Chapter 2: Image Formation and Applications of Lenses
Geometric construction of images
The three characteristic rays
To construct the image A'B' of an object AB (perpendicular to the optical axis, with B off the axis) through a thin lens, three rays from B are used whose path is known:
- The ray parallel to the optical axis emerges passing through the image focal point F'.
- The ray passing through the optical center O is not deviated (it crosses the lens in a straight line).
- The ray passing through the object focal point F emerges parallel to the optical axis.
The intersection of two of these rays (after the lens, or of their extensions) gives point B', and A' is then found on the optical axis, vertically aligned with B'.
Nature of the image depending on the object's position
| Object position (converging lens) | Image |
|---|---|
| Beyond 2f' | real, inverted, smaller |
| Between f' and 2f' | real, inverted, larger |
| Between O and f' | virtual, upright, larger (magnifying-glass effect) |
Real or virtual image?
An image is real if the light rays actually cross after the lens (it can be captured on a screen). It is virtual if only the extensions of the rays cross, before the lens (impossible to project onto a screen, but visible to the eye when looking through the lens).
Classic pitfall
For a diverging lens, the image of a real object is always virtual, upright, and smaller, regardless of the object's position: there is no table of cases to memorize, unlike for the converging lens.