Concave and convex
A spherical mirror is a piece of a sphere, curved inwards or outwards.
A shiny spoon
Look at a shiny steel spoon. Its bowl is curved, and both the inside and the back of the bowl are shiny, so both can act as mirrors.
Are the inside of the bowl and the back of the bowl the same kind of curved mirror?
What this lesson covers
The idea
A spherical mirror's reflecting surface is part of a sphere; if it is curved inwards, facing the centre of the sphere, it is a concave mirror, and if curved outwards, a convex mirror.
A shiny spoon
Look at a shiny steel spoon. Its bowl is curved, and both the inside and the back of the bowl are shiny, so both can act as mirrors.
Are the inside of the bowl and the back of the bowl the same kind of curved mirror?
- No: one curves inwards, the other bulges outwards
- Yes: both are curved, so they are the same kind
- Neither is a mirror: a spoon is not a mirror
Cut a cap, polish a face
A hollow steel ball. Cut a cap off it, then polish its inside or its outside. Watch which way the shiny face looks. Then find both kinds of mirror on a real spoon.
Curved in or curved out
The reflecting surface of a spherical mirror forms part of a sphere. If the surface is curved inwards, that is, it faces towards the centre of the sphere, the mirror is a concave mirror. If it is curved outwards, the mirror is a convex mirror.
Help for the name: concave has cave in it. You look into a cave, so the shiny face is hollow.
- What you saw | Name of the mirror
- The shiny face looks in, towards the centre of the ball | Concave mirror: the inside of the spoon's bowl
- The shiny face bulges out, away from the centre of the ball | Convex mirror: the back of the spoon's bowl
Notes
A spherical mirror's reflecting surface is part of a sphere; if it is curved inwards, facing the centre of the sphere, it is a concave mirror, and if curved outwards, a convex mirror.
Check yourself
Two caps were cut from hollow balls. The arrow shows the way the polished face looks, and the dot is the centre of its ball. Which cap is a concave mirror?
You look at the inside of a shiny steel katori (a small bowl). That surface acts as a …
You look at the outside of a small shiny steel ball. That surface acts as a …
You polish the inside of a cap to make mirror X. You polish the outside of the same cap to make mirror Y. Which is right?
- Cap A. The shiny face of Cap A bulges outwards, away from the centre of its ball. That makes it convex.
- Both of them. Being curved is not enough. A concave mirror curves inwards, towards the centre. Cap A curves outwards.
- Cap B — correct. Yes! Its shiny face looks in, towards the centre of its ball.
- concave mirror — correct. Yes! The inside of a bowl is curved inwards, towards the centre of its sphere.
- convex mirror. A convex mirror bulges outwards. The inside of a bowl curves inwards.
- plane mirror. A plane mirror is flat. The inside of a bowl is curved, so it is a spherical mirror.
- concave mirror. A concave mirror is curved inwards, facing the centre. The outside of a ball faces away from its centre.
- convex mirror — correct. Yes! The outside of a ball is curved outwards, away from its centre.
- plane mirror. A plane mirror is flat. The surface of a ball is curved.
- X is convex and Y is concave. It is the other way round. The inside faces the centre, so X is concave.
- Both are concave, as it is the same cap. The cap is the same, but the polished face is not. One face looks in, the other looks out.
- X is concave and Y is convex — correct. Yes! The inside faces the centre of the ball. The outside faces away from it.