Refraction through a lens we have seen people

Refraction through a lens

• we have seen people using spectacles for reading. • The watchmakers use a small glass to see tiny parts. • Pistol or rifle shooters use glass for clear view.

• In your biology class you must have worked with a magnifying glass. Ø Have you ever touched the surface of a magnifying glass with your hand? Ø Is the surface of above mentioned glasses is plane or curved? Ø The glasses used in spectacles and that by a watchmaker, shooter are examples of lenses.

Learning outcome v What is lens? v How lenses are made? v How lenses bend light rays? v Application of lenses.

LENS Ø A transparent material bounded by two or at least one spherically curved surface is known as lens. Ø Lenses are cut from an optical glass sphere. Ø There are two types of lenses Two types CONVEX CONCAVE

CONVEX LENS Ø Convex lens is thicker in the middle and thinner at the edges. Ø There are three types of convex lenses. CONVEX LENS BICONVEX PLANO CONVEX ØConstruction of convex lens. wmv CONCAVO CONVEX

CONVEX LENS Ø Convex lens can be considered as a set of prisms and a glass slab. Ø So, convex lens behaves as a converging lens.

CONCAVE LENS Ø Convex lens is thinner in the middle and thicker at the edges. Ø There are three types of concave lenses. CONCAVE LENS BICONCAVE PLANO CONCAVE ØConstruction of concave lens. wmv CONVEXO CONCAVE

CONCAVE LENS Ø Concave lens can also be considered as a set of prisms and a glass slab. Ø Concave lens behaves as a diverging lens.

TERMS RELATED TO LENS 1) Optical centre: The centre of a lens is called optical centre. • It is denoted by symbol O.

TERMS RELATED TO LENS 2) Centre of curvature: The centres of the spherical surfaces forming the lens are known as centre of curvature. • It is denoted by symbol C. • A lens has two centre of curvatures.

TERMS RELATED TO LENS 3) Radius of curvature: The distance between the optical centre and the centre of curvature is known as radius of curvature. • It is denoted by the symbol R. 4) Principal axis: The line joining the centres of curvature C 1 and C 2 of two spherical surfaces of the lens is called the principal axis of the lens.

TERMS RELATED TO LENS 5) First Focus: It is a point situated on the principal axis of the lens such that a beam of light starting from it in case of a convex lens, or directed towards it in case of a concave lens, becomes parallel to principal axis after refraction.

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TERMS RELATED TO LENS 6) Second Focus: It is a point situated on the principal axis of the lens such that a beam of light incident parallel to the principal axis, after refraction from the lens, pass through it (convex lens) or appear to be diverging from this point (concave lens).

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FACTORS AFFECTING FOCAL LENGTH Focal length of a lens depends on two factors. i. Refractive index of material of lens relative to its surrounding medium ii. The radii of curvature of the two surfaces of lens. (This formula is known as lens maker’s formula) (Not in the syllabus) Ø If the lens is placed in water instead of air, its focal length increases. Ø A thick lens has less focal length than a thin lens. Ø If a part of the lens is covered, • • Focal length remains unchanged Intensity of the image decreases.

To locate the position of an image in a convex lens we use two of the following rays of light 1 parallel to the principal axis emerging through focus 2 striking the centre of the lens passes straight through (if lens is thin) 3 through the focus emerging parallel to principal axis. 2 F F F 2 F

Images in Convex lens Image formed in convex lens when the object is placed at infinity 2 F F F f u Image Real, inverted & highly diminished v Use As a camera lens when object is very far, burning lens 2 F

Images in Convex lens Image formed in convex lens when the object is placed beyond 2 f object 2 F F F f u Image Real, inverted & diminished v Use As a camera lens when object is not very far 2 F

Images in Convex lens Image formed in convex lens when the object is placed at 2 f object F 2 F f u v Image Use Real, inverted & same size as object In terrestrial telescope for erecting the image

Images in Convex lens Image formed in convex lens when the object is placed between f and 2 f object 2 F F f u Image Real, inverted & magnified v Use Cinema and slide projectors

Images in Convex lens Image formed in convex lens when the object is placed at the focus object 2 F F F f u Image Use At Infinity, inverted and highly magnified In collimator of a spectrometer to obtain parallel beam 2 F

Images in Convex lens Image formed in convex lens when the object is placed between the Focus and optical centre object 2 F F F f u Image Use Virtual, magnified & upright As a reading glass 2 F

Concave lens To locate the position of an image in a concave lens we use two of the following rays of light 2 F F 1. A ray which strikes the lens travelling parallel to principal axis is refracted as if it came from focus 3. A ray heading for the focus on striking the lens is refracted parallel to principal axis F 2 F 2. A ray striking the centre of the lens passes straight through (if lens is thin)

Images in Concave lens Image formed in concave lens when the object is placed at infinity 2 F F F f Image Use Virtual, erect & diminished In Galilean telescope 2 F

Images in Concave lens Image formed in concave lens when the object is placed in front of lens object 2 F F F 2 F f v u Image Use Virtual, erect & diminished In spectacles for correcting shortsightedness

POWER OF LENS • The power of lens is a measure of deviation produced by it in the path of rays refracted through it. • Power of lens = _______1____ focal length(m) • The unit of power is dioptre. • The power of convex lens is positive. • The power of concave lens is negative.

