The optical power of corrective lenses is measured in diopters, a value equal to the reciprocal of the focal length measured in metres; with a positive focal length corresponding to a converging lens and a negative focal length corresponding to a diverging lens.
12.
When this is achieved, the screen is replaced with a mirror and the diverging lens is inserted between the converging lens and the light source at such a distance to the mirror that the light returning through the diverging and converging lenses produces a sharp image on top of the luminous object.
13.
When this is achieved, the screen is replaced with a mirror and the diverging lens is inserted between the converging lens and the light source at such a distance to the mirror that the light returning through the diverging and converging lenses produces a sharp image on top of the luminous object.
14.
One special application is to determine the focal length of a diverging lens : A light source is placed at twice the focal length of a converging lens on one side and a screen at the same distance on the other side so that the image of the light source is the sharpest possible.
15.
In it, Kepler set out the theoretical basis of double-convex converging lenses and double-concave diverging lenses and how they are combined to produce a Galilean telescope as well as the concepts of virtual images, upright vs . inverted images, and the effects of focal length on magnification and reduction.
