Curved Lenses
SNC2P – 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 Rules
Lens Rules: Convex (Converging) Lenses
Rules 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 Rules
Lens Rules: Concave (Diverging) Lenses
Rules 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