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
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.
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.
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