1、chapter 36 - lensesa powerpoint presentation bypaul e. tippens, professor of physicssouthern polytechnic state university 2007objectives: after completing this module, you should be able to:refraction in prismstwo prisms base to baseif we apply the laws of refraction to two prisms, the rays bend tow

2、ard the base, converging light.parallel rays, however, do not converge to a focus, leaving images distorted and unclear.refraction in prisms (cont.)two prisms apex to apexsimilarly, inverted prisms cause parallel light rays to bend toward the base (away from the center).again there is no clear virtu

3、al focus, and once again, images are distorted and unclear.converging and diverging lensconverging lensdiverging lensdouble-convexdouble-concavereal focusvirtual focusthe focal length of lensesconverging lensdiverging lensthe focal length f is positive for a real focus (converging) and negative for

4、a virtual focus.ffthe principal focusleft to rightright to lefttypes of converging lensestypes of diverging lenseslensmakers equationr1r2surfaces of different radiusthe lensmakers equation:the focal length f for a lens.negative (concave)positive (convex)sign conventionrsigns for lensmakers equationr

5、1r2+-r1 and r2 are interchangeabler1, r2 = radiin= index of glassf = focal length a glass meniscus lens (n = 1.5) has a concave surface of radius 40 cm and a convex surface whose radius is +20 cm. what is the focal length of the lens?12111(1)nfrr1112 1(1.5 1)20 cm( 40 cm40 cmff = 20.0 cm what must b

6、e the radius of the curved surface in a plano-convex lens in order that the focal length be 25 cm?2111(1)nfrr1=r2=?f = ?22110.500(1.5 1)25 cmrrr2 = 12.5 cmterms for image constructionconverging lensdiverging lensnear focusnear focusfar focusfar focusimage construction:converging lensdiverging lensra

7、y 1ray 1image construction:converging lensdiverging lensray 1ray 1image construction:converging lensdiverging lensray 1ray 1ray 2ray 2images tracing points3. is it enlarged, diminished, or same size?2. is the image real or virtual?1. is the image erect or inverted? real images are always on the oppo

8、site side of the lens. virtual images are on the same side.object outside 2fimage is located between f and 2freal; inverted; diminishedobject at 2freal; inverted; same sizeimage is located at 2f on other sideobject between 2f and freal; inverted; enlargedimage is located beyond 2fobject at focal len

9、gth fparallel rays; no image formedobject inside fvirtual; erect; enlargedimage is located on near side of lensreview of image formationsreal; inverted; diminishedreal; inverted; same sizereal; inverted; enlargedparallel rays; no image formedvirtual; erect; enlargeddiverging lens imagingdiverging le

10、nsdiverging lensanalytical approach to imagingpfqy-y111pqflens equation: yqmypmagnification:same sign convention as for mirrors1. object p and image q distances are positive for real and images negative for virtual images.2. image height y and magnifi-cation m are positive for erect negative for inv

11、erted images.3. the focal length f and the radius of curvature r is positive for converging mirrors and negative for diverging mirrors.working with reciprocals:alternative solutions a magnifying glass consists of a converging lens of focal length 25 cm. a bug is 8 mm long and placed 15 cm from the l

12、ens. what are the nature, size, and location of the image?p = 15 cm; f = 25 cm(15 cm)(25 cm)15 cm - 25 cmpfqpfq = -37.5 cm a magnifying glass consists of a converging lens of focal length 25 cm. a bug is 8 mm long and placed 15 cm from the lens. what is the size of the image?p = 15 cm; q = -37.5 cm(

13、 37.5 cm)8 mm15 cmy y = +20 mmyy what is the magnification of a diverging lens ( = -20 cm) if the object is located 35 cm from the center of the lens?first we find q . . . then m(35 cm)(-20 cm)35 cm - (-20 cm)pfqpfq = +12.7 cm( 12.7 cm)35 cmqmp m = +0.364 derive an expression for calculating the mag

14、nification of a lens when the object distance and focal length are given.pfpmpfsummarytheis denotedby the redsummary: lensmakers equationr1r2+-r1 and r2 are interchangeabler1, r2 = radiin= index of glassf = focal lengthsummary of math approachpfqy-y111pqflens equation: yqmypmagnification:summary of sign convention 1. object p and image q distances are positive for real and images negative for virtual images.2. image heigh