1. 2 Types of Lenses:

2.  thickest in middle & thinnest at edge  causes rays parallel to principal axis to converge through a single point (principal focus) after refraction

3.  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

4.  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

5.  rays are refracted twice, way in and way out  only incident ray entering and ray leaving are important, so we use a shortcut

6.  the thicker the converging lens, the larger the refraction

7.  3 rules: 1. ray parallel to the principal axis is refracted through the principal focus (F). 2. ray through the secondary principal focus(F’) is refracted parallel to the principal axis. 3. ray through the optical centre (O) continues straight through without being refracted.

8. 1. Object beyond 2F’ 2. Object at 2F’

9. 3. Object between F’ and 2F’

10. 4. Object at F’ 5. Object between F’ and lens

11.  thinnest in middle and thickest near edge  causes parallel incident rays to diverge (spread) after refraction

12.  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

13.  3 rules: 1. ray parallel to the principal axis is refracted as if it had come through the principal focus (F) 2. ray that appears to pass through the secondary principal focus (F’) is refracted parallel to the principal axis. 3. ray through the optical centre (O) continues straight through on its path.

14.  always the same