Knowledge Organiser: Lenses and Images
Key Terms
- Convex: converging, thicker in middle
- Concave: diverging, thinner in middle
- Focal point F: where parallel rays converge
- Real image: rays meet, can project, inverted
- Virtual image: rays appear to meet, upright, cannot project
Image Summary (Convex Lens)
- Beyond 2F → real, inverted, diminished
- At 2F → real, inverted, same size
- Between F and 2F → real, inverted, magnified
- Closer than F → virtual, upright, magnified
Key Equations
- Magnification = image height ÷ object height
- No units for magnification
Exam Tips
- Three characteristics: real/virtual, upright/inverted, magnified/diminished
- Use three rays for ray diagrams (parallel, centre, through F)
- Short-sight → concave; long-sight → convex
- Virtual images CAN be seen, just not projected
Common Mistakes
- Describing images with only one characteristic: Always give all three — real/virtual, upright/inverted, and magnified/diminished — for full marks in image description questions
- Confusing short-sight and long-sight corrections: Short-sighted (can't see far) → diverging (concave) lens; long-sighted (can't see near) → converging (convex) lens
- Saying virtual images cannot be seen: Virtual images cannot be projected onto a screen but they can be seen by the eye — a plane mirror produces a virtual image you can clearly see
- Drawing ray diagrams without using principal rays: Use the three standard rays (parallel to axis, through centre, through focal point) to accurately locate the image — guessing the image position loses marks
- Confusing focal length and focal point: The focal point (F) is where parallel rays converge; the focal length is the distance from the lens centre to F — power (D) = 1 ÷ focal length (m)
Memory Aid
- Object beyond 2F → real, inverted, diminished
- Object at 2F → real, inverted, same size
- Object between F and 2F → real, inverted, magnified
- Object closer than F → virtual, upright, magnified (magnifying glass!)