Section 1 of 4Types of Lens
A lens forms an image by refracting light at its curved surfaces.
Convex lens
- Parallel rays are brought together at a single point called the principal focus, so a convex lens is also called a converging lens.
Concave lens
- Parallel rays spread out after passing through, so they appear to come from one point, which makes it a diverging lens.
Focal length
- The distance from the centre of the lens to its principal focus, which is shorter when the lens is more strongly curved.
Central ray
- A ray passing through the centre of a thin lens is not deviated, because the two faces are almost parallel there.
| Feature | Convex (converging) | Concave (diverging) |
|---|---|---|
| Shape | Thicker in the middle | Thinner in the middle |
| Parallel rays | Brought together to a focus | Spread out |
| Focus | Real, on the far side of the lens | Virtual, on the same side as the incoming light |
Section 2 of 4Tracing Parallel Rays
1
A parallel beam entering a convex lens refracts at each curved surface and emerges converging on the principal focus on the far side.
2
A parallel beam entering a concave lens emerges spreading out, as though all the rays came from the principal focus on the same side.
3
The distance from the centre of the lens to the principal focus is the focal length, and it depends on how curved the lens surfaces are.
4
A more curved lens bends light more strongly, so its focal length is shorter and its power is greater.
Section 3 of 4Comparing the Two Lenses
Comparing the Two Lenses
Converging behaviour
- A convex lens can form real or virtual images depending on where the object is placed, as explained in the next lesson.
Diverging behaviour
- A concave lens always produces virtual, upright and diminished images, so it is used to correct some vision faults.
Refraction reason
- Light is refracted because its speed changes when it passes between air and the glass or plastic of the lens.
Symbols
- Lens diagrams use the standard symbols for each type, and these must be drawn accurately in ray diagrams.
Section 4 of 4Exam Checklist & Tips
3.6.1 – Key Points for the Exam
- Convex lens A lens that is thicker in the middle and converges parallel rays to a focus.
- Concave lens A lens that is thinner in the middle and diverges parallel rays.
- Principal focus The point where rays parallel to the axis meet, or appear to meet, after passing through a lens.
- Focal length The distance from the centre of a lens to its principal focus.
- Principal axis The straight line through the centre of a lens, perpendicular to the lens faces.
Common Mistakes
- Describing a concave lens focus as a point the rays pass through;
- the rays only appear to come from the focus.
- Mixing up the lens types, such as calling a convex lens diverging.
- Measuring focal length from the edge of the lens rather than from its centre.
- Saying a lens bends light towards the focus without stating that refraction occurs at each curved surface.