Convex vs Concave Lenses — Converging and Diverging Light
Convex (Converging) Lens
Thicker in the middle than at the edges. Parallel rays entering a convex lens converge to a real focal point on the far side — it can project a real image, like a camera or projector does.
Unit: meter (m)
Example: A magnifying glass is a convex lens: hold it at the right distance and sunlight converges to a bright, hot spot.
Concave (Diverging) Lens
Thinner in the middle than at the edges. Parallel rays spread apart as if coming from a virtual focal point on the near side — it only ever makes virtual, upright, diminished images.
Unit: meter (m)
Example: Concave lenses correct nearsightedness: they spread incoming light slightly so the eye's own lens can focus it properly on the retina.
Key differences
| Aspect | Convex (Converging) Lens | Concave (Diverging) Lens |
|---|---|---|
| Shape | Thicker at center | Thinner at center |
| Parallel rays | Converge to a real focus | Diverge as if from a virtual focus |
| Focal length sign | Positive | Negative |
| Real images? | Yes — can project onto a screen | No — only virtual images |
| Typical use | Cameras, magnifiers, projectors | Nearsightedness glasses, peepholes |
A convex lens doesn't always magnify — place the object beyond 2F and the real image is smaller than the object (that's how a camera fits a landscape onto a tiny sensor). Image size depends on object distance, not just lens type.