Page 145 - Modern Optical Engineering The Design of Optical Systems

128 Chapter Seven

Figure 7.4 (a) A direct vision prism disperses the light into
its spectral components without deviation of the beam.
(b) Hand spectroscope. The collimating lens produces a
magnified image of the slit at infinity for easy viewing.
The prism then disperses the light into a spectrum with-
out deviation of the yellow ray.

which they give will be approximations to the exact values when the
prisms are other than thin. For exact work, these approximate values
must be adjusted by exact ray tracing based on Snell’s law.

7.6 Total Internal Reflection
When a light ray passes from a higher index medium to one with a
lower index, the ray is refracted away from the normal to the surface
as shown in Fig. 7.5a. As the angle of incidence is increased, the angle
of refraction increases at a greater rate, in accordance with Snell’s law
(n n′):
n
sin I′ sin I
n′

Figure 7.5 Total internal reflec-
tion occurs when a ray, passing
from a higher to a lower index
of refraction, has an angle of
incidence whose sine equals or
exceeds n′/n.