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Physics 100 Notes – Lenses What is a lens? A lens is merely a carefully ground or molded piece of transparent material that refracts light rays in such a way as to form an image. Converging or Convex Lenses Incident rays which travel parallel to the principal axis will refract through the lens and converge to a point called the focal point. Diverging or Concave Lenses Incident rays traveling parallel to the principal axis will refract through the lens and diverge, never intersecting . VOCAB Optical center-center of the lens Principal axis-horizontal line that passes through the optical center. Focal point- point in which all parallel rays of light will converge Focal length – the distance from optical center of lens to the focal point do= object distance from optical center di = image distance from optical center ho = size of object hi= size of image Power of lens P= 1/do + 1/di Magnification of lens M= hi/ho An image location is a location in space where all the refracted light appears to come from. REAL Images Can be projected onto a screen, slide projector, camera Are formed when rays of light converge Are inverted (upside down) VIRTUAL Images Can NOT be projected onto a screen Are formed when rays of light diverge Are upright Refraction Rules for a Converging Lens Any incident ray traveling parallel to the principal axis of a converging lens will refract through the lens and travel through the focal point on the opposite side of the lens. Any incident ray traveling through the focal point on the way to the lens will refract through the lens and travel parallel to the principal axis. An incident ray that passes through the center of the lens will in affect continue in the same direction that it had when it entered the lens. Refraction Rules for a Diverging Lens Any incident ray traveling parallel to the principal axis of a diverging lens will refract through the lens and travel in line with the focal point (i.e., in a direction such that its extension will pass through the focal point). Any incident ray traveling towards the focal point on the way to the lens will refract through the lens and travel parallel to the principal axis. An incident ray that passes through the center of the lens will in affect continue in the same direction that it had when it entered the lens. The Lens Equations eye Lens Equation Conventions do = positive for real objects POSITIVE for virtual objects h = negative for inverted images positive for upright images di = positive for real images Negative for virtual images f = positive for convex lenses Negative for concave lenses Problems A 4.00-cm tall light bulb is placed a distance of 45.7 cm from a double convex lens having a focal length of 15.2 cm. Determine the image distance and the image size. A 4.00-cm tall light bulb is placed a distance of 8.30 cm from a double convex lens having a focal length of 15.2 cm. Determine the image distance and the image size. A 4.00-cm tall light bulb is placed a distance of 35.5 cm from a diverging lens having a focal length of -12.2 cm. Determine the image distance and the image size.