Thin Lenses
Section: Waves | Syllabus: Cambridge IGCSE Physics (0625)
Converging and Diverging Lenses
Action on a Parallel Beam of Light
- A converging (convex) lens is thicker in the middle than at its edges; a parallel beam of light passing through it is brought together (converged) to a single point, the principal focus
- A diverging (concave) lens is thinner in the middle than at its edges; a parallel beam of light passing through it is spread out (diverged), appearing to come from a single point - its principal focus - on the same side of the lens as the incident beam
- A diverging lens always forms a virtual, upright, diminished image, whatever the position of the object
Action of converging and diverging lenses on a parallel beam of light
Key Terms
- Principal axis: the line passing through the centre of the lens, at right angles to the lens
- Optical centre: the point at the centre of the lens where the principal axis crosses it; a ray passing through the optical centre continues without changing direction
- Principal focus (focal point, F): the point where a parallel beam converges (converging lens) or appears to diverge from (diverging lens); there is a principal focus on each side of the lens, at equal distances from the optical centre
- Focal length (f): the distance from the optical centre to the principal focus
Ray Diagrams for a Converging Lens
A ray diagram uses two rays from the top of the object to locate the top of the image:
- a ray travelling parallel to the principal axis, which refracts through the principal focus (F) on the far side of the lens
- a ray passing through the optical centre, which continues in a straight line without changing direction
An image can be described using the terms enlarged / same size / diminished, upright / inverted, and real / virtual.
Real Images: Object Beyond the Principal Focus
- When the object is beyond 2F, the two rays actually cross on the far side of the lens: the image is real, inverted, diminished, and forms between F and 2F
- When the object is at 2F, the image is real, inverted, the same size as the object, and forms at 2F on the far side
- When the object is between F and 2F, the image is real, inverted, magnified, and forms beyond 2F on the far side
Real images formed by a converging lens for different object positions
| Object Position | Image Position | Image Nature |
|---|---|---|
| Beyond 2F | Between F and 2F (far side) | Real, inverted, diminished |
| At 2F | At 2F (far side) | Real, inverted, same size |
| Between F and 2F | Beyond 2F (far side) | Real, inverted, magnified |
Extended Only
Virtual Images: Object Within the Principal Focus
- When the object is placed within F (closer to the lens than the principal focus), the two refracted rays spread apart on the far side of the lens and never actually meet
- Extending these rays backward (as dashed lines) to the same side of the lens as the object shows where they appear to meet - this locates a virtual image, formed by extrapolating the diverging rays backward; it cannot be projected onto a screen
- This image is virtual, upright, and magnified
A virtual image formed by a converging lens, object within F
Extended Only
Using a Converging Lens as a Magnifying Glass
- A converging lens is used as a magnifying glass by holding it close to a small object, so that the object lies within the focal length of the lens
- Looking through the lens produces a magnified, upright, virtual image of the object, on the same side of the lens as the object
- Muchindu holds a magnifying glass close to a stamp to examine its fine print; moving the lens further away, past the focal length, would cause the magnified image to disappear
Extended Only
Correcting Long-Sightedness and Short-Sightedness
When the eye's lens system does not focus light correctly onto the retina, a corrective lens can be used in front of the eye to restore normal vision.
Short-Sightedness (Myopia)
- Near objects are seen clearly, but distant objects appear blurred
- The eye focuses the image of a distant object in front of the retina
- Correction: a diverging (concave) lens, which diverges the light rays slightly before they enter the eye, moving the image back onto the retina
Long-Sightedness (Hyperopia)
- Distant objects are seen clearly, but near objects appear blurred
- The eye focuses the image of a near object behind the retina
- Correction: a converging (convex) lens, which converges the light rays before they enter the eye, moving the image forward onto the retina
Correcting short-sightedness with a diverging lens, and long-sightedness with a converging lens
| Condition | Image Forms | Correction |
|---|---|---|
| Short-sightedness (myopia) | In front of the retina | Diverging (concave) lens |
| Long-sightedness (hyperopia) | Behind the retina | Converging (convex) lens |
Interactive revision notes, videos and practice questions load below.