Ray Diagrams — Converging Lens
A lens is a transparent optical block that bends (refracts) light rays. A converging lens focuses parallel rays to a single point — it converges light. The thicker centre and thinner edges cause light to bend towards the principal axis:contentReference[oaicite:1]{index=1}.
Key Terms
- Principal axis — the main horizontal line through the optical centre.
- Focal point (F) — where rays parallel to the axis converge after refraction.
- Focal length (f) — measured along the principal axis between the lens centre and F.
- Optic centre (O) — midpoint of the lens; rays through it travel straight.
Finding the Focal Length
- Focus a distant object (like the Sun) onto a paper screen.
- Move the paper until a sharp image appears.
- The distance between the lens centre and the image = focal length.
Ray Construction Rules
- Ray 1 — from top of object through the centre of lens (passes straight).
- Ray 2 — from top of object through the focus then travels parallel to the axis after refraction.
- Ray 3 — from top of object parallel to axis, then passes through the focus behind lens.
- Any two rays are enough to locate the image position:contentReference[oaicite:2]{index=2}.
Image Formations by a Converging Lens
| Object Position | Image Position | Image Size | Image Nature |
|---|---|---|---|
| Beyond 2F | Between F and 2F | Diminished | Real, inverted |
| Between F and 2F | Beyond 2F | Magnified | Real, inverted |
| At 2F | At 2F | Same size | Real, inverted |
| Between F and Lens | Behind the object | Magnified | Virtual, upright |
Real and Virtual Images
- Real image — formed by rays that converge physically, inverted and screen-visible.
- Virtual image — formed by rays that appear to diverge from a point behind the lens, upright.
Worked Example
Finding Focal Length from Lens Formula
1/f = 1/u + 1/v where:
f = focal length, u = object distance, v = image distance.
Example: if u = 20 cm, v = 10 cm → f = 6.7 cm.
Exam Points
- Use arrow-topped object symbols and mark F and 2F on both sides of the lens.
- Draw only two principal rays; keep lens drawn as a thin vertical line.
- Always label the image: real or virtual, inverted or upright, and magnified or diminished.
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