Chapter 11.pdf

Keep in mind the following points when working with the thin lensequation (see also Table 11.8).• A concave lens has a negative focal length and a negative distanceto the image.• A convex lens has a positive focal length and either a positive ornegative distance to the image, depending where the object isplaced.• The image distance d iis positive if the image is real and negativeif the image is virtual.Table 11.8 Images Formed by Convex LensesLens Type Focal Length Distance to Object Distance to ImageConvex positive positive positive or negative dependingon object locationTake It FurtherLarge glass lenses can beextremely heavy. However,because refraction only occurs atthe surface of a lens, the middlematerial can be removed, as longas the lens surface remainscorrectly curved. These lenses arecalled Fresnel lenses, and they areused in lighthouses and overheadprojectors. Find out more abouthow Fresnel lenses areconstructed and used. Begin yourresearch at ScienceSource.Suggested Activity •D26 Inquiry Activity on page 461Example Problem 11.9A convex lens of a magnifying glass is held 2.00 cm above a page tomagnify the print. If the image produced by the lens is 3.60 cmaway and virtual, what is the focal length of the magnifying glass?GivenDistance of the object from the lens, d o= 2.00 cmDistance of the virtual image from the lens, d i= 3.60 cmRequiredFocal length of the lens, fAnalysis and Solution1 1The correct equation isf= +Substitute the values and their units, and solve the problem.1 1 1f=d+od i1f1f1= +2.00=0.222cmTake the reciprocal of both sides.f = 4.50 cm1–3.601 cmd oParaphraseThe focal length is about 4.50 cm.1d iPractice Problems1. A powerful magnifyingglass produces a real image4 mm from the convexlens. If the object wasplaced 28 mm away, whatis the focal length of thelens?2. Determine the focal lengthof a convex lens thatproduces a virtual image ata distance of 30 mm whenthe object is placed 15 mmaway.3. The objective lens of amicroscope is convex.The light from a specimen4.0 mm from the lensforms a real image10.0 mm from the lens.What is the focal length ofthis lens?Ray diagrams model the behaviour of light in mirrors and lenses.455