MAGNIFYING GLASS (SIMPLE MICROSCOPE)

MAGNIFYING GLASS (SIMPLE MICROSCOPE) • To see an object by the naked eye, it is necessary to place it at a minimum distance of 25 cm. (which is known as the least distance of distinct vision). • The eye is able to see the object if it subtends a minimum angle of 1’ at it. • To observe a small object which subtends an angle less than 1’ at the eye when placed at least distance of distinct vision from the eye, convex lens is used. • The object is placed between the optical centre and the focus of the lens. The image is erect, virtual, magnified and on the same side of the object.

EXPERIMENTAL DETERMINATION OF FOCAL LENGTH OF A CONVEX LENS (I) BY DISTANT OBJECT METHOD: • This is based on the following principle. • “ a beam of parallel rays incident from a distant object is converged in the focal plane of the lens. ” • The lens is kept between the distant object and a screen. The position of the lens is changed till the clear image of the object is formed on the screen. • The distance between the screen and the lens is measured. This distance is known as focal length of the lens.

EXPERIMENTAL DETERMINATION OF FOCAL LENGTH OF A CONVEX LENS (II) BY AUXILLARY PLANE MIRROR METHOD : • Place the lens L on a plane mirror kept on the horizontal surface of the vertical stand arrange the pin P horizontally in the clamp so that its tip is vertically above the centre O of the lens L.

EXPERIMENTAL DETERMINATION OF FOCAL LENGTH OF A CONVEX LENS (II) BY AUXILLARY PLANE MIRROR METHOD : • Adjust the height of the pin until it has no parallax with its inverted image as seen from vertically above the pin. • If the pin and its image shift together, then parallax is removed. • Measure the distance x of the pin from the lens and the distance y of the pin from the mirror. • Calculate the focal length using fromula • f = x+y 2

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NOTES • In galilean telescope, objective is a convex and eye piece is a concave lens. The image we get is a erect one. It is generally has a small field of view • In astronomical telescope, objective and eye piece both are convex. 35

ASSIGNMENT 1 1) Define a lens. How will you identify the nature of lens without touching it. 2) Explain why convex lens shows converging action and convex lens show diverging action with diagram ?

ASSIGNMENT 2 1) Light ray posses un-deviated through optical centre, explain with diagram? 2) If the medium is different on both side of the convex lens them how will the first and second focal length differ? If a part of lens is covered how will: 1. focal length differ 2. Intensity of image differ 3. Image differ 3) How does the action of a convex lens differ from that of a concave lens on a parallel beam of light incident on them? Draw diagrams to illustrate your answer?

ASSIGNMENT 3 1) Draw a ray diagram showing the formation of image by a convex lens, if the size of image is same as that of object. 2) Which of the following lenses would you prefer to use while reading small letters found in a dictionary- (a) Convex lens of focal length 50 cm (b) Concave lens of focal length 50 cm (c) convex lens of focal length 5 cm (d) concave lens of focal length 5 cm. 3) Parallel light rays inclined at some angle to principal axis fall on concave lens. Draw a neat ray diagram to show the position of the image. 4) Under what circumstances we obtain a virtual image with the help of convex lens. 5) An object is brought from infinity towards the pole of convex and concave lens. Mention the change in position, size and nature of image.

Cornea Suspensor ligament Retina Iris Pupil Optic nerve Lens

Short-sight defect falls short of retina light from distant object Corrected falls on retina light from distant object with help of a diverging (concave) lens 40

Long-sight defect light from near object falls ‘behind’ retina Corrected falls on retina light from near object with help of a converging (convex) lens 41

ASSIGNMENT 4 1) Identify the following uses under the category of concave lens and convex lens. a. b. c. d. e. f. g. Lens that enables our eyes to form a real and inverted image on retina. Lens used in Galillean telescope Lens used in camera Lens used in microscopes Lens used in terrestrial telescopes Lens used in spectacles to cure short sightedness ( Myopia) Lens used in spectacles to cure long sightedness (Hypermetropia). 2) State uses of concave and convex lenses. (three each) 3) Explain with ray diagram working of microscope. Write the formula for its magnification? 4) Name the lens use for correcting short sightedness and the lens used for correcting long sightedness?