Light travels in a straight line. But when it reaches another medium it will bend.
If it travels to a denser medium (e.g. air to glass), light bends towards the normal.
(normal --- the imaginary line perpendicular (90°) to the surface of the medium.)
If it travels to a less dense medium (e.g. glass to water), light bends away from the normal. (fig. 1)
But if light strikes perpendicularly on the surface of the medium, it will pass straight through.
A lens is made up of dense material, e.g. glass. It can be regarded as a series of prisms and glass blocks. (fig.2) We will talk about two common types.
The distance between the focal point and the middle of the lens is the focal distance. This distance is also the focal length of the lens.
Where comes parallel rays?
Light from a distant object can be regarded as parallel rays.
The rays from a near object are non-parallel.
When an object is moved near to a biconvex (converging) lens, the non-parallel light rays will converge at a point farther than the focal point.
! There are other rules of physical optics which govern the way light travels after passing through the lens. But only those parts which are essential to the understanding of accommodation are discussed.
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