All six cases together
Where the object is decides where the image forms, what kind it is and how big it is. Table 5.1 puts all six cases side by side.
| Object position | Image position | Real or virtual | Upright or inverted | Size |
|---|---|---|---|---|
| Less than the focal length | Behind the mirror, seen by looking into it | Virtual | Upright | Larger than the object |
| At the focus | At infinity | — | — | — |
| Between f and 2f | Beyond 2f | Real | Inverted | Larger than the object |
| At 2f (at C) | At 2f | Real | Inverted | Same size as the object |
| Beyond 2f | Between f and 2f | Real | Inverted | Smaller than the object |
| Very far away | At the focus | Real | Inverted | Much smaller than the object |
After Table 5.1 of the textbook.
Activity 5.2
You will need: a concave mirror, a stand, a candle
- Fix the mirror upright on a stand and find its focal length roughly, as in Activity 5.1.
- Place a lit candle about five focal lengths in front of the mirror, close to the principal axis.
- Move a screen until a sharp image of the flame forms on it.
- Bring the candle nearer step by step and study the image at each position — including right next to the mirror.
Only when the object is inside the focal length is the image virtual and upright. Everywhere else a concave mirror forms a real, inverted image that can be caught on a screen.