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L 33 Light and Optics [3] Light and optics • Measurements of the speed of light Å • images formed by mirrors – plane mirrors • The bending of light – refraction Å – curved mirrors • Total internal reflection Å • concave •Dispersion • convex • Images formed by lenses • DispersionÅ • the human eye • Rainbows – correcting vision problems • Atmospheric scatteringÅ • nearsightedness • farsightedness • Blue sky •astigmatism • red sunsets – depth perception light and optics Image formation with lenses • effects related to the wave nature of light • converging lens • lenses are relatively – polarization (positive lens) simple optical devices – interference • diverging lens • the principle behind (negative lens) the operation of a • thin film interference lens is refractionÆ • diffraction • the human eye the bending of light as • resolving close objects – correcting for nearsightedness it passes from air into – correcting for glass (or plastic) farsightedness • optical instruments converging lens Diverging lens F focal point F Æa converging lens focuses parallel rays to A diverging lens causes a point called the focal point. parallel rays to diverge as if they came from a Æ a thicker lens has a shorter focal length focal point F 1 Image formation by a A converging lens is used to converging lens focus rays from the sun to a point image object 2F F ÎIf the object is located at a distance ofat least 2F from the lens, the image is inverted and smaller than the object. since the sun is very ÎThe image is called a REAL image since light rays far from the lens, the actually converge at the image location rays are nearly parallel converging lens is used in a Image formation by a camera to focus light onto the film diverging lens Object image when you focus a camera, ÎThe diverging lens produces an image that is upright you adjust the distance and diminished in size. between the lens and the ÎIt is a VIRTUAL image, since light rays do not film depending on the actually pass through the image point object location. a magnifying lens Sight – the human eye • Physics of the human eye • Abnormal vision F • Nearsightedness • Farsightedness F Object • astigmatism virtual image • Depth perception By placing the lens close to the object we get a magnified virtual image. 2 • light enters through the The Eye cornea the physics of the human eye • the iris controls the amount of light that gets in, a muscle can close it or open it, the iris is determimes your eye color • the lens is filled with a jelly-like substance; the ciliary muscle can change the shape of the lens and Æby changing the focal The relaxed eye can easily focus on distant thus change its focal length, (accommodation) the lens objects. To focus on close objects the lens is squeezed to length is able to focus light onto the shorten it’s focal length, making it possible to converge the retina for objects located at rays onto the retina. The near point is the distance at which various distances the closest object can be seen clearly. It recedes with age. When a nearsighted person views a distant object, the lens When a farsighted person tries to focus on a close object cannot relax enough to focus at the retina. The rays converge the lens cannot be squeezed enough to focus on the retina. too quickly. The remedy is to place a diverging lens in front The focusW point is behind the retina. The remedy is to place e of the eye to first diverge the a convergingh lens in front of the eye to converge the rays before they enter the eye. How does the eye judge distance? astigmatism Right eye • Astigmatism means that the cornea is D B oval like a football instead of spherical like a basketball. Most astigmatic corneas Left eye have two curves – a steeper curve and a • Our brain interprets the images formed on the flatter curve. This causes light to focus on retinas of both eyes as a single image Æ this is more than one point in the eye, resulting in called binocular vision blurred vision at distance or near • Our eyes roll inward slightly to focus on the distant point D. Our brain interprets the distance BD by the muscular effort required to roll the eyes inward. 3 Astigmatism Astigmatism can be corrected with specially shaped lenses or, in extreme cases, with surgery 4.

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