Grind, polish, coat
Where the shiny coating goes decides concave or convex.
Cut from a glass ball?
A spherical mirror is shaped like a part of a hollow sphere. So is it made by slicing a hollow glass ball into pieces?
How do you think spherical mirrors are made?
What this lesson covers
The idea
Spherical mirrors are made by grinding and polishing flat glass into a curved surface; a reflective coating on the outer curved surface gives a concave mirror, and on the inner curved surface a convex mirror.
Cut from a glass ball?
A spherical mirror is shaped like a part of a hollow sphere. So is it made by slicing a hollow glass ball into pieces?
How do you think spherical mirrors are made?
- Flat glass is ground and polished into a curve
- A hollow glass ball is sliced into pieces
- A plane mirror is simply bent
Make two mirrors
Grind and polish a flat piece of glass. Then choose which curved surface gets the shiny aluminium coating. Make both kinds of mirror.
The coating decides
Spherical mirrors are made by grinding and polishing flat glass into a curved surface. A reflective coating on the outer curved surface gives a concave mirror. On the inner curved surface, it gives a convex mirror.
Why? Light passes through the clear glass and comes back from the coating. You look from the other side, so the coating on the outer surface curves inwards as you see it. A spherical mirror can be *thought of* as a part of a hollow sphere, but it is not made by slicing one.
- Coating (thin aluminium layer) on | Mirror made
- Outer curved surface | Concave mirror
- Inner curved surface | Convex mirror
Notes
Spherical mirrors: flat glass is ground and polished into a curve. Coating on the outer curved surface gives a concave mirror; on the inner curved surface, a convex mirror.
Check yourself
How are spherical mirrors made?
The aluminium coating is put on the outer curved surface. Which mirror is made?
To make a convex mirror, where does the coating go?
- By grinding and polishing flat glass into a curved surface — correct. Yes. Then a reflective coating is added.
- By slicing a hollow glass sphere. Its shape fits a sphere, but it is not cut from one. Flat glass is ground and polished.
- By painting a flat sheet of glass silver. That would give a flat mirror. The glass must first be made curved.
- A convex mirror. A common mix-up. The coating is on the outer surface, but you look at it from the hollow side, through the glass. So it curves inwards: concave.
- A concave mirror — correct. Yes. You look from the hollow side, and the coating curves inwards as you see it.
- A plane mirror. The glass is curved, so the mirror is curved too.
- On the inner curved surface — correct. Yes. Seen from the bulging side, the coating bulges outwards.
- On the outer curved surface. Coating the outer surface makes a concave mirror.
- Nowhere. Curved glass is already a mirror. Clear glass lets most light pass through. The reflective coating makes it a mirror.