Effects of a lens on a beam of light parallel to the principal axis
A beams of
light whose rays are parallel to the principal axis propagates toward a
thin diverging lens.
After the lens, the beam of light widens and rays move away from the
principal axis: the beam of light is diverging.
If the diverging lens is replaced by a converging lens then the beam of
light gets less wide and rays get closer to the principal axis: the
beam of light is converging.
- The beam
of light before the lens is called " incident beam "
- The beam of light after the lens is called " emergent beam "
Characteristics of a converging lens
where converge the rays of an incident beam parallel to the principal
axis is denoted by F and called focal point.
Distance between the optical center and the focal point is denoted by f
and called optical lenght.
The focal length can easily be determinate using a parallel incident
beam. A screen, after the lens, must be moved until the spot of light
gets the tiniest: the screen is then placed at the focal point.
Formation of an image with a converging lens
To form an
image of a bright object with a converging lens, a screen must be
placed behind the lens and its distance has to be adjusted to obtain a
formation of an image is possible only if the distance between the
screen and the object is greater than the focal length.
formed is then inverted, and placed after the focal point of the lens.