2 Module Objectives Lens – How Light navigate’s through it? Concept of Refraction from Curved SurfacesFormation of images in Convex and Concave LensesUses of LensesRectify defects in Eye sightWorking and uses of different optical instruments like camera, microscope, telescope and binoculars
3 Why certain things happen? What causes fascination Rainbow and Mirages?A coin placed in a glass trough filled with water appears to be slightly higher level than it really is. Why?A ruler kept inclined and partly dipped in water appears to be bent. Why?ABOVE PHENOMENON IS THAT LIGHT GETS REFRACTED WHEN IT ENTERS A DIFFERENT MEDIUM.
4 Properties of Light Light travels in straight lines Light travels much faster than soundWe see things because they reflect light into our eyesShadows are formed when light is blocked by an object
5 What is Refraction?Refraction is the bending of waves as they pass from one medium to another, due to a change in their speed.The phenomenon is most commonly associated with light, but can also apply to sound, or even water, waves.It happens when a series of waves travels toward the new medium at an angle, so that one side experiences a change in speed before the other, causing it to turn toward the slower side in the same way that a moving vehicle will tend to turn if one side is slowed more than the other.Refraction can cause objects to appear displaced, and may amplify distant sounds. It has many uses in the context of light, such as lenses and prisms.
6 Lenses – Refraction of Light Transparent material piece which has at least one curved surface.Commonly used lenses have two smooth curved surfacesCurved surfaces refract light in a particular patternSome lenses converge light and some others diverge lightConvex Lens – Central portion is thicker than the edgesConcave Lens – Central portion is thinner than the edges
7 Refraction of Light through Lenses Light gets refracted when it passes from a denser medium to rarer medium and vice versaLens is a transparent, dense r mediumIn the ‘Water’ example, Light enters a denser medium (Glass) from a rarer medium and vice versa. Therefore, Light gets refracted by a lens twice.
8 Thin Lens as an Array Of Prisms Consider Prisms to be arrangement of Thin LensesAs convex lens is thicker at the center it looks as though it is made by joining two prisms at their bases.Concave lens is thicker at the edges and hence it appears as though it is made by joining two prism at their vertices so that their bases are away from each other.Light rays bend as they do while passing through prism ie.e rays of light bend towards the base of prism
9 Thin Lens as an Array Of Prisms Contd… Principal Axis – All Refracted rays bends towards this axis and meet at a specific pointRefracted light from both the prisms reach the same point.Rays that come along the principal axis do not bend even when they get refractedConvergence of Light – Rays of light refracted through a convex lens meet at one pointConvex Lens is a Converging Lens.Divergent Beam of Light – Rays of light bend twice while passing through concave lens. Separation between the refracted rays goes on increasing as the rays move aways from the lens.Concave Lens is a Diverging Lens.
10 Some parameters pertaining to lenses – A. Center of Curvature Centre of Curvature – Each face of a convex or concave lens is a part of sphere.Centres of Curvature are the centres of those imagines spheres of which the surfaces of the lens are the parts
11 Some parameters pertaining to lenses – B. Principal Axis Lens has two center of curvature, one on each of its sides.Imaginary line joining these centres of curvature is the Principal Axis.Principal axis is the imaginary line running through the centre of the lens. Here light is neither reflected or refracted.
12 Some parameters pertaining to lenses – C. Radius of Curvature Radius of a sphere of which a surface of a lens is a part, is the radius of curvature of the lens.Radii of the curvature of two surfaces of a convex lens or a concave lens are equal.
13 Some parameters pertaining to lenses – D. Optic Centre It is the geometric of the Lens.The ray of light passing through this point emerges in the same direction without bending.
14 Some parameters pertaining to lenses – E. Principal Focus Principal Focus of Convex LensIt is a point on the principal axis of a convex lens where parallel beam of light rays, travelling parallel to principal axis, after passing through the lens actually meet.Principal Focus of Concave LensRays of light parallel to the principal axis enter the lens and get refracted and then appear to diverge from a particular point on the principal axis.
15 Some parameters pertaining to lenses – E. Focal Length It is the distance between optic centre and principal focus of a Lens.
