Schooling · Class 8 · Science · Light: Mirrors and Lenses · Chapter on a page
CBSE Class 8 · Science · NCERT Curiosity
Light: Mirrors and Lenses: chapter on a page
A cut of Shishya's notes for this chapter: the first paragraph of the introduction and the key ideas, formulas and facts, common mistakes and quick revision lists, word for word. Nothing is added or reworded; the worked examples are left out (they are on the chapter page).
About the notes: Written by Shishya's AI on 26 Sept 2026 from the chapter's title and class level, in Shishya's own words — not a copy or summary of the textbook. Read the official chapter for the book's own text, activities and exercises.
This note's current text has no applied fact-check on record.
Mind map
Drawn from the same notes by a fixed rule (no AI): each label is the item's own words — its bold term, its label before a colon, a short formula as written, or its first words.
The mind map as a list
- Key ideas
- Light travels in straight lines…
- When light bounces off a surface…
- A plane mirror forms a virtual image…
- A concave mirror curves inward like…
- A convex mirror curves outward. It…
- The focal length of a curved mirror…
- A lens is a transparent piece of…
- Formulas and facts
- Law of reflection
- For a spherical mirror, focal length…
- Real images are formed when rays…
- Virtual images are formed where rays…
- A convex mirror always gives a…
- A concave mirror can magnify…
- A convex lens is called a converging…
- Watch out for
- Confusing concave and convex
- Thinking virtual images are not real…
- Measuring angles from the mirror…
- Believing a plane mirror magnifies
- Mixing up converging and diverging
What this chapter is about
This chapter explores how light behaves when it bounces off mirrors or passes through lenses. You will learn why you see your reflection in a mirror, how a magnifying glass makes things look bigger, and why spectacles help people see clearly.
Key ideas
- Light travels in straight lines called rays. A group of rays together is a beam.
- When light bounces off a surface, the angle of incidence equals the angle of reflection. This is the law of reflection.
- A plane mirror forms a virtual image that is the same size as the object, at the same distance behind the mirror as the object is in front.
- A concave mirror curves inward like the inside of a spoon. It can form real images (which can be caught on a screen) or virtual images depending on where the object is placed.
- A convex mirror curves outward. It always forms a smaller, virtual and upright image, which is why it is used as a rear-view mirror in vehicles.
- The focal length of a curved mirror is the distance from its centre to the point where parallel rays meet (concave) or appear to come from (convex).
- A lens is a transparent piece of glass or plastic with curved surfaces. A convex lens is thicker in the middle and bends light inward. A concave lens is thinner in the middle and spreads light outward.
- Convex lenses can form real or virtual images; concave lenses always form virtual, upright and smaller images.
Formulas and facts to remember
- Law of reflection: angle of incidence (i) = angle of reflection (r). Both angles are measured from the normal (a line perpendicular to the surface).
- For a spherical mirror, focal length f ≈ R/2, where R is the radius of curvature.
- Real images are formed when rays actually meet; they can be projected on a screen and are usually inverted.
- Virtual images are formed where rays only appear to meet; they cannot be caught on a screen and are usually upright.
- A convex mirror always gives a diminished, virtual, upright image—useful in shops and vehicle mirrors for a wide view.
- A concave mirror can magnify; dentists and makeup mirrors use it.
- A convex lens is called a converging lens; a concave lens is called a diverging lens.
- Power of a lens (in dioptres, D) = 1 / focal length in metres. A lens with f = 0.5 m has power 2 D.
Common mistakes
- Confusing concave and convex → Remember: concave curves inward (like a cave), convex bulges outward.
- Thinking virtual images are not real at all → Virtual images are real to your eyes; they just cannot be projected on a screen.
- Measuring angles from the mirror surface instead of the normal → Always measure angles from the normal, the perpendicular line at the point where light strikes.
- Believing a plane mirror magnifies → A plane mirror produces an image of exactly the same size; only curved mirrors or lenses change size.
- Mixing up converging and diverging → Convex lens converges light; concave lens diverges light. For mirrors it is the opposite: concave converges, convex diverges.
Quick revision
- Angle of incidence = Angle of reflection (measured from the normal).
- Plane mirror: virtual, same-size, laterally inverted image.
- Concave mirror: can form real or virtual images; used for magnification.
- Convex mirror: always virtual, smaller, upright image; gives a wide field of view.
- Convex lens converges light; concave lens diverges light.
- Focal length f = R/2 for spherical mirrors.