Presentation on theme: "Image formation by Lenses"— Presentation transcript:
1 Image formation by Lenses Chitranjan KumarLecturer, G.P.C.G.,Jalandhar
2 In daily life, you may have come across two types of lenses that are 1. thicker or 2. thinner in the middle.Which of the following items do you think may contain lenses?Eye glasses.Peep-hole.Camera.Magnifying glasses.Security mirror.
3 What happens to the light when it When you see through lenses,do the objects appear the same as they are?No !What happens to the light when itpasses through a lens
4 Introduction Lenses are very useful. e.g. in cameras, projectors, telescopes, microscopes and eyesThe most common lenses:spherical lenses(i.e. the surface is spherical)
5 1 Convex and concave lenses Convex lens - thicker at the centre than at the edge.Bi-convexPlano-convexConvex-concave
6 1 Convex and concave lenses Concave lens - thinner at the centre than at the edge.Bi-concavePlano-concaveConvex-concave
7 1 Convex and concave lenses Converging or Diverging?convex lensconcave lens(converging lens)(diverging lens)
8 1 Convex and concave lenses Converging or Diverging?convex lensconcave lens(converging lens)(diverging lens)
9 1 Convex and concave lenses Converging lensbend the light inwardsYou may remember ‘converging’ as ‘concentrating’.
10 1 Convex and concave lenses Diverging lensbend the light outwardsYou may remember ‘diverging’ as ‘dividing’.
11 1 Convex and concave lenses Converging or diverging?Recall that light travels more slowly in glass than in air.Light converges or diverges as some parts of the wavefronts lag behind.convergesdiverges
13 Parallel rays are refracted inwards. Convex LensParallel rays are refracted inwards.Refracted rays meet at a point calledprincipal focusF
14 Centre of the lens is called the optical centre C.
15 The distance of F from C is the focal length f of the lens.
16 A convex lens has 2 principal foci. 1 on each sideFF'FF'Cprincipal axisThe line through the optical centre and 2 foci is called the principal axis.
17 Parallel rays are refracted outwards. Concave LensParallel rays are refracted outwards.Refracted rays appear to spread from a point calledprincipal focus
18 Centre of the lens is called the optical centre C.
19 The distance of F from C is the focal length f of the lens.
20 A concave lens has 2 principal foci. 1 on each side principal axisThe line through the optical centre and 2 foci is called the principal axis.
21 Q1 A light ray is incident on a convex lens. Which one represents the path of the light ray?XYZA Path X.B Path Y.C Path Z.
22 Q2 A light ray is incident on a concave lens. Which one represents the path of the light ray?XYZA Path X.B Path Y.C Path Z.
23 2 Image formation by a lens When the light from a point of an object enters our eyes,we can see the point.screenOI
24 2 Image formation by a lens a Real imagesLight rays converge to a point.OI
25 2 Image formation by a lens Image can be captured by a screen.Hence called ‘real’.screenOI
26 2 Image formation by a lens b Virtual imagesLight rays diverge from a point.IONo rays actually come from the image.concave lensHence called ‘virtual’.
27 2 Image formation by a lens b Virtual imagesLight rays diverge from a point.IOconvex lens
28 Since only convex lenses converge light rays, real images can only be formed by convex lenses. screenOI
29 3 Graphical construction of image formation a Construction rulesIn ray diagrams, we use symbols to represent lenses.convex lensesconcave lenses
30 …is refracted through F. Construction rules for convex lensesRule 1A ray parallel to the principal axis...FC…is refracted through F.
31 …is refracted parallel to the principal axis. Rule 2A ray passing through F'...F'FC…is refracted parallel to the principal axis.
32 Rule 3A ray passing through C...F'FC…travels straight on.
33 The 3-ray diagram againRay 1: Parallel to axis, then passes through far focal pointRay 2: Passes unchanged through center of lensRay 3: Passes through near focal point, then parallel to axis
34 Real image, inverted, smaller FfObjecthodoReal image, inverted, smallerhidi
35 Construction rules for concave lenses A ray parallel to the principal axis...CF'F…is refracted so that it appears to come from F'.
36 …is refracted parallel to the principal axis. Rule 2A ray directed towards F...CF'F…is refracted parallel to the principal axis.
37 Rule 3A ray directed towards C...CF'F…travels straight on.
38 The 3-ray diagram againRay 1 Parallel to axis, virtual ray passes through near focal pointRay 2 Straight through center of lensRay 3 Virtual ray through far focal point, virtual ray parallel to axis
58 The Thin-Lens Equation Sign conventions for thin lenses:
59 Defects in LensSpherical Aberration – Peripheral rays and axial rays have different focal points (caused by spherical shape of the lens surfaces.causes the image to appear hazy or blurred and slightly out of focus.The spherical aberration is caused by the spherical shape of the lens surfaces, hence spherical aberration. The center remains more in focus than the edges of the image and the intensity of the edges falls relative to that of the center. This defect appears in both on-axis and off-axis image points
60 Defects in LensChromatic Aberration Blue light is refracted to the greatest extent followed by green and red light, a phenomenon commonly referred to as dispersion.A converging lens can be combined with a weaker diverging lens, so that the chromatic aberrations cancel.The combination – achromatic doubletLight is not monochromatic. Light of different wavelengths is brought to focus at different distances from the center of the lens. This occurs because the refractive index of a transparent isotropic material is greater for light of shorter wavelength than for light of longer wavelength.-dispersion.
61 ExampleAn object of height 5 cm is placed at 15 cm from a convex lens of focal length 10 cm.Find the position and magnification of the image.CF'2FFO5 cm
62 Height of image= 10 cmHeight of object= 5 cm5 cm5 cmIOF2FF'C=10 cm5 cmm= 2
63 Alternatively, Image distance = 30 cm Object distance = 15 cm 5 cm F2FOF'C=30 cm15 cmm= 2
64 Q1 Which of the following... Which of the following incident rays is mentioned in the construction rules?AFF’BFF’CFF’
65 Q2 Which of the following light… Which of the following light rays is correct ?F’FABC
66 Q3 A boy holds a magnifying... A boy holds a magnifying glass at arm’s length.He looks at a poster through the glass and sees a magnified erect image.What happens to the image if he moves the lens closer to his eyes?
67 Q3 A boy holds a magnifying... What happens to the image if he moves the lens closer to his eyes?A It gets larger till it gets totally blurred at some distance.B It gets larger, keeping erect all the way.C It gets smaller and becomes totally blurred at some distance.D It gets smaller, keeping erect all the way.
68 Q4 If you can capture an image... If you can capture an image of a doll on a screen using a lens,which of the following may NOT be correct?A The lens you use is a convex lens.B The image is magnified.C The image is real.D The image is erect.
69 Q7 Both convex and concave... Both convex and concave lenses can produce _______ images, which must be _________ than the object if convex lenses are used.virtuallarger
70 An object is positioned between F’ and 2F’ of convex lens. Complete the rays.2F'F’’OF2F(a) Locate the image.(b) Is it a virtual or a real image?