One way to classify lenses is how the lenses bend the light rays. There are two kinds of lenses – one that is thick at centre and thin at base (convex); the other which is thin at centre and thick at base (concave).
When a beam of light passes through a convex lens, light will converge at a point on the other side of lens. A convex or positive lens will converge and focus light to a certain point.
When the same beam of light is allowed to pass through a concave lens, light diverge as they emerge from the other side. A concave or negative lens will spread or diverge light on the other side of the lens.
Focus is the point where the image is formed. Focal point (f) is a spot where the refracted light meets in case of convex lenses. Concave lenses will have a negative focal point from where the rays originate before diverging through the lens. The focal length is the distance from the center of the lens to the focal point. The principal axis is a horizontal imaginary line drawn through the center of the lens. In a perfect lens, the focal point will reside on the principal axis at a distance of the focal length from the center of the lens.
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