Class 10 Science Solutions English Medium

Refraction by Spherical Lenses: 

Lens: A piece of glass or other transparent material bound by two
surfaces, of which one or both surfaces are spherical, forms a lens. This means that a lens is bound by at least one spherical surface.

Category of Lens: There are two types of lens.

(i) Convex lens: A lens which spherical surfaces bulging outward, such a lens is called a double convex lens. It is simply called a convex lens. It converges light rays hence convex lenses are also called converging lenses.

Features of convex lens:

(i) It is thicker at the middle as compared to the edges.

(ii) Convex lens converges light rays. 

(ii) Concave Lens: A lens which is bounded by two spherical surfaces and curved inwards such lens is called concave lens. Concave lens is thicker at the edges than at the middle and it diverge light rays so it is called diverging lens. A double concave lens is simply called a concave lens.

Features of concave lens: 

(i) Concave lens is thicker at the edges than at the middle.

(ii) It diverge light rays

Parts of Spherical Lens:

(i) Centers of Curvature: A lens, either a convex lens or a concave lens, has two spherical surfaces. Each of these surfaces forms a part of a sphere. The centres of these spheres are called centres of curvature of the lens.

The centre of curvature of a lens is usually represented by the letter C.

Since there are two centres of curvature, we may represent them as C1
and C2.

(ii) Principal axis: An imaginary straight line passing through the two centres of curvature of a lens is called its principal axis.

(iii) Optical Centre: The central point of a lens is its optical centre. It is usually represented by the letter O.

(iv) Aperture: The effective diameter of the circular outline of a spherical lens is called its aperture.

(v) Thin lens with small aperture: Lenses whose aperture is much less than its radius of curvature. Such lenses are called thin lenses with small apertures.

(vi) Principal Focus of convex lens: A point on the principal axis on which light rays after refraction from the lens converge, this point is called principal focus of convex lens. 

(vii) Principal Focus of concave lens: Light rays after refraction from the lens, are appearing to diverge from a point on the principal axis. This point on the principal axis is called the principal focus of the concave lens. 

Letter F is usually used to represent principal focus. However, a lens has two principal foci. They are represented by F₁ and F₂. 

See the fig.

Image of sun on paper sheet by using lens: 

When we point a lens over a paper sheet for sometimes. The paper begins to burn producing smoke. It may even catch fire after a while. The light from the Sun constitutes parallel rays of light. These rays were converged by the lens at the sharp bright spot formed on the paper. In fact, the bright spot you got on the paper is a real image of the Sun. The concentration of the sunlight at a point generated heat. This caused the paper to burn.

(viii) Focal Length: The distance of the principal focus from the optical centre of a lens is called its focal length. The letter f is used to represent the focal length.