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Refraction through a lens

Explore the world of lenses - from the basics of lens construction and types, to understanding how lenses bend light and their various applications. Discover the difference between convex and concave lenses and how they are used in different industries.

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Refraction through a lens

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  1. Refraction through a lens

  2. 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.

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

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

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

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

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

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

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

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

  11. 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.

  12. 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 C1 and C2 of two spherical surfaces of the lens is called the principal axis of the lens.

  13. 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.

  14. PATH OF LIGHT

  15. 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).

  16. PATH OF LIGHT

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

  18. 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. F F 2F 2F

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

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

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

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

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

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

  25. 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 F 2. A ray striking the centre of the lens passes straight through (if lens is thin) Concave lens To locate the position of an image in a concave lens we use two of the following rays of light 2F 2F

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

  27. Images in Concave lens F F f v u Image formed in concave lens when the object is placedin front of lens object 2F 2F Image Virtual, erect & diminished Use In spectacles for correcting short-sightedness

  28. 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.

  29. MAGNIFYING GLASS (SIMPLE MICROSCOPE)

  30. 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.

  31. EXPERIMENTAL DETERMINATION OF FOCAL LENGTH OF A CONVEX LENS 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.

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

  33. EXPERIMENTAL DETERMINATION OF FOCAL LENGTH OF A CONVEX LENS 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

  34. THANK YOU

  35. 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.

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

  37. ASSIGNMENT 2 Light ray posses un-deviated through optical centre, explain with diagram? 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?

  38. ASSIGNMENT 3 Draw a ray diagram showing the formation of image by a convex lens, if the size of image is same as that of object. 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. 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. Under what circumstances we obtain a virtual image with the help of convex lens. An object is brought from infinity towards the pole of convex and concave lens. Mention the change in position, size and nature of image.

  39. Suspensor ligament Cornea Retina Iris Pupil Optic nerve Lens

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

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

  42. ASSIGNMENT 4 Identify the following uses under the category of concave lens and convex lens. 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). State uses of concave and convex lenses. (three each) Explain with ray diagram working of microscope. Write the formula for its magnification? Name the lens use for correcting short sightedness and the lens used for correcting long sightedness?

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