Class 8 › Science › Chapter 10
Light: Mirrors and Lenses
Concave and convex mirrors, the images they form, laws of reflection, how mirrors and lenses converge or diverge light, and the uses of mirrors and lenses.
1. Spherical Mirrors
Spherical mirror: a curved mirror whose reflecting surface is part of a hollow sphere. A shiny spoon is a simple example: the inner (curved in) side gives an inverted image; the outer (bulging) side gives an erect, smaller image.
Concave mirror
Curves inwards
The reflecting surface curves inwards (like the inside of a bowl).
Convex mirror
Curves outwards
The reflecting surface curves outwards (bulges towards you).
- Identifying them: place the mirrors on a table with reflecting surface facing up and view from the side at eye level to see if the surface curves in or out. You can also tell plane, concave and convex apart by the image they form.
- Representation: the non-reflecting side is drawn shaded.
- How they are made: by grinding and polishing a flat glass piece into a curved surface (not by slicing a hollow sphere). A reflective coating (e.g. aluminium) on the outer curved surface gives a concave mirror; on the inner curved surface gives a convex mirror.
2. Images Formed by Spherical Mirrors
| Mirror | Object close to mirror | Object far from mirror |
|---|---|---|
| Plane | Always erect, same size as object | |
| Concave | Erect and enlarged | Inverted; initially enlarged, then gets smaller as the object moves away |
| Convex | Always erect and diminished (size decreases only slightly as the object moves away) | |
- In plane mirrors the image size does not depend on distance; in spherical mirrors it changes with the object's distance.
- Lateral inversion (left-right swap) is seen in all three types of mirrors.
- Concave mirror image can be enlarged, diminished or same size; erect or inverted, depending on object distance.
3. Uses of Concave and Convex Mirrors
Concave
Uses
Reflectors of torches and headlights of cars and scooters; dentist's mirror (enlarged view of teeth when held close); main mirror of reflecting telescopes; solar concentrators (e.g. solar cookers, solar furnaces).
Convex
Uses
Side-view mirrors of vehicles; road safety mirrors at sharp bends and intersections; surveillance mirrors in big stores.
- Why convex for side-view mirrors: always gives an erect image of traffic behind, and as it curves outwards it gives a much wider field of view than a plane mirror. Images look smaller, which is why the mirror warns that objects are closer than they appear.
- Convex mirrors at bends let drivers on both sides see each other and prevent collisions.
4. Laws of Reflection
The normal is perpendicular (90°) to the mirror at the point of incidence O.
First law
Angle of incidence (i) = Angle of reflection (r)
Second law
The incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane.
- Terms: ray = straight line with an arrow showing the path of light; incident ray falls on the mirror; reflected ray comes back; normal = line at 90° to the mirror at the point of incidence; i and r are measured from the normal, not from the mirror.
- Along the normal: if the ray falls along the normal, i = 0° and r = 0°, so the ray returns along the same path. Reflection still takes place.
- Second law activity: the reflected beam is visible on a flat sheet but disappears when the sheet is bent (a new plane is created) and reappears when flattened.
- The laws hold for all mirrors: plane, concave and convex.
- Heritage: astronomers in the time of Bhaskara II (800+ years ago) used shallow bowls of water as reflectors to observe star and planet positions.
5. Parallel Beams: Converging and Diverging
Each ray obeys the laws of reflection, but the curved surface makes parallel rays meet (concave) or spread out (convex).
| Device | Effect on parallel beam |
|---|---|
| Plane mirror | Reflected beams stay parallel |
| Concave mirror | Converges the beams |
| Convex mirror | Diverges the beams |
| Thin flat glass plate | Beam passes through unchanged |
| Convex lens (converging lens) | Converges the beams |
| Concave lens (diverging lens) | Diverges the beams |
- Burning paper: a concave mirror (or a convex lens) held facing the Sun concentrates light into a small bright spot that can heat and ignite paper. Safety: never look at the Sun or its reflection, and never focus the light towards anyone's eyes.
- Solar concentrators: devices that use mirrors and lenses to concentrate sunlight; the heat makes steam to generate electricity or provides heat for large-scale cooking and solar furnaces (even for melting steel).
6. What Is a Lens?
Lens: a piece of transparent material (usually glass or plastic) with curved surfaces. A drop of water on a flat surface acts like a simple lens, and a magnifying glass is a lens.
Convex lens
Thicker at the middle
Converging lens. Thin at the edges, thick in the centre.
Concave lens
Thicker at the edges
Diverging lens. Thick at the edges, thin in the centre.
- Mirror vs lens: light passes through a lens, so we see things through a lens, but in a mirror.
- Lenses and mirrors both can be convex or concave, but a convex lens converges while a convex mirror diverges. Do not mix them up.
7. Images Seen Through Lenses
| Lens | Object close to lens | Object far from lens |
|---|---|---|
| Convex | Erect and enlarged | Inverted; initially enlarged, then diminishes as the object moves away |
| Concave | Always erect and diminished (size changes with distance) | |
| Flat glass | Same size and shape | |
- Pattern: the convex lens behaves like the concave mirror, and the concave lens behaves like the convex mirror, as far as the images are concerned.
- A magnifying glass is a convex lens; text looks bigger when it is close to the lens and appears inverted when the lens is moved far away.
8. Uses of Lenses
- Eyeglasses, cameras (including smartphone cameras), telescopes, microscopes and magnifying glasses.
- The eye has a convex lens that can change its shape, allowing us to read a book or see far away.
- A convex lens in sunlight can burn paper by concentrating light (never look at the Sun through it).
- Pencil in a water-filled tumbler: the curved water surface acts like a lens, so the part of the pencil in water looks changed (enlarged and altered in shape).
9. Quick Revision
| Device | Light | Image |
|---|---|---|
| Plane mirror | Parallel stays parallel | Erect, same size |
| Concave mirror | Converges | Erect and enlarged (near); inverted (far) |
| Convex mirror | Diverges | Always erect, diminished |
| Convex lens | Converges | Erect and enlarged (near); inverted (far) |
| Concave lens | Diverges | Always erect, diminished |
Exam tip
Reflection problems
Find the normal first (90° to the mirror). Angle of reflection = angle of incidence. If i = 40°, the reflected ray makes 90° − 40° = 50° with the mirror.
Remember
Matching shortcuts
Wide view of traffic: convex mirror. Enlarged view of teeth: concave mirror. Magnifier: convex lens.
Common mistakes:
• Measuring the angle of incidence or reflection from the mirror surface instead of the normal.
• Saying no reflection happens when light falls along the normal (i = r = 0°).
• Mixing up convex mirror (diverges) with convex lens (converges).
• Saying a convex mirror can form an inverted image (it is always erect and diminished).
• Forgetting that laws of reflection apply to spherical mirrors too.
• Forgetting the safety rule: never look at the Sun or its reflection.
• Measuring the angle of incidence or reflection from the mirror surface instead of the normal.
• Saying no reflection happens when light falls along the normal (i = r = 0°).
• Mixing up convex mirror (diverges) with convex lens (converges).
• Saying a convex mirror can form an inverted image (it is always erect and diminished).
• Forgetting that laws of reflection apply to spherical mirrors too.
• Forgetting the safety rule: never look at the Sun or its reflection.