One of the most fascinating principles of beam optics is the power of a lens. Merely stated, a lens’s capacity to bend light is its stamin in ray Optics. The capability of a lens come refract light the passes v it is proportional to its power. The converging ability of a convex lens is characterized by the strength, if the diverging ability of a concave lens is identified by that diverging ability.Are you mindful of the connection between focal length and light beam bending? The variety of light bends rises as the focal length decreases. Together a result, the focal size of a lens is inversely proportional come its intensity. High optical strength is aided through a brief focal range. Therefore, let’s very first discuss some an easy concepts related to the strength of the lens.

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What is Focus and Focal size of a Lens?

A lens is a item of transparent material, mainly glass, the is offered to emphasis or disperse a beam the light. It offers the refraction residential property of light. Refraction is the readjust observed in light’s path when it travel from one tool to another. The lens focuses the light to a point and develops the image. It’s largely used as a call lens in our specs, in the magnifying glass, microscopes, etc.The focal length is a measure up of exactly how sharply the system converges or quarter light; it is the train station of the lens’s optical power.While a focus or focal length point is a suggest at which light from infinity gets converged after passing with the lens. The is denoted by F.


Therefore, the distance or the length of the pole the the lens to its focal suggest is referred to as its Focal length. It is denoted through f.Based on the place of the focal point near a spherical lens, the focal length is of two types: an unfavorable Focal length and also Positive focal length.Negative focal length Length: A an unfavorable focal length,means that the focal suggest is on the exact same side that the lens together the object. Generally, that is acquired in a concave lens together the parallel beam of light traveling parallel to the principal axis shows up to diverge native the suggest or second focus, the emphasis is dubbed virtual focus and its focal size is taken as negative.Positive focal length: A hopeful focal length method that the focal allude of the lens is ~ above the various other side of the lens from whereby the thing is placed. Generally, that is it was observed in a convex lens together the parallel beam of light traveling parallel come the major axis actually meets at a point, the focus is referred to as real focus and its focal length is taken as positive.Thus, a positive focal length denotes that the lens converges light, if a an adverse focal length denotes the the lens quarter light. A lens through a much shorter focal size bends the rays more sharply, pass them to a focus in a much shorter distance or diverging them an ext quickly.

Power of a Lens

The capacity of a lens to bending light is yes, really what provides it its strength in beam Optics. The better a lens’s strength, the greater its capacity to refract irradiate passing v it. Power specifies the converging capability of a convex lens and also the diverging ability of a concave lens. The number of light bends rises as the focal size reduces. As a result, we can assume that the strength of a lens is inversely proportional to its focal distance length. A short focal length, in essence, leader to high optical power.Mathematically, the power of a lens is characterized as:Power (P) = 1 / Focal length (f)or


P = 1 / fIf the focal size is given in meter (m), the power of the lens is calculate in Diopters (D), together the lens’s unit of power is diopter. Another thing come remember is the the optical power of a converging lens is positive, if the optical power of a diverging lens is negative.For instance, if the focal length of a lens is 15 cm, we obtain 0.15 m as soon as we analyze it to meters. Take it the mutual of 0.15 to gain the power of this prism, which is 6.67. As a result, the strength of this lens is 6.67 D. This assumes that the power of a lens have the right to be calculated using the radii of curvature of two surfaces and the refractive index of the lens material.The strength of a lens is inversely proportional to its focal length. Therefore, a short focal size lens has much more power, conversely, a lens of lengthy focal length has actually less power.The power of a convex lens (converging lens) is optimistic as that is focal size is positive.The strength of a concave lens (diverging lens) is negative as its focal size is negative.The power of a aircraft glass plate is 0.

Refractive index

A refractive index is a worth that is regularly used in optical physics. Furthermore, there is no unit. The Refractive index of a substance shows its capability to refract light. To far better understand the meaning of refractive index, one must very first know the idea that refraction. Just put, refraction occurs together light or other radiation changes the medium.The proportion of the amplitude of one electromagnetic tide in a vacuum come its rate in an additional medium is referred to as the refractive index. The denotes the amount of refraction that will certainly exist in a provided substance. In addition, the refractive table of contents in a vacuum is 1.The normal deserve to be defined as the perpendicular line to the interface’s surface. In this case, a higher refractive table of contents value indicates that the beam is bent towards the average. Refractive index refers to the calculation of the bending of a ray of light as it travels from one tool to one more one. The bulk of visible irradiate refractive index values are between 1 and 2. Furthermore, refractive index values for infrared waves can be slightly higher.Formula because that Refractive Index:The refractive table of contents formula is offered as follows:n = c / v


where n is the refractive table of contents of the medium, c is the speed of light when in a vacuum and v is the rate of light once in the medium.

