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. (viii) Draw a line AB, perpendicular (downwards) from A to meet the principal axis at B. NCERT Books. Example 10.4 - A 2.0 cm tall object is placed perpendicular to the principal axis of a convex lens of focal length 10 cm. It is found that CB' = 3.3 cm and A'B' = 0.7 cm. (x) Then, measure CB' and A'B'. We are given a concave mirror. Here, Object size, h = + 7.0 cmObject distance, u = - 27 cmFocal length, f = - 18 cm Image distance, v = ? (vi) Draw a line from A to C (centre of the lens), which goes straight without deviation. The image is real, inverted and enlarged in size. An optical lens is generally made up of two spherical surfaces. (iv) Join any point D (nearly at the top of lens) and F by a dotted line. 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. Magnification, m = -vu= h'h Therefore,  Image size, h' = -vhu                          = -8.6 × 5-20                         = 2.15 ≃ 2.2 cm. More generally these are often used in compound lenses used in various instruments such as magnifying devices like microscopes, telescopes and camera lenses. Ltd. Download books and chapters from book store. If the equation shows a negative image distance, then the image is a virtual image on the same side of the lens as the object. BNAT; Classes. A concave lens will always produce diminished, upright and virtual image of the object in front of it. We draw the ray diagram as follows:(i) Draw the principal axis (a horizontal line). This lens formula is applicable to both the concave and convex lens. How far is the object placed from the lens? We are given a convex mirror. Now, we draw the ray diagram as follows:(i) Draw a horizontal line to represent the principal axis of the convex lens. (vi) Draw a line A'B', perpendicular to principal axis from B'. Magnification, m = h'h = -vu ∴ Image size,                         h' = -vhu                              =-(-54)×(+7)(-27)= -14 cm The image is real, inverted and enlarged in size. These types of lenses can converge a beam of light coming from outside and focus it to a point on the other side. As the distances given in the question are large, so we choose a scale of 1: 5, i.e., 1 cm represents 5 cm. (iii) Mark two foci F and F' on two sides of the lens, each at a distance of 2 cm from the lens. An object of size 7.0 cm is placed at 27 cm in front of a concave mirror of focal length 18 cm. Find the position of the image, its nature and size. So the most common use of the lens is that it helps us to see. The major differences between concave and convex lens two are: These are used for a variety of purposes in our day-to-day lives. should be placed at a distance of 54 cm on the object side of the mirror to obtain a sharp image. If those surfaces are bent outwards, the lens is called a biconvex lens or simply convex lens. Lens formula: The lens formula is a mathematical relation between the object distance u, image distance v and focal length f of a spherical lens. Find the nature, position and size of the image. (vii) Draw a line CA', backwards, so that it meets the line from D parallel to principal axis at A. 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. Delhi - 110058. convex lens can converge a beam of parallel rays to a point on the other side of the lens. Measure distance BC. At what distance from the mirror should a screen be placed so that a sharp focussed image can be obtained? Is the same formula applicable to both convex and concave lenses? (ix) The AB is position of object. 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. 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. It is used in cameras because it focuses light and produces a clear and crisp image. This formula is applicable to both convex and concave lenses. BO = – u, DI = +v, we get .....(10) Equation (10) is the familiar thin lens formula.

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