Terms for Lenses
Terms for Lenses
The lenses are made of two curved surfaces which are a part of the sphere. The diagram below shows important terms related to Convex Lens.
The diagram below shows important terms related to Concave Lens.
Optical Centre (O):
The central point O of the lens is called its optical centre. A ray incident towards the optical centre passes almost without any deviation through the lens.
Centres of Curvature:
The centres of the two imaginary spheres of which the lens is a part are called centre of curvature of the lens. It represented by . A lens has two centres of curvature with respect to its two curved surfaces.
Radii of Curvature:
The radii of the two imaginary spheres of which the lens is a part are called radii of curvature of the lens. A lens has two radii of curvature. These may or may not be equal.
Principal Axis:
Lens is bounded by two spherical surfaces, and therefore, has two centres of curvature and
. The imaginary line joining them is called the Principal Axis of the lens.It passes through the optical centre.
Principal Focus
A lens has two surfaces and hence two focal points first focal point is an object on the principal axis foe which image is at infinity while second focal point is an image point on the principal axis for which object is at infinity.
First Principal Focus It is a point on the principal axis of lens, the rays starting from or directed to which become parallel to principal axis after refraction.
Second Principal Focus It is the point on the principal axis at which the rays coming parallel to the principal axis converge on the other side of lens (convex) or appear to meet on the same side of lens (concave) after refraction from the lens.
Both the foci of convex lens are real while that of concave lens are virtual.
Focal Length:
The distance between optical centre of a lens and the point where the parallel beam of light converge or appear to diverge from i.e. focus is called focal length.
Focal Plane:
The plane passing through the focus and perpendicular to the principal axis is called focal plane.
Aperture :
The effective diameter of its light transmitting area is called the aperture of lens. It controls the brightness i.e. intensity of image formed by a lens which depends on the light passing through the lens. It is proportional to the square of aperture.i.e.
Which of the following are correct ? (a) The lenses are made of two curved surfaces which are a part of the sphere. (b) Lens is bounded by two spherical surfaces, and therefore, has two centres of curvature (c) The effective diameter of its light transmitting area is called the aperture of lens. | |||
| Right Option : D | |||
| View Explanation | |||
Which of the following are correct ? (a) The centres of the two imaginary spheres of which the lens is a part are called centre of curvature of the lens. (b) The radii of the two imaginary spheres of which the lens is a part are called radii of curvature of the lens. | |||
| Right Option : C | |||
| View Explanation | |||
The centers of the two imaginary spheres of which the lens is a part are called _______________________ . | |||
| Right Option : A | |||
| View Explanation | |||
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