Thin LensesCambridge IGCSE Physics: Revision notes
Section 1
Converging and diverging lenses
A converging (convex) lens is thicker in the middle and bends parallel rays of light inwards so they meet at a point. A diverging (concave) lens is thinner in the middle and spreads parallel rays outwards, as if they all came from a single point behind the lens.
Key terms used to describe a lens:
- Principal axis — the line passing through the centre of the lens, perpendicular to it
- Principal focus (focal point) — the point where parallel rays converge (or appear to diverge from)
- Focal length — the distance from the centre of the lens to the principal focus
Converging = convex = thicker in the middle. Diverging = concave = thinner in the middle. Mixing these up is one of the most common lens errors.
Section 2
Images formed by a converging lens
A converging lens can form two different kinds of image, depending on where the object is placed relative to the focal length:
- Beyond a certain distance, the lens forms a real image — light rays actually converge and the image can be captured on a screen
- Very close to the lens (nearer than the focal length), it forms a virtual image — the rays only appear to diverge from a point, and cannot be projected on a screen
Ray diagrams are used to construct these images, tracing rays through the lens to find where they meet (real) or where they appear to come from when extended backwards (virtual).
Section 3
Describing an image
Any image formed by a lens can be fully described using three pairs of terms:
| Property | Options |
|---|---|
| Size | Enlarged / same size / diminished |
| Orientation | Upright / inverted |
| Type | Real / virtual |
A diverging lens always produces a virtual image — it can never form a real image, no matter where the object is placed. A converging lens can produce either a real or a virtual image depending on the object's distance from the lens.
If asked to describe an image, always give all three properties: size, orientation, and real/virtual — a partial answer loses marks.
Section 4
The magnifying glass
A single converging lens can be used as a magnifying glass. When the object is placed closer to the lens than the focal length, the lens produces an image that is:
- Virtual
- Upright
- Enlarged
This is the arrangement used whenever you look through a magnifying glass held close to a small object.
Holding a magnifying glass very close to text on a page produces a large, upright, virtual image of the text that you view directly through the lens.
Section 5
Correcting sight problems with lenses
Lenses are used to correct common vision problems by changing where light is focused relative to the retina:
- Long-sightedness (hyperopia) — light from near objects focuses behind the retina; corrected with a converging (convex) lens, which adds extra convergence
- Short-sightedness (myopia) — light from distant objects focuses in front of the retina; corrected with a diverging (concave) lens, which spreads the rays out before they enter the eye
Match the fix to the lens type: converging lens fixes long-sight (adds convergence power), diverging lens fixes short-sight (removes convergence power).
Must Know
- Converging (convex) lenses bring parallel rays together; diverging (concave) lenses spread them apart
- Focal length is the distance from the lens to the principal focus
- A converging lens can form a real image (further than focal length) or a virtual image (closer than focal length)
- A diverging lens always forms a virtual image
- Describe any image using size (enlarged/same/diminished), orientation (upright/inverted) and type (real/virtual)
- A magnifying glass is a converging lens producing a virtual, upright, enlarged image
- Long-sightedness is corrected with a converging lens; short-sightedness with a diverging lens
That's the notes covered.
Carry on to the next subtopic.