Test Your Understanding and Answer These Questions: The power of a lens is defined as the ability of the lens to converge or diverge a beam of light falling on it. In other words, a lens of small focal length is said to have greater power. it is said to be more powerful. In other words, a lens of large focal length is said to be of lesser power. The power of a convex lens is positive as a convex lens has a positive focal length, while the power of a concave lens is negative as concave lens has a negative focal length. Lens power is measured in dioptres (D). The power for a convex lens is positive and the power for a concave lens is negative. Converging (convex) lenses have positive focal lengths, so they also have positive power values. Read about our approach to external linking. Diverging (concave) lenses have negative focal lengths, so they also have negative power values. Ans: P= -0.5D Q13. It is given as, \frac {1} {i} + \frac {1} {o} = \frac {1} {f} i= distance of the image from the lens. The SI unit of power of a lens is dioptre and often denoted by D. Note that as the focal length of a concave lens is negative, the power of this type of lens is negative (-), whereas the power of a convex lens is positive (+) as the focal length of this lens is positive. At what distance should the object from the lens be placed so that it forms an image at 10 cm from the lens? We will start wit A combination of convex and concave lenses has power 4 D.If the convex lens has power 5 D focal length of the concave lense will be View Answer A bi-convex lens is formed with two thin plano-convex lenses as shown in the figure. Rules for Obtaining Images by Spherical Lenses. First convert focal length into metres: f = 25 cm = 0.25 m, \[lens \\ power = \frac{1}{0.25} = 4.0 \\ D\]. \[power \\ of \\ a \\ lens \\ (D) = \frac{1}{focal \\ length \\ (m)}\] \[P = \frac{1}{f}\] Converging ( convex ) lenses have positive focal lengths, so they also have positive power values. Reflection of Light ... Power of a lens NCERT Questions → Class 10. For instance, if the focal length (f) of a lens is 1 m, the power of the lens (p) is equal to 1/f = 1/1 = 1 dioptre. In other words, a lens of large focal length is said to be of lesser power. The focal point is the point where parallel light rays meet. Using lens maker s formula 1f = (μ-1) 1R1-1R2 ∴ Power of lens, P = 1f = (μ-1) 1R1-1R2Refractive index of violet light is greater than refractive index of red light.Therefore, power of the lens will be increased when red light is replaced by violet light. Also, find the power of the lens. They separate but appear to come from a principle focus on the other side of the lens. The power of a lens is defined as the, of the focal length. Power of a lens. Our tips from experts and exam survivors will help you through. So, when f = 1 meter, then, Power = or Power = = 1 dioptre. What will be the resultant power? Thus, the power of a lens is the reciprocal of focal length of the lens, i.e., where, p = power of the lens f = focal length of the lens in meters. o= distance of the object from the lens. Diverging (concave) lenses have negative focal lengths, so they also have negative power values. Calculate the focal length and state what type of lens it is. lenses have positive focal lengths, so they also have positive power values. ) Sample exam questions - light and the electromagnetic spectrum, Home Economics: Food and Nutrition (CCEA). Define lens formula. VIEW MORE. Due to the differences in curvature, image formation is different in concave and convex lenses. It is represented by the letter P. The SI unit of power of a lens is ‘dioptre’. A convex lens converges the light rays towards the principal axis whereas a concave lens diverges the light rays away from the principal axis. Lens Formula Power of a lens NCERT Questions → Facebook Whatsapp. f = focal length of the lens in meters. An object 5 cm high is held 25 cm away from a converging lens of focal length 20 cm.