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A ray of light bends whenever it passes through a lens (except when it passes
through the optical center). Convergence is the bending of light rays towards the
principal axis, and divergence is the bending of light rays away from the principal
axis. The power of a lens expresses its degree of convergence or divergence. A
lens with a short focal length deviates the rays more than a lens with a long focal
length. As a result, a lens's power is defined as the reciprocal of its focal length in
meters.

1
Power of a lens 
Focal length in metres

The unit of power is dioptre.

If a convex lens has a power of one D, its focal length is one meter.

Dispersion and Scattering of light

Newton's Experiment - Dispersion of Light

Sir Isaac Newton discovered that the images of heavenly bodies formed by a lens
were colored at the edges while studying them. In order to investigate this, he
conducted an experiment with a prism in 1665. Newton's room at Trinity College,
Cambridge, was darkened, and a beam of sunlight passed through a small circular
hole in the shutter, forming a white circular patch on the opposite wall. He then
placed a triangular prism in the path of the light beam and observed that the white
light was split into seven colors, which resembled the colors of a rainbow, namely
violet, indigo, blue, green, yellow, orange, and red (VIBGYOR).

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