Lenses Lesson 19 (CSEC)

LENSE S K. BELLANFANTE CSEC  OBJECTIVES illustrate the effect of converging and diverging lenses on a beam of parallel rays; define the terms: (a) principal axis; (b) principal focus; (c) focal length; (d) focal plane; (e) magnification; Differentiate between real and virt...

LENSE S K. BELLANFANTE CSEC  OBJECTIVES illustrate the effect of converging and diverging lenses on a beam of parallel rays; define the terms: (a) principal axis; (b) principal focus; (c) focal length; (d) focal plane; (e) magnification; Differentiate between real and virtual images. apply the equations for magnification; determine the focal length of a converging lens. determine the focal length of a converging lens.  Lenses are viewed as composing of several prisms which Principal focus For converging lens – it is the point which all incident rays converge after refraction from lens. For diverging, it is the point from which all incident ray appear to diverge after refraction by the lens. This focus is  Characteristics of a Lens ANATOMY OF principal axis - line through LENSE optical center principal focus (F) - a point on the principal axis where incident rays of light converge after refraction. focal length - the distance between the principal focus and the optical centre of the lens optical centre (center of lens) - an imaginary point inside a lens through which a light ray is able to travel Basic ray diagram for a convex lens without being deviated  Object at infinity is a fancy way of saying that all the light rays coming from the object are parallel(almost) to each other. why is 2f center of curvature. The 2F point is called the centre of curvature. If we take the whole lens radius and draw a circle then the centre of the circle will be at point 2F from the pole of the lens , thus the 2F point is called the centre of curvature of the lens.1 WHAT IS FOCAL LENGTH. Focal length is the distance measured in millimeters, between the optical center of the lens and the camera sensor Lens focal length tells us the angle of view—how much of the scene will be captured— and the magnification—how large individual elements will be. The longer the focal length, the narrower the angle of view and the higher the magnification. The shorter the focal length, the wider the angle of view and the lower the magnification. Shorter focal length more powerful lense TELESCOPE Peepholes S Telescopes Flashlight sends out parallel beams- so light start from small source and don’t converge just spread out REAL IMAGE VIRTUAL IMAGE REAL IMAGES CAN BE FORMED IMAGE CANNOT BE FORMED ON ON A SCREEN A SCREEN LIGHT RAYS TRAVEL TO A REAL OCCUR WHERE LIGHT RAYS IMAGE FROM THE OBJECT APPEAR TO COME FROM AFTER CHANGING DIRECTION Real images are always on the Virtual images are on the same opposite side of the lens. side. IMAGES ARE INVERTED IMAGES ARE ERECTED MAGNIFICATION Magnification (m) = height of image(h) height of object (H) = image distance (v) object distance (u).  QUESTIO N A building is 6.0m high and 80m from a converging camera lens. If the camera forms an image which is 6. 0 mm high.. What is the magnification 2. How far must the camera film be behind the lens for a focused image to be formed? M = height of image m = v/u height of object v= m x u = 0.006 m 0.001 x 80m 6m = 0.001 X or =0.08m 1 x 10-3X If your value is less than 1, it means that your image is smalle object, if more than one means image is larger than object.  RAY DIAGRAMS & GEOMETRIC OPTICS All lenses and mirrors can use ray diagrams to find images. A ray diagram is a diagram that traces the path that light takes in order for a person estimate location of image.  Constructing Ray CONSTRUCTING RAY DIAGRAMS scale diagrams There are three principal rays and they behave in 1. Draw 2 the following ways: perpendicular lines to In parallel, out represent the through principal axis and the focus. lens. Place points, F, to Straight scale in position to through represent the optical center principal focus. without Draw the object to deviation scale, in size and In through position to stand on focus, the principal axis. out parallel. Draw lines to represent the DRAWING RAY DIAGRAMS When drawing ray diagram to find image, you need to draw at least two rays base on any two rules. Then apply rules to find image and describe it. Properties Use Acronym SALT (S) Size – smaller than object (A) Attitude- Inverted (L) Location – Between F2 and 2 F2 (T) Type – Real Image RAY DIAGRAMS FOR OBJECT POSITIONS RELATIVE TO A CONVEX LENS If object appear If object at greater between 2f and f, distance than 2 object gets bigger focal length then as in projector. image appear Image larger than smaller as in object. camera. a real image is formed at Object less than infinity. The size focal length, of the image is virtual image much larger than forms larger than that of the real object as in object magnifying glass.  THE LENS FORMULA NOTE:- 1.All the distances are measured from the Optical Center. 2.Distance measured in the direction of incident light ray is taken as Positive and vice- versa. *a negative value for image mean image For Example ,In this case :virtual image are Is not real and for positive value it is real. given negative values QUESTION If you get a negative value in your calculation it is virtal and real if you get a positive value  QUESTION Use a ruler to draw ray diagrams to locate the images of the following objects: (a) an 2cm tall object that is 30 cm from a convex lens of +10- cm focal length, (b) an object that is 14 cm from the same lens, and (c) an object that is 5 cm from the same lens. (Choose a scale so that your drawing fills a significant portion of the width of a paper.) Measure the image locations on your drawings and indicate if they are real or virtual, upright or inverted. WHY IMAGES ARE UPSIDE DOWN/INVERTED Because the front part of the eye is curved, it bends the light, creating an upside down image on the retina. The brain eventually turns the image the right way up.