Power the a lens using the refractive index

There is a relation in between the focal length of the lens and it’s the refractive index, it’s recognized as Lens maker’s formula. It consists of the radius that curvatures that both surfaces. The lens is a part of the hollow sphere of glass, Radius that curvature of the lens is the radius of the sphere. Each lens has actually two radii the curvature.
Radii of curvature the a convex lens
According to the Lens maker’s formula:1/f = (n-1) × (1/R1 – 1/R2)where, n is the Refractive index of the material, f is the focal size of the lens, R1 is the radius of curvatures of an initial surface and also R2 is the Radius the curvature of 2nd surface.Also the strength of a lens is offered by,P = 1 / fNow, from both the over equations:P = (n-1) × (1/R1 – 1/R2)This is the forced relation between the power and also the refractive index of the lens. This formula have the right to be used to discover the power of the lens making use of the refractive index of the material and radius that curvatures of the lens.


Power of combination of lenses

Two or more lenses have the right to be combined in order to boost or decrease the power of lenses. The formula of the mix is simple and is described with the assist of an image.
Combination of lenses
Let there be two lenses A and also B with focal lengths f1 and f2 respectively. These two lenses are put in call with each other such that their primary axes coincide through each other. Things is put at O top top the primary axis that the combination. Lens A produces picture of the thing at E1. This photo acts as things for lens B and also the last image is created at E.PO = u, i.e. Object distance for lens APE = v, i.e. Last image distancePE1 = V1, i.e. Image distance because that lens A and also object street for lens B.Using Lens formula on the image formed by lens A:1/v1 – 1/u = 1/f1 ……(1)Using Lens formula ~ above the image created by lens B:1/v – 1/v1 = 1/f2 ……(2)Adding equation (1) and (2) as:


1/v – 1/u = 1/f1 + 1/f2 ……(3)Replace the mix with a solitary lens of focal length F, such the the final image is developed at E.1/v – 1/u = 1/F ……(4)Now indigenous equation (1) and (2) as:1/F = 1/f1 + 1/f2 ……(5)where, F is the focal length of the mix of lenses A and B.Since,P = 1 / fTherefore, equation (5) transforms toP = P1 + P2


where, p is the strength of the combination of lenses, P1 is the power of lens A, P2 is the strength of lens B.This is the formula because that the merged power the lenses. Proper sign convention demands to be followed when substituting the values of P1 and P2.

Sample Problems

Problem 1: how does the strength of a lens adjust if the focal length is doubled?Solution:Power gets halved as strength is inversely proportional come focal length.Problem 2: What is the strength of convex lens (with sign) the focal size 40cm?Solution:Since, power = 1 / fSubstituting the given values as,P = 100/40 = 2.5DSince it’s a convex lens, therefore power will certainly be positive.Thus, the strength of convex lens is +2.5D.Problem 3: determine the kind of lens, and also its focal length if its strength is 0.2D.Solution:Since the focal length, f = 1 / strength (P)Therefore, substituting the given values in the above expression as:f = 1 / (0.2D) = 5 mSince the power is positive, because of this given lens is a convex lens.Problem 4: Sunglasses have actually curved surface but still perform not have any kind of power. Why?Solution:Sunglasses have two bent surfaces, one is convex and another is concave. Both surfaces room of equal power yet of the contrary signs, so both the power cancels the end each other and resultant strength is 0.Problem 5: A convex lens the focal length 50 centimeter is in call with a concave lens of 20 cm focal length. Find the power of the combination of lenses.Solution:For a combination of lenses,P = P1 + P2orP = 1/f1 + 1/f2P = 100/50 + 100/(-20) (concave lenses have an adverse focal length) = -3 D
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