Answer . unit of power of a lens is $m^{-1}$. Lens formula in terms of power. 5 years ago. Define lens formula. Draw the ray diagram and find the position, size and nature of the image formed. The object lies close to principal axis. Anonymous. An object 5 cm high is held 25 cm away from a converging lens of focal length 20 cm. Power of lens calculation. Assumptions made: The lens is thin. 5 cm. (b) Now, Focal length of concave lens, f = –16 cm Object distance, u = 12 cm ∴ How to find the power of lens using the focal length. Power of lens and its unit . The incident rays make small angles with the lens surface or the principal axis. The S.I. Power of a Lens. A concave lens of focal length 15 cm forms an image 10 cm from the lens. Optical power. For a concave mirror: In figure (a), ∠ B P ′ C = ∠ P ′ C F (alternate angles) and ∠ B P ′ C = ∠ P ′ F (law of reflection, ∠ i = ∠ r) Hence ∠ P ′ C F = ∠ C P ′ F ∴ F P ′ C is isosceles. Diopter formula. Focal length, f = 20 cm (a) Using the lens formula, 1 v = 1 f + 1 u ∴ 1 v = 1 20 + 1 12 = 3 + 5 60 = 8 60 i.e., v = 60 / 8 = 7. The power of a lens is specified as P = $\frac{1}{F}$, where f is the focal length. The formula delivers the focal length within the paraxial approximation, not considering spherical aberrations, for ... A mirror with a curvature radius R of the surface has a focal length f = R / 2, if the beam axis is normal to the mirror surface. In the sign convention used in optical design, a concave mirror has negative radius of curvature, so = −, where R is the radius of curvature of the mirror's surface. Image is at a distance of 7.5 cm to the right of the lens, where the beam converges. This is also known as diopter. 0 0. (For a spherically curved mirror, the focal length is equal to half the radius of curvature of the mirror. (We take positive signs for concave curvatures and focusing mirrors.) MEDIUM. This is a basic equation used to calculate the focal length of a lens given the radius of curvature of the lens and the refractive index of the lens relative to the medium. The radius r for a concave mirror is a negative quantity (going left from the surface), and this gives a positive focal length, implying convergence. From the geometry of the spherical mirror, note that the focal length is half the radius of curvature: Show: As in the case of lenses, the cartesian sign convention is used here, and that is the origin of the negative sign above. Video Explanation. Is lens formula applicable only for convex lens? Definition: The equation relating the object distance (u), the image distance (v) and the focal length (f) of the lens is called the lens formula. The focal length is positive for a concave mirror, and negative for a convex mirror.) The focal length is positive for a concave mirror, and negative for a convex mirror. Write the relation between focal length and radius of curvature of a spherical mirror? It is only an approximation and works best for thin lenses. For a spherically curved mirror in air, the magnitude of the focal length is equal to the radius of curvature of the mirror divided by two. The lens has a small aperture.