3.2.6Waves
Ray Diagrams — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions
5 min read 10 parts Lesson 20 of 35 1 quiz
Lesson summary
3.2.6 Ray Diagrams — IGCSE PHYSICS notes: Use lens terms and the three construction rules to draw ray diagrams for converging and diverging lenses.Lens — a piece of equipment that forms an image by refracting lightPrincipal axis — a line which passes through the centre of a lensP…
- Principal focus
- Focal length
- Converging lens
- Memory aid
- Diminished image
- Central ray
Part 1 of 8Lens terms
Lens
- Lens a piece of equipment that forms an image by refracting light
Principal axis
- Principal axis a line which passes through the centre of a lens
Principal focus
- Principal focus the point at which rays parallel to the principal axis meet, or from which diverging rays appear to come
- Also called the focal point
Focal length
- Focal length the distance between the centre of the lens and the principal focus
Curvature
- Curvature the more curved the lens, the shorter the focal length
Part 2 of 8Converging and diverging lenses
Converging lens
- Converging lens brings parallel rays to a focus at the principal focus, also called a convex lens
Diverging lens
- Diverging lens makes parallel rays spread out as if from a point, also called a concave lens
- The principal focus is the point the rays appear to come from
Memory aid
- Memory aid convex lens = converging
Symbols
- Symbols drawn as a line or thin shape with arrow heads that point outwards for convex and inwards for concave
Part 3 of 8Three rules for a converging lens
Rule 1
- Rule 1 a ray through the optical centre passes straight through undeviated
Rule 2
- Rule 2 a ray parallel to the principal axis is refracted through the focal point
Rule 3
- Rule 3 a ray through the focal point emerges parallel to the principal axis
Part 4 of 8Method for a ray diagram
1
Draw the principal axis and the lens as a line or very thin ellipse.
2
Mark and label the focal points on both sides.
3
Draw and label the object further than the focal length from the lens.
4
Draw a ray from the top of the object through the optical centre.
5
Draw a second ray parallel to the principal axis and then through the focal point.
6
Draw and label the image where the rays meet.
7
Check that the object, image and focal points are all labelled.
- Diminished image
- place the object at least 2f from the lens
Part 5 of 8Diverging lens rays
Central ray
- Central ray the ray through the centre of the lens does not change direction
Parallel ray
- Parallel ray refracted as though it came from the virtual focus, so extend it backwards as a dashed line
Image
- Image where the central ray and the dashed line cross
Part 6 of 8Image position and size
| Object position | Image 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 7 of 8Checking the diagram
Labels
- Labels object, image, principal axis and focal points
Rays
- Rays use a ruler and arrows to show direction
Magnification
- Magnification image height divided by object height
Part 8 of 8More from the specification
1Ray tracing virtual images and fibres
- Ray tracing virtual images and fibres divergent light rays trace backward to locate virtual images,
- whereas total internal reflection confines rays inside optical fibres.
2Law of Reflection at Plane Mirrors
- For a light ray incident on a plane mirror, the angle of incidence equals the angle of reflection (i = r), both measured relative to the normal perpendicular to the boundary.
3Specular vs. Diffuse Reflection
- Specular reflection at a smooth, flat mirror produces a sharp virtual image at equal reflection angles,
- whereas reflection from a rough boundary scatters rays in multiple random directions.
4Refraction Across Optical Boundaries
- When light crosses a boundary between media of different optical densities, the change in wave speed bends the ray towards the normal if entering a medium of higher refractive index.
5Conditions for Total Internal Reflection
- Total internal reflection occurs only when light propagates in a higher refractive index material towards an interface with a lower index medium,
- and the angle of incidence exceeds the critical angle.
6Law of Reflection at Plane Mirrors
- For a light ray incident on a plane mirror, the angle of incidence equals the angle of reflection (i = r), both measured relative to the normal perpendicular to the boundary.
7Specular vs. Diffuse Reflection
- Specular reflection at a smooth, flat mirror produces a sharp virtual image at equal reflection angles,
- whereas reflection from a rough boundary scatters rays in multiple random directions.
8Refraction Across Optical Boundaries
- When light crosses a boundary between media of different optical densities, the change in wave speed bends the ray towards the normal if entering a medium of higher refractive index.
9Conditions for Total Internal Reflection
- Total internal reflection occurs only when light propagates in a higher refractive index material towards an interface with a lower index medium,
- and the angle of incidence exceeds the critical angle.
10Optically dense
- Having a high refractive index
11Reflection
- Light bouncing off a surface.
12Boundary
- Surface between two media at which waves are partially reflected
13Normal
- Line at 90° to a surface.
14Refractive index
- Ratio of the speed of light in a vacuum to its speed in a material
15Critical angle
- Incline angle at which sliding just begins.
16Virtual image
- Image formed where rays only appear to meet, not projectable on a screen
17Optical fibre
- Thin transparent strand that guides light by TIR
18Plane mirror
- Flat mirror.
Key terms
| Term | Meaning |
|---|---|
| Principal axis | Line through the centre of a lens at right angles to it |
| Principal focus | Point where parallel rays meet or appear to come from |
| Focal length | Distance from the centre of the lens to the principal focus |
| Converging lens | Convex lens that brings parallel rays to a focus |
| Diverging lens | Concave lens that spreads parallel rays out |
Avoid these mistakes
1
Drawing rays that miss the focal point — the parallel ray must pass through a labelled focal point.
2
Confusing converging and diverging lenses — convex converges and concave diverges.
3
Forgetting to label the image and the object — marks are lost.
4
Measuring focal length from the edge of the lens — measure from the centre.
Marked practiceQuizzes for this lesson
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