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.
Light travels in straight lines, and when it meets a shiny surface it bounces back. This bouncing is called reflection. When light passes from air into glass or water, it bends. This bending is called refraction. Mirrors use reflection; lenses use refraction.
After studying this chapter, you should be able to draw ray diagrams for plane and curved mirrors, understand what focal length means, describe the images formed by convex and concave mirrors and lenses, and connect these ideas to everyday objects like torches, car mirrors and camera lenses.
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.
Worked examples
Example 1: Finding the reflected ray
A ray of light hits a plane mirror at an angle of 35° to the normal. At what angle does it reflect?
Solution: By the law of reflection, angle of reflection = angle of incidence. So the ray reflects at 35° to the normal.
Example 2: Image in a plane mirror
Riya stands 1.5 m in front of a plane mirror. How far from her does her image appear to be?
Solution: The image in a plane mirror forms at the same distance behind the mirror as the object is in front. Image distance behind mirror = 1.5 m. Distance from Riya to her image = 1.5 m + 1.5 m = 3 m.
Example 3: Choosing the right mirror
A watchmaker wants a mirror that shows an enlarged image of tiny parts. Which type of curved mirror should she use, and where should she place the watch part?
Solution: A concave mirror gives a magnified, upright, virtual image when the object is placed between the pole and the focus. The watchmaker should use a concave mirror and keep the watch part close to it, inside the focal length.
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.