Power of a lens is expressed in dioptre when the focal length is expressed in metre. Power of lens is 1 diopter if its focal length is 1 m. Consider 2 lenses with focal length.When lens combine,they behave as a single lens of focal length. Thus, the power of a converging lens is positive and that of the diverging lens is negative. 5)Concave lens is used to correct myopia defect of human eye. Power of a lens. Dioptre formula is used to calculate the optical power of a lens or curved mirror. Example. If the combination is replaced by a single lens of focal length F such that it forms the image of O at the position I, \[\frac{1}{v}\] - \[\frac{1}{u}\] = \[\frac{1}{F}\]……(4). The power of a lens is measured as the reciprocal of the focal length of the lens. Suppose, a point object is placed at a point O beyond the focus of lens \[L_{1}\] such that OP = u (object distance) on the common principal axis (coaxially). 32. Question 4 A convex and concave lens of focal length 50 cm and 40 cm are combined together,what is the power of the combination? The first lens \[L_{1}\] alone forms an image at \[I_{1}\] where P1 = \[v_{1}\] (image distance). The focal length (f) of a converging lens is considered positive and that of a diverging lens is considered negative. 4)It is used to correct hypermetropia defect. The power of a lens is specified as the inverse of the focal length in meters, or D =\[\frac{1}{F}\], where D is the power in dioptres. Consider 2 lenses with focal length.When lens combine,they behave as a single lens of focal length. Live class of electricity : Understanding series and parallel resistors. Is the prescribed lens diverging or converging? The thick convex lens has more power than a thin one because the thick one has a greater curvature and less focal length. (2), \[\frac{1}{v_{1}}\] -\[\frac{1}{u}\] + \[\frac{1}{v}\] - \[\frac{1}{v_{1}}\] = \[\frac{1}{F_{1}}\] + \[\frac{1}{f_{2}}\], \[\frac{1}{v}\] - \[\frac{1}{u}\] = \[\frac{1}{F_{1}}\] + \[\frac{1}{f_{2}}\]…. On this Page, we will Learn About the Following: The Focal Length of the Combined Lens is Given by. \[\frac{1}{F}\] = (v - 1) (\[\frac{1}{R_{1}}\] - \[\frac{1}{R_{2}}\]), P = (v - 1) (\[\frac{1}{R_{1}}\] - \[\frac{1}{R_{2}}\]), Here, v = refractive index of the material, \[R_{1}\] = Radius of curvature of the first surface of the lens, \[R_{2}\] = Radius of curvature of the second surface of the lens. Question 6 What is the SI unit of power of lens? Your email address will not be published. It is represented by the letter P. The power P of a lens of focal length f is given by \(P = \frac{1}{f}\) Power of a convex lens is positive and that of a concave lens is negative. When f = 1 meter, P = \[\frac{1}{F}\] = 1/ 1 = 1 dioptre. Pro Lite, Vedantu Filed Under: Class 10, Light-Reflection and Refraction Tagged With: application of lens, diopter, power of lens. The focal lengths of the lenses are \[f_{1}\] and \[f_{2}\] respectively. Power of Lens … How to Find the Power of the Lens Using the Focal Length? Thus, a lens having 1 metre of focal length has power equal to 1 dipotre. \[\frac{1}{F}\] = \[\frac{1}{f_{1}}\] + \[\frac{1}{f_{2}}\]……(5), Here, F is the focal length of the equivalent lens for the combination. It is denoted by the letter D. Pro Lite, Vedantu ; The lens has a small aperture. Chapter-4 Refraction of Light at plane surface. 31. Also, find the magnification produced by the lens. A convex lens converges the light rays towards the principal axis whereas a concave lens diverges the light rays away from the principal axis. 2)Convex lens of small focal length is used in microscope to study biological specimen. In general, the lenses come in two shapes: convex (curved outward) and concave (curved inward). When f is in 1 cm, P = \[\frac{1}{F}\] / 100 = 100/ f. So we get the formulas to describe the relationship between P and f. Optical power is defined as the degree to which a lens, mirror, or other optical system converges or diverges the light.