magnifying glass work?
A magnifying glass is a convex lens. It refracts light rays so they converge, and when an object is placed close to it, we see a larger, upright, virtual image of the object. - Why does a pencil in water look broken at the surface?
Light coming from the underwater part of the pencil refracts — bends — when it leaves the water and enters air. Our eyes follow the bent rays back in a straight line, so the pencil appears shifted and broken.
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.