What the mirror does to an image
A dentist's small mirror shows a big, upright tooth. Move it away and the picture changes.
Each image below is real or virtual. It is also upright (the right way up) or inverted (upside down).
- Beyond C, the image is real, inverted, smaller.
- At C, the image is real, inverted, the same size.
- Between C and F, the image is real, inverted, bigger.
- At F, the rays leave parallel. The image is at infinity.
- Inside F, the image is virtual, upright, bigger.
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The rule
A real image forms where reflected rays truly meet. A screen can catch it. A virtual image forms where rays only seem to meet. Your eye sees it. A screen cannot catch it.
The place of the object decides the image. The dentist's mirror is concave. With the mirror close, the tooth is inside F. So the image is upright and bigger. With the tooth farther than F, the image is inverted.
The distances in the table are measured from the mirror. Here f = 20 cm, so C is at 2f = 40 cm. An image at infinity means the reflected rays do not meet.
| Object stands | The image |
|---|---|
| very far away | at F, real, inverted, tiny |
| beyond C (60 cm) | between F and C, real, inverted, smaller |
| at C (40 cm) | at C, real, inverted, same size |
| between C and F (30 cm) | beyond C, real, inverted, bigger |
| at F (20 cm) | at infinity: the rays leave parallel, so a screen catches nothing |
| inside F (10 cm) | behind the mirror, virtual, upright, bigger |
A convex mirror
A convex mirror makes a virtual, upright, smaller image. This is true for a real object, wherever it stands. A real object is an actual thing in front of the mirror.
Vehicles use convex mirrors as rear-view mirrors. They show a wide view of the road. A convex mirror curves outwards. So it covers more road than a flat one.
Worked example
An object stands between C and F. Its image is beyond C. It is real, inverted and magnified, which means bigger.
The mirror in a dentist's hand works inside F. There the image is upright and bigger.