16 Rules of Signs in obtaining images by lenses Focal Length, object and image distance are all measured from the Optic center.Distance between lens and object is always positive (+)Distance of real image is always positive (+)Distance of virtual image is always negative (-)Focal length of Convex lens is written with (+) signFocal length of Concave lens is written with (-) sign
17 Activity Hold a sheet of paper against sunlight Keep the convex lens in the path of lightChange the lens position till a sharp point of light appears on the sheet of paperMeasure distance between lens and white sheet. What is this length?What happens if the lens and sheet of paper are held in this position for long period?
18 Formation of image in lenses Images are formed when rays of light reflected by an object meet or appear to meet at a point after intersection in a lensPosition, size and other properties of the image depends on the distance between object and lens as well as nature of the lensLight rays determine the nature and position of images formed by lenses
19 Formation of image in lenses – Normal Ray Ray of light passing through the Optic CenterLight ray that hits a surface at 90 degrees, or perpendicular.
20 Formation of image in lenses – Parallel Ray Incident rays parallel to principal axis pass through principal focus of a convex lensIn case of concave lens it appears to emerge from the principal focus after the refraction
21 Formation of image in lenses – Zero Angle of Incidence Light ray travels along the principal axis after refractionIf angle of Incidence is zero, then light ray reflects in the same path
22 Formation of image in lenses – Incident ray of light passing through the principal focus Convex Lens – Travel parallel to principal axis after refractionConcave Lens – Travel parallel to principal axis
23 Formation of image by Convex lens – Image of an object at Infinity Materials –Convex LensWhite ScreenLens standHalf metre scaleProcedureIdentify distant object which is clearly visible through your class room window. Example - TreePlace a table in a position where the reflected rays coming from this object are receivedDraw a straight line on the table along the direction of these raysPlace a lens holder in this line and convex lens on this holderMark mid point of the lens holder on the table to represent optic centreObtain the image of the distant object on the white screen placed on the other side of lensAdjust the screen to obtain sharp imageMark the position of the base of the screen to represent the position of the imageNotice that the sharp image of the distant object is real
24 Formation of image by Convex lens – Image of an object at Infinity Contd. Can you explain why it is call real image?Compare the sizes of the object and image. What conclusion will your draw?Measure the distance between optic centre and position of the image.Answer – This is the focal length of the lens
25 Formation of image by Convex lens – Image of an object placed beyond 2F MaterialsConvex LensLens holderWhite screenCandleMatch boxMeter scaleProcedureDraw a straight line on the tablePlace lens holder on this straight line and fix convex lensMark the mid point of the base of lens holder on the straight line to represent optic centreMark Focus F and 2F on the straight line on both sides of the lens holderPlace burning candle about 3 to 4 cms beyond 2F on the straight linePlace the screen on the straight line on the other side of lens and obtain image on the screenMove the screen until sharp image is obtainedMark the mid point of the position of the screen on the table to represent image position
26 Formation of image by Convex lens – Image of an object placed beyond 2F Contd. Can you describe the position and characteristics of the obtained image?
27 Formation of image by Convex lens – Image of an object placed beyond 2F Objects placed at 2FObjects placed between F1 and F2Objects place at F
28 Formation of image by Concave lens Images formed by Concave lens is always VIRTUALParallel and normal rays coming from an object are sufficient to study the image formed by a lensParallel rays appear to emerge from principal focus after refractionParallel and Normal Ray Do Not Meet after refraction. Hence, image formed is Virtual.Position of the image is determined by virtual meeting point of Parallel and Normal Rays.When the object is at infinity the virtual image is formed at principal focus on the same side of incident ray.Image is very small and erect.Above image cannot be caught on the screen. Why?
30 Uses of Lens – Short Sightedness or Myopia People who can see nearer clearly objects but not far objects are said to have Myopia DefectIn such people, parallel rays of light coming from a distant object get focused at point in front of our eye retina instead of getting focused on the retinaFocal length of the eye lens will be less than the distance between the lens and retinaAbove may be due to increased lens thickness or due to elongation of eye ballThis defect is corrected by making the parallel rays of light from distant object made fall obliquely on the eye lensDiverging or Concave lens has to be used for the above correctionBy using a concave lens of suitable focal length the image can be focused on the retina of the eye.
