(^294) Aptitude Test Problems in Physics
to the geometrical path difference 8 due to the
deflection of the rays from the initial direction of
propagation. In this case, the interference of the
Fig. 228
rays will result in their augmentation (Huygens'
principle). It follows from Fig. 228 that
8 opt [n (r Ar) — n (r)] /,^8 = Ar sin cp.
Hence
sincp-
6 In (r+ n (r)]
Ar Ar
This leads to the following conclusion. If we con-
sider a narrow beam of light such that the deflection
angle y is small, then y vc r, i.e. the rotating vessel
will act as a diverging lens with a focal length
F = (21c1)-4.
Therefore, for the maximum deflection angle,
we obtain
sin Truax = 2k/ri„am•
Consequently, the required radius of the spot on
the screen is
R = rbeam L tan (Nal,•
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