1. Can you list out other applications of lenses?
What is a lens?
A contact lens
Lenses are used in telescopes
Can you list out other applications of lenses?
microscopes
cameras

2. Convex and concave lenses
convex lenses
concave lenses
thicker at the centre than at the edge
thinner at the centre than at the edge

3. What happens when parallel light rays pass through a convex lens?
The rays converge to a single point.
convex lens = converging lens

4. What happens when parallel light rays pass through a concave lens?
The rays diverge and appear to come from a single point.
concave lens = diverging lens

5. Terms used to describe convex lenses
optical centre C
optical centre – the centre of the lens

6. principal
axis C
principal axis – a straight line perpendicular to the lens and passing through the optical centre

7. A convex lens has 2 principal foci.
principal focus F’ C F
Rays passing through convex lens parallel to the principal axis converge to the principal focus.
A convex lens has 2 principal foci.

8. focal length – the distance between the focus and the optical centre
focal length f
focal length – the distance between the focus and the optical centre

9. focal plane
principal axis C F
focal plane – a plane through the principal focus and is perpendicular to the principal axis.
Simulation: Parallel rays through convex lens

10. Terms used to describe concave lenses
optical centre
principal focus
principal
axis F’ C F
focal length f

11. C principal axis F’ focal plane
Simulation: Parallel rays through concave lens

12. How lenses work Why does a convex lens converge light rays?
Why does a concave lens diverge light rays?

13. Think of a convex lens as a number of prisms.
The rays near the edge are refracted more. F
When light rays leave the prisms, they are refracted away from the normal.
When light rays enter the prisms, they are refracted towards the normal.

14. Think of a concave lens as a number of prisms.
The rays near the edge are refracted more. F’
When light rays enter the prisms, they are refracted towards the normal.
When light rays leave the prisms, they are refracted away from the normal.

15. No. It is laterally displaced.
Does the ray passing through the optical centre of a lens change direction?
No. It is laterally displaced.
smaller lateral displacement
larger lateral displacement

16. That’s the end of Section 15.1
Check Point
Key Ideas
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Section 15.2
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