Technical terms related to lens - class-VIII

A lens is a piece of transparent glass bounded by two spherical surfaces.

focal length is basically length from the optic centre to the focus, so it is expressed in metres

Principal axis is a straight line passing through the two centres of curvature of lens.

The principal axis is also called optical axis of a lens, passing through the optical center of the lens.

Aperture of a lens is the length of the surface of lens exposed for incident light.Focal length is the distance between focus of the lens and lens optical center.Principal focus is the focus from which a ray of light traveling parallel to optical axis passes through or appears to pass through after refraction.There is no term like efficiency of a lens.but there is magnification of a lens.

When the object is placed at infinity, i.e., when the rays are parallel to the principal axis, the value of focal length is equal to the value of the image distance.

Principal axis is also called optical axis of the mirror or lens .

The focal length is associated with distance. So, it can be expressed in metre.

The term momentum is not associated with a lens. It is the product of mass and velocity of a body.

For a silvered plano-convex lens, the focal length is given by 1/f = 2/f_lens + 1/f_mirror. With f_mirror = infinity (plane surface) and f_lens = R/(n-1), the effective focal length is R/(2(n-1)). Setting this to 0.2m with n=1.5 yields R = 0.2m.

${R _1} = 60,cm,,,{R _2} = \infty ,,,\mu  = 1.6$

A point on the principal axis of a lens, to which light rays parallel to the principal axis converge after passing through the lens, is  called focus of the lens.

Centre point of a lens is called the Optical Centre of the lens. Any ray of light passing through it goes un-deviated.

$\dfrac{1}{f _r}=\dfrac{1}{f _1}+\dfrac{1}{f _2}-\dfrac{d}{f _1f _2}$