3.2.3Waves
THIN Lenses — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions
5 min read 9 parts Lesson 13 of 35 1 quiz
Lesson summary
3.2.3 Thin Lenses — IGCSE PHYSICS notes: Describe converging and diverging lenses, their images and their use in correcting sight.Lens — a piece of equipment that forms an image by refracting lightConverging lens — brings parallel rays to a focus at the principal focus and is als…
- Focal length
- Real image
- Virtual image
- Long sight
- Short sight
- Magnifying glass
Part 1 of 7Types of lens
Types of lens
Lens
- Lens a piece of equipment that forms an image by refracting light
Converging lens
- Converging lens brings parallel rays to a focus at the principal focus and is also called a convex lens
Diverging lens
- Diverging lens makes parallel rays spread out as if from a point and is also called a concave lens
Focal length
- Focal length the distance from the centre of the lens to the principal focus, and the more curved the lens the shorter it is
Part 2 of 7Construction rules
Rule 1
- Rule 1 rays through the optical centre pass through undeviated
Rule 2
- Rule 2 rays parallel to the principal axis are refracted through the focal point
Rule 3
- Rule 3 rays through the focal point emerge parallel to the principal axis
Part 3 of 7Real and virtual images
Real image
- Real image formed when rays converge and meet, can be projected on a screen and is always inverted
Virtual image
- Virtual image formed when rays do not meet but appear to meet behind the lens, cannot be projected and is always upright
Part 4 of 7Converging lens images
| Object position | Image | Nature | Orientation | Size |
|---|---|---|---|---|
| Beyond 2f | Between f and 2f | Real | Inverted | Diminished |
| At 2f | At 2f | Real | Inverted | Same size |
| Between f and 2f | Beyond 2f | Real | Inverted | Magnified |
| Closer than f | Same side as object | Virtual | Upright | Magnified |
Part 5 of 7Diverging lens images
Always
- Always virtual, upright and diminished, on the same side of the lens as the object
Reason
- Reason the central ray is undeviated and the parallel ray appears to come from the virtual focus
Part 6 of 7Correcting sight
Long sight
- Long sight the focus lies behind the retina, corrected by a converging lens
Short sight
- Short sight the focus lies in front of the retina, corrected by a diverging lens
Magnifying glass
- Magnifying glass a converging lens with the object closer than f
Part 7 of 7More from the specification
More from the specification
Focal length determination and optical fibres
- Focal length determination and optical fibres focusing light beams onto ends of thin optical fibres requires calibrated convex glass lenses.
Thin lenses
- Thin lenses a converging lens refracts parallel rays to a principal focus, while a diverging lens spreads them out.
Ray diagrams
- Ray diagrams an image is located where rays from one point of the object meet, using rays through the centre and parallel to the axis.
Refraction at a boundary
- Refraction at a boundary a ray bends towards the normal when entering a denser medium, with the angle of incidence larger than the angle of refraction.
Reflection and total internal reflection
- Reflection and total internal reflection the angle of incidence equals the angle of reflection,
- and above the critical angle in a dense medium light is wholly reflected.
Mirror
- Mirror a plane mirror gives an upright, virtual image the same size as the object and as far behind the mirror as the object is in front.
Thin lenses
- Thin lenses a converging lens refracts parallel rays to a principal focus, while a diverging lens spreads them out.
Ray diagrams
- Ray diagrams an image is located where rays from one point of the object meet, using rays through the centre and parallel to the axis.
Refraction at a boundary
- Refraction at a boundary a ray bends towards the normal when entering a denser medium, with the angle of incidence larger than the angle of refraction.
Reflection and total internal reflection
- Reflection and total internal reflection the angle of incidence equals the angle of reflection,
- and above the critical angle in a dense medium light is wholly reflected.
Mirror
- Mirror a plane mirror gives an upright, virtual image the same size as the object and as far behind the mirror as the object is in front.
Refractive index
- Ratio of the speed of light in a vacuum to its speed in a material
Critical angle
- Incline angle at which sliding just begins.
Optical fibre
- Thin transparent strand that guides light by TIR
Plane mirror
- Flat mirror.
Optically dense
- Having a high refractive index
Reflection
- Light bouncing off a surface.
Boundary
- Surface between two media at which waves are partially reflected
Normal
- Line at 90° to a surface.
Key terms
| Term | Meaning |
|---|---|
| Converging lens | Lens that brings parallel rays to a focus |
| Diverging lens | Lens that spreads parallel rays out |
| Principal focus | Point where parallel rays meet or appear to come from |
| Focal length | Distance from the lens centre to the principal focus |
| Real image | Image formed by converging rays that can be shown on a screen |
Avoid these mistakes
1
Confusing convex and concave — convex converges and concave diverges.
2
Saying every lens image is inverted — only real images are inverted.
3
Measuring focal length from the lens edge — measure from the centre.
4
Saying a diverging lens makes real images — its images are always virtual.
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