To know the formation of image by lenses, we need to know the
behaviour of light rays when they meet a lens.
Though we know that the lens has two surfaces; while drawing ray
diagrams, we can consider the lens as a single surface element because we
assume that the thickness of the lens is very small and show the net refraction
at only one of the surfaces, as shown in the fig.- 8(c) and 8(d).
Behaviour of certain light rays when they are incident on a lens
The behaviour of a light ray when it passes through a lens can be
understood by observing its behaviour in the following situations:
Note:
: The points C1, C2 are not centre of curvatures. They are the points
at ‘2f’distances from optic centre.
situation:I
Ray passing along the principal axis
Any ray passing along the principal axis is undeviated. (see Figures 9a
and 9b)
situation:II
Ray passing through the optic
centre.
Any ray passing through the optic centre is
also undeviated.
(see Figure 10a and 10b)
situation:III
: Rays travelling parallel to the principal axis.
We know that the rays passing parallel to the principal axis converge
at the focus or appear to diverge from the focus as shown in fig.- 8(c) and
8(d).
• If we allow a light ray to pass through the focus, which path does it take?
situation:IIII
: Ray passing through focus.
Light rays obey the principle of least time. Hence the ray passing
through the focus will take a path parallel to principal axis after refraction.
(see Figure 11(a) and 11(b))
