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

1. Converging Lenses

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

Converging Lens Terms

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

Path of Light Through a Lens

the thicker the converging lens, the larger the refraction

Thickness of Lens

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.

Locating Images in Converging Lenses

1. Object beyond 2F’

2. Object at 2F’

SALT of Converging Lenses p558

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

(spread) after refraction

2. Diverging Lenses

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

Diverging Lens Terms

Locating Images in Diverging Lenses 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.

always the same

SALT of Diverging Lenses