Presentation on theme: "Image formation by Lenses Chitranjan Kumar Lecturer, G.P.C.G., Jalandhar."— Presentation transcript:
Image formation by Lenses Chitranjan Kumar Lecturer, G.P.C.G., Jalandhar
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.
When you see through lenses, do the objects appear the same as they are? No ! What happens to the light when it passes through a lens
Introduction Lenses are very useful. e.g. in cameras, projectors, telescopes, microscopes and eyes The most common lenses: – spherical lenses (i.e. the surface is spherical)
1Convex and concave lenses Convex lens - thicker at the centre than at the edge. Bi-convex Plano-convex Convex-concave
1Convex and concave lenses Concave lens - thinner at the centre than at the edge. Bi-concave Plano-concave Convex-concave
1Convex and concave lenses convex lens (converging lens) concave lens (diverging lens) Converging or Diverging?
1Convex and concave lenses Converging or Diverging? convex lens (converging lens) concave lens (diverging lens)
1Convex and concave lenses Converging lens You may remember ‘converging’ as ‘concentrating’. bend the light inwards
1Convex and concave lenses Diverging lens You may remember ‘diverging’ as ‘dividing’. bend the light outwards
1Convex and concave lenses Converging or diverging? Light converges or diverges as some parts of the wavefronts lag behind. convergesdiverges Recall that light travels more slowly in glass than in air.
Key features of lenses
Refracted rays meet at a point called principal focus Parallel rays are refracted inwards. F Convex Lens
Centre of the lens is called the optical centre C. optical centre F c
The distance of F from C is the focal length f of the lens. focal length F C
A convex lens has 2 principal foci. 1 on each side FF' The line through the optical centre and 2 foci is called the principal axis. principal axis FF' C
Refracted rays appear to spread from a point called Parallel rays are refracted outwards. principal focus Concave Lens
F optical centre Centre of the lens is called the optical centre C.
F focal length The distance of F from C is the focal length f of the lens. C
F'F F A concave lens has 2 principal foci. 1 on each side The line through the optical centre and 2 foci is called the principal axis. principal axis C
Q1 A light ray is incident on a convex lens. Which one represents the path of the light ray? APath X. BPath Y. CPath Z. X Y Z
Q2 A light ray is incident on a concave lens. Which one represents the path of the light ray? APath X. BPath Y. CPath Z. X Y Z
2Image formation by a lens When the light from a point of an object enters our eyes, we can see the point. O I screen
2Image formation by a lens aReal images O I Light rays converge to a point.
Image can be captured by a screen. screen O I Hence called ‘real’. 2Image formation by a lens
Light rays diverge from a point. bVirtual images O I concave lens No rays actually come from the image. Hence called ‘virtual’.
2Image formation by a lens Light rays diverge from a point. bVirtual images O I convex lens
screen O I Since only convex lenses converge light rays, real images can only be formed by convex lenses.
3Graphical construction of image formation aConstruction rules In ray diagrams, we use symbols to represent lenses. convex lensesconcave lenses
A ray parallel to the principal axis... …is refracted through F. C F Rule 1 Construction rules for convex lenses
F' C F A ray passing through F'... …is refracted parallel to the principal axis. Rule 2
F' C F A ray passing through C... …travels straight on. Rule 3
The 3-ray diagram again Ray 1: Parallel to axis, then passes through far focal point Ray 2: Passes unchanged through center of lens Ray 3: Passes through near focal point, then parallel to axis
F F f Object hoho dodo Real image, inverted, smaller hihi didi
…is refracted so that it appears to come from F'. C F'F Construction rules for concave lenses A ray parallel to the principal axis... Rule 1
C F'F Rule 2 A ray directed towards F... …is refracted parallel to the principal axis.
A ray directed towards C... …travels straight on. C F'F Rule 3
The 3-ray diagram again Ray 1 Parallel to axis, virtual ray passes through near focal point Ray 2 Straight through center of lens Ray 3 Virtual ray through far focal point, virtual ray parallel to axis
F F f Object hoho dodo Virtual image, upright, smaller hihi didi
Images formed by a convex lens Object : Image: At infinity at F, real, inverted and diminished C 2F'F' 2F2F F I
Object : Image: Beyond 2F ’ Between F and 2F, real, inverted and diminished C 2F' F' 2F2F F O I
Object : Image: At 2F ’ At 2F, real, inverted and same size C 2F'F' 2F2F F O I
Object : Image: Between F ’ and 2F ’ Beyond 2F,real, inverted and magnified C 2F'F' 2F2F F O I
Object : Image: At F ’ At infinity C 2F'F' 2F2F F O
Object : Image: < F ’ On the same side as the object virtual, erect and magnified O I C 2F'F' 2F2F F
Images formed by a concave lens C F' F I Object : Image: At infinity At F’ 2F' 2F2F
C F' F Object: Image: Within 2F ’ (or near object) Between F ’ and 2F ’, on the same side of object 2F' 2F2F O I virtual, erect and diminished
magnification m = height of image (image size) height of object (object size) Magnification F' C F I O hoho hihi hihi hoho =
C F I O magnification m = image distance object distance v u Also, m = vu
The Thin-Lens Equation
Sign conventions for thin lenses:
Defects in Lens Spherical 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.
Chromatic 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 doublet Defects in Lens
Example An 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. C F' 2F2FF O 5 cm
C F' 2F2FF O 5 cm Height of object Height of image = 10 cm = 5 cm m = 10 cm 5 cm = 2 I
C F' 2F2FF O 5 cm I Object distance Image distance = 30 cm = 15 cm m = 30 cm 15 cm = 2 Alternatively,
Q1Which of the following... Which of the following incident rays is mentioned in the construction rules? A FF’ C F B F
Q2Which of the following light… Which of the following light rays is correct ? F’F A B C
Q3A 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?
Q3A boy holds a magnifying... What happens to the image if he moves the lens closer to his eyes? AIt gets larger till it gets totally blurred at some distance. BIt gets larger, keeping erect all the way. CIt gets smaller and becomes totally blurred at some distance. DIt gets smaller, keeping erect all the way.
Q4If 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? AThe lens you use is a convex lens. BThe image is magnified. CThe image is real. DThe image is erect.
Q7Both convex and concave... Both convex and concave lenses can produce _______ images, which must be _________ than the object if convex lenses are used. virtual larger
An object is positioned between F’ and 2F’ of convex lens. Complete the rays. (a)Locate the image. (b)Is it a virtual or a real image? 2F' F’’ O F 2F2F
Example 6 (a) (b)It is a real image. 2F' F 2F2F F’’ O I
Example 7 An object is positioned between F ’ and 2F ’ from concave lens. Complete the rays. (a)Locate the image. (b)Is it a virtual or a real image? 2F' F’’ O F2F2F
Example 7 (a) (b)It is a virtual image. 2F' F’’ O F2F2F I