Learning Goals
(13.1) Draw, label AND explain ray diagrams for terminology in converging and
diverging lenses like the ones in Fig. 4 and 5.
(13.3) Draw, label AND explain ray diagrams for locating images in converging and
diverging lenses like the ones in Fig. 3 to 7.
SNC 2DI
Lenses and Lens Images
Remember that a concave mirror is a converging mirror (rays come together to a
real focal point) and a convex mirror is a diverging lens (rays come together at a
virtual focal point). There are also converging and diverging lenses as well. The
biggest difference with mirrors and lenses is that rays cannot pass through
mirrors and there is only one focal point. Rays can pass through lenses in both
directions and there are 2 focal points.
Converging lens
Diverging lens
Finding images in lenses also has similarities to the mirror. Like the mirror, there is
a principal axis through the centre (called optical centre instead of vertex) and a
focal point along the PA through which all rays running parallel to the PA will pass.
In order to pass through, the ray refracts instead of reflecting. It refracts twice
(once at each interface) but for simplicity, we’ll refract it once along the dotted
line drawn through optical centre.
Images in Converging Lenses
Three imaging rules are shown in the above diagram. Note that the image formed
is always real and always inverted. It’s size and location, however, depend on the
objects position. See Figure 3 on page 558 for details.
Images in Diverging Lenses
The imaging rules for diverging lenses are similar to converging lenses. Only this
time the image formed is ALWAYS smaller, upright, same side as the object and
virtual.