(^304) Aptitude Test Problems in Physics
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4.13. The light entering the camera is reflected
from the surface of the facade. It can be assumed
that the reflection of light from the plaster is
practically independent of the angle of reflection.
In this case, the luminous energy incident on the
objective of the camera is proportional to the solid
angle at which the facade is seen from the objective.
As the distance from the object is reduced by half,
the solid angle increases by a factor of four, and
a luminous energy four times stronger than in the
former case is incident on the objective of the same
area.
For such large distances from the object, the
distance between the objective and the film in the
camera does not practically change during the
focussing of the object and is equal to the focal
length of the objective. The solid angle within
which the energy from the objective is incident
on the surface of the image depends linearly on
the solid angle at which the facade is seen, i.e.
on the distance from the object. In this case, the
illuminance of the surface of the image (which,
by hypothesis, is uniformly distributed over the
area of this surface), which determines the ex-
posure, is directly proportional to the correspond-
ing energy incident on the objective from the facade
and inversely proportional to the area of the image.
Since this ratio is practically independent of the
distance from the object under given conditions
there is no need to change the exposure.
4.14*. The problem is analogous to the optical
probleth in which the refraction of a plane wave
in a prism is analyzed. According to the laws of
geometrical optics, the light ray propagating from
point A to point B (Fig. 240) takes the shortest
time in comparison with all other paths.
The fisherman must move along the path of a
"light ray", i.e. must approach point E of the bay
at an angle y, cross the bay in a boat at right angles
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