As the distances given in the question are large, so we choose a scale of 1: 5, i.e., 1 cm represents 5 cm. More generally these are often used in compound lenses used in various instruments such as magnifying devices like microscopes, telescopes and camera lenses. Ans. This point is known as the focus and the distance between the center of the lens to the focus is called the focal length of convex lens. An optical lens is generally made up of two spherical surfaces. These types of lenses can converge a beam of light coming from outside and focus it to a point on the other side. (ii) Centre line is shown by DE. The lens in the human eyes is the prime example. (iii) Mark points F and B on the left side of lens at a distance of 3 cm and 2 cm respectively. should be placed at a distance of 54 cm on the object side of the mirror to obtain a sharp image. BO = – u, DI = +v, we get .....(10) Equation (10) is the familiar thin lens formula. Second Formula for Magnification There is another formula of magnification Note : - If magnification (m) is positive , It means image formed is virtual and erect If magnification (m) is negative, It means image formed is real and inverted Questions Example 10.1 - A convex mirror used for rear-view on an automobile has a radius of curvature of 3.00 m. A concave lens of focal length 15 cm forms an image 10 cm from the lens. For correcting the distortions and aberrations, it is better to combine both types of lenses. convex lens can converge a beam of parallel rays to a point on the other side of the lens. It will be found to be equal to 6 cm.Thus, object is placed at a distance of 6 cm × 5 = 30 cm from the lens. For example. The image is real, inverted and enlarged in size. However, if one of the surfaces is flat and the other convex, then it is called a plano-convex lens. (x) Then, measure CB' and A'B'. Now, we draw the ray diagram as follows:(i) Draw a horizontal line to represent the principal axis of the convex lens. This formula is applicable to both convex and concave lenses. There is another type of lens known as concave lens. If those surfaces are bent outwards, the lens is called a biconvex lens or simply convex lens. This relation is: 1v-1u = 1f In words, we can say that 1Image distance - 1Object distance =1Focal length This formula is applicable to both convex and concave lenses. Image size, h' = ? An object 5.0 cm in length is placed at a distance of 20 cm in front of a convex mirror of radius of curvature 30 cm. (v) Draw a line AD, parallel to principal axis. Now, using the mirror formula, 1u+1v = 1f∴ 1v = 1f-1u ⇒ = 1-18-1-27 = -3+254 = -154i.e., v = -54 cm The screen should be placed at a distance of 54 cm on the object side of the mirror to obtain a sharp image. (vii) Draw a line CA', backwards, so that it meets the line from D parallel to principal axis at A. How far is the object placed from the lens? Magnification, m = h'h = -vu ∴ Image size, h' = -vhu =-(-54)×(+7)(-27)= -14 cm The image is real, inverted and enlarged in size. (ix) The AB is position of object. A simple kind of these lenses can focus light into an image, but that image won’t be of a high quality. 2020 Zigya Technology Labs Pvt. Here, Object size, h = + 7.0 cmObject distance, u = - 27 cmFocal length, f = - 18 cm Image distance, v = ? Explore more about other related concepts at BYJU'S. View Answer NCERT Question 9 - One-half of a convex lens is covered with a black paper. Find the size and the nature of the image. It can form both real and virtual images depending on where the light is coming from. The convex lens is a lens that converges rays of light that convey parallel to its principal axis (i.e. (xi) Thus the final position, nature and size of the image A'B' are: (a) Position of image A'B' = 3.3 cm × 5 = 16.5 cm from the lens on opposite side. 232, Block C-3, Janakpuri, New Delhi,
https://www.zigya.com/share/U0NFTjEwMDUyNjMy. This lens is thinner at the centre and gets thicker as we move towards the edges. This lens is thicker at the centre and gets thinner as we move towards the edges. It is used to correct Hypermetropia or long-sightedness. For refraction at the convex lens, we haveu = –20 cm; f1 = 10 cm; v = v1 = ?Using lens formula, we have The convex lens produces converging rays trying to meet at I1, 20 cm from the convex lens, i.e., 15 cm behind the concave lens.I1 will serve as a virtual object for the concave lens.For refraction at the concave lens, we haveFor concave lens As per sign convention i.e., (iv) Draw an arrow AB of height 1 cm on the left side of lens at a distance of 5 cm from the lens. Measure distance BC. BNAT; Classes. The distance of the object from the lens is 15 cm. Stay tuned with BYJU’S to learn more about the lens, convex lens, and much more. At what distance from the mirror should a screen be placed so that a sharp focussed image can be obtained? NCERT Books. Is the same formula applicable to both convex and concave lenses? A concave lens will always produce diminished, upright and virtual image of the object in front of it. Hi I am Girish Chandra.Welcome to my youtube channel Chandra Mathematics. But the nature of the image produced by a these lenses depends upon the position of the object. Drawing the ray diagram: Using a scale of 1: 5, we get v = - 2 cm, f = - 3 cm. (viii) Draw a line AB, perpendicular (downwards) from A to meet the principal axis at B. Focal length, f = - 15 cm [f is - ve for a concave lens], Image distance, v = - 10 cm [Concave lens forms virtual image on same side as the object, so v is - ve]. Draw the ray diagram and find the position, size and the nature of the image formed. (iii) Mark two foci F and F' on two sides of the lens, each at a distance of 2 cm from the lens. So the most common use of the lens is that it helps us to see. This lens diverges a straight beam of light. NCERT Books for Class 5; NCERT Books Class 6; NCERT Books for Class 7; NCERT Books for Class 8; NCERT Books for Class … Also find its magnification. A virtual and erect image of height 2.2 cm is formed behind the mirror (because v is positive) at a distance of 8.6 cm from the mirror. Find the nature, position and size of the image. This lens formula is applicable to both the concave and convex lens. (ix) Now AB', represents the real, but inverted image of the object AB. It is used in cameras because it focuses light and produces a clear and crisp image. Delhi - 110058. Read about the convex lens along with its use and difference between the concave and convex lens. (v) Draw a line AD parallel to principal axis and then, allow it to pass straight through the focus (F') on the right side of the lens. converges the incident rays towards the principal axis) which is relatively thick across the middle and thin at the lower and upper edges. (b) Nature of image A’B’: Real and inverted. A virtual and erect image of height 2.2 cm is formed behind the mirror (because v is positive) at a distance of 8.6 cm from the mirror. Though we derived it for a real image formed by a convex lens, the formula is valid for both convex as well as concave lenses and for both real and virtual images. We are given a convex mirror. Focal length, f = - 15 cm [f is - ve for a concave lens]Image distance, v = - 10 cm [Concave lens forms virtual image on same side as the object, so v is - ve]As, Object distance, u = -30 cm. Find the position of the image, its nature and size. Draw the ray diagram. Required fields are marked *. 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