1. In connection with spherical lenses, the following terms are defined as:
(i) Centre of curvature: It refers to the center point of the sphere from which the lens is derived. For a convex lens, the center of curvature is located on the side opposite to the object.
(ii) Radius of curvature: It is the distance between the center of curvature and the surface of the lens. It is denoted by the symbol 'R'.
(iii) Principal focus: It is the point on the principal axis where the parallel rays of light converge or appear to diverge after passing through the lens. It is denoted by the symbol 'F'.
(iv) Optical centre: It is the center point of the lens and coincides with the intersection of the principal axis and the lens. Light rays passing through the optical center do not undergo any deviation.
(v) Aperture: It refers to the opening or the diameter of the lens through which light passes. It determines the amount of light entering the lens.
2. Formation of image in a concave mirror:
(i) At infinity: When the object is placed at infinity, the rays of light are parallel to the principal axis. After reflection, they converge at the focal point, forming a real and inverted image.
(ii) Beyond the centre of curvature: In this case, the object is placed beyond the center of curvature. The reflected rays converge and form a real, inverted, and diminished image between the focal point and the center of curvature.
(iii) Between the center of curvature and the focus: When the object is placed between the center of curvature and the focus, the reflected rays converge and form a real, inverted, and enlarged image beyond the center of curvature.
(iv) Between the pole and the focus: In this case, the object is placed between the pole and the focus. The reflected rays diverge, and the image formed is virtual, upright, and enlarged.
(v) At the principal focus: When the object is placed at the principal focus, the reflected rays become parallel and do not converge. As a result, no image is formed.
3. Formation of image in a convex lens:
(i) At infinity: When the object is placed at infinity, the rays of light coming from the object are parallel. After passing through the lens, they converge and form a real, inverted, and diminished image at the focal point.
(ii) Beyond 2F1: In this case, the object is placed beyond twice the focal length (2F1). The rays of light converge after passing through the lens and form a real, inverted, and diminished image between the focal point and the optical center.
(iii) Between F1 and 2F1: When the object is placed between the focal point (F1) and twice the focal length (2F1), the rays of light converge and form a real, inverted, and enlarged image beyond 2F1.
(iv) At focus F1: If the object is placed at the focal point, the rays of light after passing through the lens become parallel and do not converge. Hence, no image is formed.
(v) Between F1 and the lens (O): When the object is placed between the focal point (F1) and the lens (O), the rays of light diverge after passing through the lens. The image formed is virtual, upright, and enlarged.