- Optics 103
Determine the minimum refractive index
n of the cylinder material for which none of
the rays entering the base will emerge from
the lateral surface.4.12. Two converging lenses are mounted
at the ends of a tube with a blackened inner
lateral surface. The diameters of the lenses
are equal to the diameter of the tube. The
focal length of one lens is twice that of the
other lens. The lenses are at such a distance
from each other that parallel light rays
incident along the axis of the tube on one
lens emerge from the other lens in a parallel
beam. When a wide light beam is incident
on the lens with the larger focal length, a
bright spot of illuminance E 1 is formed onthe screen. When the tube is turned through
1800 , the bright spot formed on the screen
has an illuminance E 2.
Determine the ratio of illuminances on
the screen.
4.13. An amateur photographer (who is an
expert in geometrical optics) photographs
the façade of a building from a distance of
100 m with a certain exposure. Then he
decides to make a photograph from a dis-
tance of 50 m (to obtain a picture on a larg-
er scale). Knowing that the area of the
image will increase by a factor of four, he
decides to increase accordingly the exposure
in the same proportion. After developingthe film, he finds out that the first picture
is good, while the exposure for the second
photograph has been chosen incorrectly.
Determine the factor by which the expo-