Section 1 of 5Refraction Revisited
A lens works because light changes direction when it passes between materials in which it travels at different speeds.
Refraction
- The change of direction of a wave when it crosses a boundary because its speed changes.
Entering glass
- Light slows down in glass and bends towards the normal when it enters a block at an angle.
Leaving glass
- Light speeds up again on leaving and bends away from the normal.
Frequency
- The frequency of the light stays the same, while the speed and wavelength change, so the colour does not change.
Section 2 of 5Convex (Converging) Lens
Convex (Converging) Lens
Shape
- A convex lens is thicker in the middle than at the edges and bulges outwards on both sides.
Effect on rays
- Parallel rays are bent inwards and brought together at a point called the principal focus.
Focal length
- The distance from the centre of the lens to the principal focus;
- a more curved lens has a shorter focal length.
Uses
- Magnifying glasses, cameras, projectors and spectacles to help people who are long-sighted.
Section 3 of 5Concave (Diverging) Lens
Concave (Diverging) Lens
Shape
- A concave lens is thinner in the middle than at the edges and curves inwards.
Effect on rays
- Parallel rays are spread apart, so they appear to come from a point behind the lens.
Principal focus
- The point from which the diverging rays appear to come, found by tracing the emerging rays backwards.
Uses
- Spectacles to correct short sight, and lenses in some peepholes and cameras where a smaller image is wanted.
| Lens | Shape | Action on parallel rays | Focus |
|---|---|---|---|
| Convex | Thick in middle | Converges them | Real focus on far side |
| Concave | Thin in middle | Diverges them | Apparent focus behind lens |
Section 4 of 5Drawing Lenses
Ray diagrams use simple symbols and a ruler.
1
Draw the principal axis as a horizontal line through the centre of the lens.
2
Represent a convex lens as a vertical line with outward-pointing arrowheads, and a concave lens with inward-pointing arrowheads.
3
Mark the principal focus on both sides of the lens at the same distance from the centre.
4
Draw rays as straight lines using a ruler, changing direction only once at the middle of the lens.
5
Add arrows to every ray to show the direction in which the light travels.
Section 5 of 5Exam Checklist & Tips
5.3.2 – Key Points for the Exam
- Convex lens A lens that is thicker at the centre and converges parallel rays to a focus.
- Concave lens A lens that is thinner at the centre and diverges parallel rays.
- Principal focus The point where parallel rays meet, or appear to come from, after passing through a lens.
- Focal length The distance from the lens to its principal focus.
- Principal axis The line through the centre of the lens at right angles to it.
Common Mistakes
- Mixing up the two lenses; convex converges and concave diverges.
- Saying the focal length is measured from the object instead of from the lens to the principal focus.
- Drawing curved rays; rays are straight lines that change direction at the lens.
- Forgetting that the frequency of light does not change when it is refracted by a lens.