On the other hand, a lens is said to be of lesser power if it diverges or converges a beam of light to less extent. Here, P =. $power \\ of \\ a \\ lens \\ (D) = \frac{1}{focal \\ length \\ (m)}$ $P = \frac{1}{f}$ Converging ( convex ) lenses have positive focal lengths, so they also have positive power values. it is said to be more powerful. First convert focal length into metres: f = 25 cm = 0.25 m, $lens \\ power = \frac{1}{0.25} = 4.0 \\ D$. 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. Ans: P= -0.5D Q13. 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. Sign in, choose your GCSE subjects and see content that's tailored for you. The power of a lens is defined as the, of the focal length. The focal point is the point where parallel light rays meet. unit of power of a lens. 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. $power \\ of \\ a \\ lens \\ (D) = \frac{1}{focal \\ length \\ (m)}$. 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. It is denoted by the symbol D. One dioptre is the power of a lens whose focal length is 1 meter. where, p = power of the lens. Calculate the power of the lens. The S.I. Concepts. Example 10.3 - A concave lens has focal length of 15 cm. A concave lens of power -2D is placed in contact with a convex lens of power +1.5D. A concave lens is thinner in the middle than it is at the edges. Find the power of a concave lens of focal length 4 m. Find the focal length of a concave lens of power – 4 D. A convex lens forms a real and inverted image of a needle at a distance of 50 cm from the lens. Lens power is measured in dioptres (D). f= focal length of the lens. An object 5 cm high is held 25 cm away from a converging lens of focal length 20 cm. We will start wit Reflection of Light ... Power of a lens NCERT Questions → Class 10. What will be the resultant power? Diverging (concave) lenses have negative focal lengths, so they also have negative power values. In other words, a lens of small focal length is said to have greater power. 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. In other words, a lens of large focal length is said to be of lesser power. A convex lens has a focal length of 5.0 cm. 1 F. The power of a lens is defined as the inverse of its focal length (f) in meters (m). 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. If a lens has a shorter focal length it is said to be more powerful. In other words, a lens of large focal length is said to be of lesser power. If you’ve previously read about the lens maker’s formula, you would’ve realized that what we are calculating there is actually the power of the lens. lenses have negative focal lengths, so they also have negative power values. Lenses are precisely shaped pieces of glass that have been developed and used in corrective glasses, telescopes, microscopes, binoculars, and magnifying glasses. 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