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 Rationalised 2023-24
Ray Optics and
Optical Instruments
233
 EXAMPLE 9 5
tan ÐNCM = MN
MC
tan ÐNIM  = MN
MI
Now, for DNOC, i is the exterior angle