730
15 Optics
Fig. 15.17 Refraction in a
prism
15.8 The net deviation of the ray
δ = (i 1 − r 1 ) + (360
◦
− 2r 2 ) + (i 3 − r 3 )
(1)
In the minimum angle position, r 1 = r 2 = A/2 = 60/2 = 30 ◦
From the geometry of Fig. 15.18, r 3 = 30 ◦ .
Fig. 15.18 Deviation of a ray
incident in the minimum
deviation position of the
prism after suffering one
internal reflection
By Snell’s law
Sin i 1 = n sin r 1 = 1.5 × sin 30
◦
= 0.75
∴ i 1 = 48.59
◦
Similarly, i 3 = 48.59 ◦
∴
δ = (48.59 − 30
◦
) + (360 − 2 × 30
◦
) + (48.59 − 30
◦
) = 337.18
◦