Science Define :

The minimum distance from the normal eye, at which the objects can be seen clearly and distinctly without any strain to the eye is known as the distance of distinct vision.

The ability of the lens of adjusting focal length is known as power of accommodation.

The small opening of variable diameter at the centre of iris is called the pupil.

The dark muscular diaphragm behind cornea is called iris.

The tendency of the pupil to adjust the opening for light is called adaptation.

The light sensitive screen in the human eye is called retina.

A lens is a transparent medium bounded by two surfaces; out of which atleast one surface is spherical.

The lens which diverges the light incident on it is called diverging lens.

The lens which converges the light incident on it is called converging lens.

The image obtained when the refracted rays appear to meet is called the virtual image.

The image obtained when the refracted rays actually meet each other is called a real image.

A lens having both the spherical surfaces curved inwards is a concave lens.

A lens having both surfaces spherical, bulging outward, is a convex lens.

The distance between optical centre and principal focus is called focal length of a lens.

The point on the principal axis where all the incident rays parallel to the principal axis actually meet or appear to meet is called principal focus.

The imaginary point on the lens through which the light ray pass undeviated is called optical centre.

The imaginary line passing through the centres of curvatures is called principal axis.

The radii of the spheres whose parts form surfaces of the lenses are called radii of curvatures.

The centres of the spheres whose parts form surfaces of the lenses are called centres of curvature.