31 Uses of Lens – Short Sightedness or Myopia Contd.
32 Uses of Lens – Long Sightedness or Hypermetropia or Hyperopia People who can see nearer clearly objects but not far objects are said to have Myopia DefectIn such people, parallel rays of light coming from a distant object get focused at point in front of our eye retina instead of getting focused on the retinaFocal length of the eye lens will be less than the distance between the lens and retinaAbove may be due to increased lens thickness or due to elongation of eye ballThis defect is corrected by making the parallel rays of light from distant object made fall obliquely on the eye lensDiverging or Concave lens has to be used for the above correctionBy using a concave lens of suitable focal length the image can be focused on the retina of the eye.
33 Uses of Lens – Short Sightedness or Myopia Contd.
34 Optical Instruments – Construction and Working of Simple Camera Contd. Camera has a metal box which allows light only through the lensAt the end of the box a convex lens of shorter focal length is fixedOther end of the box has a provision to place photosensitive filmImage formed on the film remains due to photo chemical reactionDistance between convex lens and the film can be adjustedObtain clear image of the object to be photographed by adjusting the bellowsPlace the film where the clear image is formedOperate the camera and obtain the image on the filmFilm with the image is processed to obtain photosIt is possible to form real object on the screen using Convex lens. However, the image disappears if the object is removed. Which instrument can make the image remain permanently on the screen even when the object is removed?Images – Google Images
35 Optical Instruments – Simple Microscope Watch repairers and goldsmiths use a reading lens to see finer detailsThis device helps to see and study minute parts and particles which the naked eye cannotSimple microscope is a convex lens of shorter focal lengthPlace an object between the lens and its principal focus and observeVertical erect and magnified image is formedImages – Google Images
36 Optical Instruments – Compound Microscope Two convex lens of different focal length are used in a Compound MicroscopeLens with lesser focal length is used as objective and another lens with larger focal length is used as eye pieceObjective and eye piece are arranged in co-axial tubes to have a common principal axisDistance between objective and eye piece can also be adjustedTiny object to be observed should be placed beyond the principal focus of the objectiveImage formed is real, inverted and magnifiedAdjust the distance between objective and eye piece to get this image within the focal length of the eye pieceMagnified Image is formed by the eye pieceObjective of the compound microscope forms an image which is magnifiedCompound microscope is used to observe and study micro organisms and smaller particles invisible to the naked eye
38 Optical Instruments – Telescope Using two convex lens of suitable focal lengths we can make Compound micrscopeSimilarly we can arrange convex lenses and observe distant object – Such device is called TelescopeTelescope can be used to observe distant stars, planets and their satellites – Called Astronomical telescope
39 Optical Instruments – Astronomical Telescope Two convex lens of different focal lengths are used in an Astronomical TelescopeLenses are mounted in co-axial pipes such that their principal axis is same and the distance between them is adjustableLens of greater focal length is called the Objective and the other with lesser focal length is called Eye Piece.Light rays from a distant object enter the telescope through the objective of the telescope and form the image at the principal focus after refractionDistance between the objective and the eye piece is now adjusted to get the image of the distant object within the focal length of the eye pieceVirtual magnified image of the object is formed from the eye pieceImage formed by a telescope is always smaller than the object but it brings the image of the distant object closer to the eyes so that it can be studied
41 Optical Instruments – Binoculars It is an optical device used for observing cricket matches, birds, movement of enemies in war field etc.Binoculars has two telescopes in which total internal reflecting prisms are fixedObserve a clear image of a distant object, the focal length of the object should be much longerNormally when the focal length increases the length of the tube is required moreBut we do not require longer tubes when prisms are usedGab between the two eye pieces normally equal to the distance between the eyesAs the distance between two objects increases the field of vision also increasesBinoculars give three dimensional vision and the image is always erect
42 References http://www.physchem.co.za/OB11-wav/eye.htm Google Images