Presentation on theme: "Image formation by Lenses"— Presentation transcript:
1Image formation by Lenses Chitranjan KumarLecturer, G.P.C.G.,Jalandhar
2In 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.
3What 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
4Introduction Lenses are very useful. e.g. in cameras, projectors, telescopes, microscopes and eyesThe most common lenses:spherical lenses(i.e. the surface is spherical)
51 Convex and concave lenses Convex lens - thicker at the centre than at the edge.Bi-convexPlano-convexConvex-concave
61 Convex and concave lenses Concave lens - thinner at the centre than at the edge.Bi-concavePlano-concaveConvex-concave
71 Convex and concave lenses Converging or Diverging?convex lensconcave lens(converging lens)(diverging lens)
81 Convex and concave lenses Converging or Diverging?convex lensconcave lens(converging lens)(diverging lens)
91 Convex and concave lenses Converging lensbend the light inwardsYou may remember ‘converging’ as ‘concentrating’.
101 Convex and concave lenses Diverging lensbend the light outwardsYou may remember ‘diverging’ as ‘dividing’.
111 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
58The Thin-Lens Equation Sign conventions for thin lenses:
59Defects 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
60Defects 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.
61ExampleAn 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
62Height of image= 10 cmHeight of object= 5 cm5 cm5 cmIOF2FF'C=10 cm5 cmm= 2
63Alternatively, Image distance = 30 cm Object distance = 15 cm 5 cm F2FOF'C=30 cm15 cmm= 2
64Q1 Which of the following... Which of the following incident rays is mentioned in the construction rules?AFF’BFF’CFF’
65Q2 Which of the following light… Which of the following light rays is correct ?F’FABC
66Q3 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?
67Q3 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.
68Q4 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.
69Q7 Both convex and concave... Both convex and concave lenses can produce _______ images, which must be _________ than the object if convex lenses are used.virtuallarger
70An 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?