Types of Lenses
| Lens type | Shape | Effect on light |
|---|---|---|
| Convex (converging) | Thicker in the middle | Bends rays towards each other |
| Concave (diverging) | Thinner in the middle | Bends rays away from each other |
The Focal Point
Focal point (F): the point where rays converge (convex) or appear to diverge from (concave).
Focal length: the distance from the centre of the lens to the focal point.
A shorter focal length = a more powerful lens.
Convex Lenses (Converging)
Parallel rays from a distant object converge at the focal point on the other side of the lens.
| Object position | Image type | Image size |
|---|---|---|
| Beyond 2F | Real, inverted | Smaller |
| At 2F | Real, inverted | Same size |
| Between F and 2F | Real, inverted | Larger |
| Inside F | Virtual, upright | Larger (magnifying glass) |
A magnifying glass is a convex lens. When you place an object inside the focal length, you get a magnified, upright, virtual image.
Concave Lenses (Diverging)
Parallel rays spread out after passing through a concave lens. The rays appear to come from a virtual focal point on the same side as the incoming light.
Concave lenses always produce a virtual, upright, smaller image.
They are used in spectacles (glasses) for short-sightedness (myopia).
Real vs. Virtual Images
- Real image: light rays actually meet; can be projected onto a screen.
- Virtual image: rays only appear to meet (when you trace them back); cannot be projected.
Real world: Glasses for short-sightedness (myopia)
A short-sighted person cannot focus on distant objects because their eye converges light in front of the retina. Concave (diverging) lenses in glasses spread the light rays slightly before they enter the eye, shifting the focal point back onto the retina. The weaker the lens (longer focal length), the milder the short-sightedness.
Real world: Camera lens and photography
A camera lens is a convex (converging) lens. When a distant scene is photographed, light from the scene converges through the lens and forms a real, inverted, smaller image on the camera sensor. The photographer adjusts focus by moving the lens closer to or further from the sensor — changing the effective object distance.
Real world: Magnifying glass in science class
A magnifying glass (convex lens) works when the object is placed inside the focal length. The lens produces a virtual, upright, enlarged image — which is why small print on a map or tiny insects look much bigger through a hand lens.
Practice Problems
What type of image does a magnifying glass produce?
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A student is short-sighted. What type of lens is used in their glasses?
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What is the focal length of a lens?