### Properties of Lenses

```2 Types of Lenses:


thickest in middle & thinnest at edge
causes rays parallel to principal axis to
converge through a single point (principal
focus) after refraction



Optical Centre (O)
– centre of the lens
Principal Axis
– line through the
optical centre
Principal Focus (F) – point on principal
axis where light rays that are parallel to
principal axis converge after refraction

-
Secondary Principal Focus (F’)
light can strike lens from either side
both sides of lens can focus parallel light
F’ is focus on same side of lens as incident
rays
F and F’ alternate sides depending on which
side the incident rays are coming from


rays are refracted twice, way in and way out
only incident ray entering and ray leaving are
important, so we use a shortcut

the thicker the converging lens, the larger the
refraction

1.
2.
3.
3 rules:
ray parallel to the principal axis is refracted
through the principal focus (F).
ray through the secondary principal focus(F’) is
refracted parallel to the principal axis.
ray through the optical centre (O) continues
straight through without being refracted.
1.
Object beyond 2F’
2.
Object at 2F’
3. Object between F’ and 2F’
4. Object at F’
5. Object between F’ and lens


thinnest in middle and thickest near edge
causes parallel incident rays to diverge







same as Converging Lens:
Optical Centre (O)
Principal Axis
Principal Focus (F)
Secondary Principal Focus (F’) – different
F is on same side of lens as incident rays
F’ is on opposite side as incident rays

1.
2.
3.
3 rules:
ray parallel to the principal axis is refracted as if
it had come through the principal focus (F)
ray that appears to pass through the secondary
principal focus (F’) is refracted parallel to the
principal axis.
ray through the optical centre (O) continues
straight through on its path.

always the same
```