Together or apart
Curved mirrors bring parallel rays together or spread them apart.
Many beams at once
Several parallel beams of light fall on a concave mirror together.
After reflection, what do you think the beams do?
What this lesson covers
The idea
The laws of reflection hold for all mirrors, but on reflection parallel rays stay parallel from a plane mirror, converge from a concave mirror and diverge from a convex mirror because of the curved surface.
Many beams at once
Several parallel beams of light fall on a concave mirror together.
After reflection, what do you think the beams do?
- They come closer together
- They stay parallel
- They spread out
Three mirrors, one beam
Send the same beam of parallel rays to each mirror. Watch the reflected rays. Then show the normals on a curved mirror: does every ray still obey i = r?
The curve does it
The laws of reflection hold for all mirrors. But on reflection, parallel rays stay parallel from a plane mirror, converge from a concave mirror and diverge from a convex mirror, because of the curved surface.
On a curved mirror, the normal points a different way at every point. Each ray obeys i = r at its own point, so together the reflected rays come closer or spread apart.
- Mirror | Parallel rays after reflection
- Plane | Stay parallel
- Concave | Converge: come closer
- Convex | Diverge: spread out
Notes
The laws of reflection hold for all mirrors. Parallel rays stay parallel from a plane mirror, converge from a concave mirror and diverge from a convex mirror, because of the curved surface.
Check yourself
What does a concave mirror do to a beam of parallel rays?
Do the laws of reflection hold for a convex mirror?
Why do parallel rays converge after reflection from a concave mirror?
Parallel beams fall on a mirror. The reflected beams spread out. What kind of mirror is it?
- It diverges the beam. That is what a convex mirror does.
- It converges the beam — correct. Yes. The reflected rays come closer together.
- It keeps the rays parallel. That is what a plane mirror does.
- Yes. Every ray obeys them at its own point — correct. Yes. The laws of reflection hold for all mirrors.
- No. They hold only for plane mirrors. The normals showed it: every ray on a curved mirror has i = r.
- Only at the middle of the mirror. Every point obeys the laws. The normal just points a different way at each point.
- Its curved surface makes the normal point a different way at each point — correct. Yes. Each ray obeys i = r, but the normals differ, so the rays come together.
- A concave mirror does not obey the laws of reflection. It does obey them. The curved surface is the reason.
- A concave mirror is bigger than a plane mirror. Size does not matter here. The curve does.
- A convex mirror — correct. Yes. A convex mirror diverges a beam.
- A concave mirror. A concave mirror makes the beams come closer.
- A plane mirror. A plane mirror keeps parallel beams parallel.