Curved LensesSNC2P – Optics

Lenses

• Lenses are thin pieces of glass or plastic that have at least one curved side.

There are two basic types of lenses:• convex (converging – cause light rays to be refracted toward the

principal axis)

Lenses

• Lenses are thin pieces of glass or plastic that have at least one curved side.

• concave (diverging – cause light rays to be refracted away from the principal axis).

From our lesson and demos on refraction we know that light bends when it enters and exits a material. For our purposes, we will understand that lenses are very thin and assume that light refracts only once in the material, at the axis of symmetry.

Important Terms• Principal axis - a straight line that passes through the

optical centre, O.• Focal point (lenses have 2 because light can pass through

in either direction)• The point where light rays converge after refraction is designated

the principal focal point - F • The focal point on the other side of the lens is designated the

secondary focal point - F’ (“F prime”). • If light were coming from the other direction, F and F’ would be

reversed.

• Optical centre (O) – centre of lens (vertically and horizontally)• Axis of symmetry – vertical line through optical centre

Diagrams

Finding Images in Lenses – The RulesLens Rules: Convex (Converging) LensesRules for drawing ray diagrams:1. A light ray travelling parallel to the principal axis is refracted

through the principal focus, F.2. A light ray travelling toward the secondary focus, F’, refracts

parallel to the principal axis.3. A light ray that passes through the optical centre goes

straight through, without refracting.

Examples

Examples

Examples

Examples

Examples

Examples

Finding Images in Lenses – The RulesLens Rules: Concave (Diverging) LensesRules for drawing ray diagrams:1. A ray parallel to the principal axis is refracted as if it had

come through the principal focus (F).2. A ray that appears to pass through the secondary principal

focus (F’) is refracted parallel to the principal axis.3. A ray through the optical centre (O) continues straight

through on its path.

• All images using diverging lenses have the same characteristics

Homework• Complete the lenses drawing practice sheet