As before, we take the aperture
(or the lateral size) of the surface to be small compared to other distances
involved, so that small angle approximation can be made In particular,
NM will be taken to be nearly equal to the length of the perpendicular
from the point N on the principal axis We have, for small angles,
tan ÐNOM = MN
OM
FIGURE 9 14 Light undergoes successive total
internal reflections as it moves through an
optical fibre