What is a lens?
An image-forming device that works by refraction at two surfaces.
What materials are lenses made from?
Glass, quartz, fused silica, and plastics.
How can lenses be classified?
As converging or diverging, and thin or thick.
What defines a converging lens?
It is thicker in the middle than at the edges.
.What does a converging lens do to parallel rays?
It focuses them to a point.
Why is a convex lens called converging?
Because it brings parallel rays to a focus.
List forms of converging lenses.
Equi-convex, bi-convex, plano-convex, convex meniscus.
What defines a diverging lens?
It is thinner in the centre than at the edges.
What does a diverging lens do to parallel rays?
It spreads them out.
Why is a concave lens called diverging?
Because it causes rays to diverge.
List forms of diverging lenses.
Equi-concave, bi-concave, plano-concave, concave meniscus.
What defines a lens geometrically?
The intersection of two spherical surfaces.
What are r₁ and r₂?
Radii of curvature of the first and second surfaces.
What is the optical axis?
The line joining the centres of curvature.
What are A₁ and A₂?
Front and back vertices of the lens.
What is centre thickness?
?The distance between A₁ and A₂.
What is the first principal focus?
Point from which rays diverge and emerge parallel.
What is the second principal focus?
Point where parallel rays converge.
First focal length (convex lens)?
f=−n/F
First focal length (concave lens)?
f=n/F
Second focal length (convex lens)?
f′=n/F
Second focal length (concave lens)
f′=−n/F
Ray 1 rule?
Passes undeviated through optical centre.
Ray 2 rule?
Parallel ray passes through F’ (or appears from F’ for minus lens).
