3.2.7Waves
Real and Virtual Images — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions
5 min read 9 parts Lesson 21 of 35 1 quiz
Lesson summary
3.2.7 Real and Virtual Images — IGCSE PHYSICS notes: Distinguish real and virtual images and describe images by size, orientation and nature.Real or virtual — the nature of the imageUpright or inverted — the orientation compared with the objectEnlarged, same size or diminished — …
- Complete description
- Definition
- Can it be put on a screen
- Dashed lines
- Real image
- Refraction
Part 1 of 7Describing images
1Real or virtual
- Real or virtual the nature of the image
2Upright or inverted
- Upright or inverted the orientation compared with the object
3Enlarged, same size or diminished
- Enlarged, same size or diminished the size compared with the object
4Complete description
- Complete description give all three properties
Part 2 of 7Real images
Definition
- Definition an image formed when light rays from an object converge and meet each other
Screen
- Screen a real image can be projected onto a screen or paper
Orientation
- Orientation real images are always inverted
Cause
- Cause convergence of light towards a focus
Example
- Example the image on a cinema screen
Part 3 of 7Virtual images
Definition
- Definition an image formed when rays do not meet but appear to meet behind the lens
Screen
- Screen a virtual image cannot be projected onto a screen or paper
Orientation
- Orientation virtual images are always upright
Cause
- Cause divergence of light away from a point
Example
- Example a person's reflection in a mirror
Part 4 of 7Image characteristics for a converging lens
| Object position | Image position | 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 the object | Virtual | Upright | Magnified |
Part 5 of 7Comparing devices
| Device | Image | Use |
|---|---|---|
| Plane mirror | Virtual, upright, same size | Seeing yourself |
| Magnifying glass | Virtual, upright, magnified | Reading small print |
| Camera lens | Real, inverted, diminished | Photography |
| Projector | Real, inverted, magnified | Showing slides on a screen |
| Diverging lens | Virtual, upright, diminished | Correcting short sight |
Part 6 of 7Quick test
Quick test
Can it be put on a screen
- Can it be put on a screen if yes the image is real
Rays
- Rays real images are made where rays actually meet, virtual images where they only appear to meet
Dashed lines
- Dashed lines virtual images are drawn with dashed construction lines
Part 7 of 7More from the specification
More from the specification
Image transmission along flexible optical fibres
- Image transmission along flexible optical fibres digital endoscopy relays virtual images of internal organs through flexible bundles of thin optical fibres.
Refraction and Snell's Law
- When light passes across a boundary between media of differing optical densities, it bends relative to the normal line according to Snell's law n₁ sin(i) = n₂ sin(r),
- where i is the angle of incidence, r is the angle of refraction,
- and n is the refractive index.
Total Internal Reflection and Critical Angle
- Total internal reflection occurs when light travelling within a medium of higher refractive index strikes a boundary at an angle of incidence exceeding the critical angle c,
- where sin(c) = 1/n.
Focal Length and Converging Lens Imaging
- A converging lens focuses parallel rays through its principal focal point;
- objects placed farther than the focal length produce real inverted images,
- while objects placed within the focal length produce magnified virtual upright images.
Refraction and Snell's Law
- When light passes across a boundary between media of differing optical densities, it bends relative to the normal line according to Snell's law n₁ sin(i) = n₂ sin(r),
- where i is the angle of incidence, r is the angle of refraction,
- and n is the refractive index.
Total Internal Reflection and Critical Angle
- Total internal reflection occurs when light travelling within a medium of higher refractive index strikes a boundary at an angle of incidence exceeding the critical angle c,
- where sin(c) = 1/n.
Focal Length and Converging Lens Imaging
- A converging lens focuses parallel rays through its principal focal point;
- objects placed farther than the focal length produce real inverted images,
- while objects placed within the focal length produce magnified virtual upright images.
Focal length
- Distance from the lens centre to the principal focus
Optically dense
- Having a high refractive index
Refraction
- Change in direction of light when it changes speed between media.
Boundary
- Surface between two media at which waves are partially reflected
Normal
- Line at 90° to a surface.
Distance
- Total length of path travelled, a scalar.
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
Principal focus
- Point where parallel rays meet or appear to come from
Key terms
| Term | Meaning |
|---|---|
| Real image | Image formed where light rays actually converge, which can be projected on a screen |
| Virtual image | Image formed where rays only appear to meet, which cannot be projected |
| Inverted | Upside down compared with the object |
| Upright | The same way up as the object |
| Diminished | Smaller than the object |
Avoid these mistakes
1
Saying virtual images are not there — they are seen but cannot be projected on a screen.
2
Describing an image with only one property — state real or virtual, upright or inverted and size.
3
Saying all lens images are inverted — only real images are inverted.
4
Thinking a real image must be larger — its size depends on the object position.
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