P3.2.3P3. Waves
Thin lenses — Combined Science 0653 - Physics Revision Notes & Topical Questions
5 min read 10 parts Lesson 5 of 9 1 quiz
Lesson summary
P3.2.3 Thin lenses — COMBINED SCIENCE notes: Describe converging and diverging lenses, draw ray diagrams for real and virtual images, and explain how lenses correct sight defects.Lens = a piece of transparent material that forms an image by refracting light.Principal axis = the l…
- Principal focus (focal point)
- Focal length
- Real image
- Describing an image
- Magnification
- Object closer than f to a converging lens
Part 1 of 8Lens terms
Lens terms
Lens
Lens a piece of transparent material that forms an image by refracting light.
Principal axis
Principal axis the line passing through the centre of the lens at right angles to it.
Principal focus (focal point)
Principal focus (focal point) the point where rays parallel to the principal axis meet, or appear to come from.
Focal length
Focal length the distance from the centre of the lens to the principal focus.
Curvature
- Curvature the more curved the lens, the shorter its focal length.
Part 2 of 8Converging and diverging lenses
| Feature | Converging lens | Diverging lens |
|---|---|---|
| Other name | Convex | Concave |
| Effect on parallel rays | Brings them to a focus at the principal focus | Spreads them out so they appear to come from the principal focus |
| Images | Real or virtual | Always virtual |
Part 3 of 8Real and virtual images
Real image
Real image an image formed where light rays actually meet, which can be projected onto a screen.
- Real images are always inverted, as in a cinema screen.
Virtual image
Virtual image an image formed where rays only appear to meet, which cannot be projected onto a screen.
- Virtual images are always upright, as in a mirror.
Describing an image
- Describing an image state whether it is enlarged, same size or diminished, upright or inverted, and real or virtual.
Magnification
Magnification image height ÷ object height, which has no unit.
Part 4 of 8Rules for ray diagrams with a converging lens
Rules for ray diagrams with a converging lens
A ray through the optical centre of the lens passes through undeviated.
A ray parallel to the principal axis is refracted through the principal focus.
A ray through the principal focus on the object side emerges parallel to the principal axis.
Drawing
- Drawing use two of these rays from the top of the object and mark the image where they cross.
Labels
- Labels label the object, the image and the focal points on both sides of the lens.
Part 5 of 8Real images from a converging lens
| Object position | Image position | Image nature |
|---|---|---|
| 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 |
Part 6 of 8Virtual images
1Object closer than f to a converging lens
- Object closer than f to a converging lens the lens acts as a magnifying glass.
- The image is virtual, upright and magnified, and is on the same side of the lens as the object, further away than the object.
2Diverging lens
- Diverging lens for any object position the image is virtual, upright, diminished and on the same side of the lens as the object.
- The rays are extended backwards to a virtual focus, so the image cannot be projected.
Part 7 of 8Correcting sight defects
Long sight
- Long sight the eyeball is too short or the lens too weak, so close objects focus behind the retina and only distant objects are seen clearly.
- A converging lens makes the rays converge before they reach the eye, so the image forms on the retina.
Short sight
- Short sight the eyeball is too long or the lens too strong, so distant objects focus in front of the retina and only close objects are seen clearly.
- A diverging lens makes the rays diverge before they reach the eye, so the image forms on the retina.
Fitting
- Fitting the lenses are used in glasses and contact lenses.
Part 8 of 8More from the specification
More from the specification
Thin lenses
- A converging lens refracts parallel rays to a principal focus;
- the focal length is the distance from lens to focus.
Refraction and refractive index
- Refraction and refractive index n = sin i / sin r, with the normal as reference for angle of incidence.
Reflection
- The angle of reflection equals the angle of incidence for a reflected ray measured from the normal.
Total internal reflection
- Occurs at a boundary when light travels in a dense medium, such as a glass block, and the angle exceeds the critical angle.
Critical angle
- Critical angle sin c = 1/n, used in optical fibres and prisms.
Thin lenses
- A converging lens refracts parallel rays to a principal focus;
- the focal length is the distance from lens to focus.
Refraction and refractive index
- Refraction and refractive index n = sin i / sin r, with the normal as reference for angle of incidence.What the symbols mean: n = refractive index (no unit) · i or θ1 = angle of incidence · r or θ2 = angle of refraction (both measured from the normal)
Reflection
- The angle of reflection equals the angle of incidence for a reflected ray measured from the normal.
Total internal reflection
- Occurs at a boundary when light travels in a dense medium, such as a glass block, and the angle exceeds the critical angle.
Critical angle
- Critical angle sin c = 1/n, used in optical fibres and prisms.What the symbols mean: c = critical angle · n = refractive index of the denser material (no unit)
Refractive index
- The ratio of the speed of light in a vacuum to its speed in a material, n = c ÷ v.What the symbols mean: n = refractive index (no unit) · c = speed of light in a vacuum (3.00 × 108 m s−1) · v = speed of light in the material (m s−1)
Critical angle
- The angle of incidence in the denser medium at which the refracted ray travels along the boundary.
Optical fibre
- A thin glass fibre that carries light by total internal reflection.
Plane mirror
- A flat mirror.
Reflection
- Bouncing of light off a surface.
Reflected
- Bounced off a surface.
Normal
- A line at right angles to a surface.
Key terms
| Term | Meaning |
|---|---|
| Converging lens | A lens that brings parallel rays to a focus. |
| Diverging lens | A lens that spreads parallel rays out. |
| Principal focus | The point where rays parallel to the principal axis meet or appear to come from. |
| Focal length | The distance from the centre of the lens to the principal focus. |
| Real image | An image formed by rays that meet, which can be projected onto a screen. |
| Virtual image | An image formed where rays only appear to meet, which cannot be projected. |
| Magnification | Image height divided by object height. |
Avoid these mistakes
1
Saying a real image is upright — real images from a single lens are inverted.
2
Saying a diverging lens can form a real image — its image is always virtual.
3
Measuring focal length from the edge of the lens — measure from the centre of the lens.
4
Mixing up the corrections — converging lenses correct long sight and diverging lenses correct short sight.
5
Drawing rays that do not meet at the image — rays must cross at one point for a real image.
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