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L 32 Light and Optics [2] the Rainbow Why Is It a Rain BOW ? Atmospheric Scattering

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L 32 Light and Optics [2] the Rainbow Why Is It a Rain BOW ? Atmospheric Scattering The index of refraction (n) depends L 32 Light and Optics [2] of the color (wavelength) of the light • Measurements of the speed of light Å color Wavelength (nm) n • The bending of light – refraction Å Red 660 1.520 • Total internal reflection Å •Dispersion orange 610 1.522 • Dispersion yellow 580 1.523 • Rainbows green 550 1.526 • Atmospheric scattering • Blue sky and red sunsets blue 470 1.531 • Mirrors n violet 410 1.538 1 nm = 0.000000001 m Different colors are refracted (bent) The rainbow by different amounts • Rainbows are caused by dispersion of sunlight from water droplets which act as tiny prisms White light red violet contains all Glass prism wavelengths (colors) Why is it a rain BOW ? Atmospheric scattering The drops must be At just the right • Why is the sky blue and sunsets red? Angle (42 degrees) • It is due to the way that sunlight is Between your eyes scattered by the atmosphere (N and O ) And the sun to see 2 2 The rainbow. This • ScatteringÆ atoms absorb light energy Angle is maintained and re-emit it but not at the same Along the arc of a wavelength Circle. • Sunlight contains a full range of wavelengths in the visible region 1 Atmospheric scattering: blue sky Atmospheric scattering: red sunset • Short wavelengths are • At sunset, the sun scattered more than long is low on the wavelengths horizon • Blue light (short) is scattered 10 times more • When looking at the than red light sun it appears red • The light that we see in because much of the sky when not looking the blue light is directly at the sun is scattered out scattered blue light leaving only the red Mirrors Î reflection The law of reflection • Light does not pass thru metals – it is reflected • The angle of reflection = angle of incidence at the surface • Incident ray, reflected ray and normal all lie • Two types of reflection: diffuse and specular in the same plane normal i r reflected Incident ray ray Diffuse reflection: specular reflection: Fuzzy or no image Sharp image mirror You only need a mirror half as tall image formation by plane mirrors as you are to see your whole self The rays appear to originate from the image behind the Homer’s image Homer mirror. Of course, there is no light behind the mirror Æ this is called a virtual image Mirrors appear to make rooms look larger. 2 The image of your right Spherical or curved mirrors hand is your left hand Concave mirror Focus parallel light rays are focused to one point AMBULANCE is painted backward so that you see it correctly in your real-view mirror convex mirror Dish antennas signal from satellite focus detector at the focal parallel rays diverge from a focus behind point of the dish the mirror Magnifying mirrors Convex mirrors: wide angle view Homer’s Homer image Object Image A convex lens provides a wide • when something placed angle view. Since it sees more, within the focus of a concave the images are reduced in size. mirror, an enlarged, upright Passenger side mirrors are often of image is formed. this type with the warning: “objects • this principle is used in a appear further than they actually shaving or makeup mirror are". Because they appear smaller they look further away. 3 Image formation with lenses converging lens • converging lens • lenses are relatively (positive lens) simple optical devices • diverging lens • the principle behind (negative lens) the operation of a lens is refractionÆ • the human eye focal the bending of light as – correcting for point F nearsightedness it passes from air into – correcting for glass (or plastic) Æa converging lens focuses parallel rays to farsightedness a point called the focal point. • optical instruments Æ a thicker lens has a shorter focal length Image formation by a Diverging lens converging lens image F object 2F F A diverging lens causes ÎIf the object is located at a distance ofat least 2F from the parallel rays to diverge lens, the image is inverted and smaller than the object. ÎThe image is called a REAL image since light rays as if they came from a actually converge at the image location focal point F A converging lens is used to converging lens is used in a focus rays from the sun to a point camera to focus light onto the film when you focus a camera, since the sun is very you adjust the distance far from the lens, the between the lens and the rays are nearly film depending on the parallel object location. 4 Image formation by a a magnifying lens diverging lens F Object image F Object virtual image ÎThe diverging lens produces an image that is upright and diminished in size. By placing the lens close to the object ÎIt is a VIRTUAL image, since light rays do not actually pass through the image point we get a magnified virtual image. • light enters through The Eye Sight – the human eye the cornea • the iris controls the • Physics of the human eye amount of light that • Corrections for abnormal vision gets in, a muscle can close it or open it, the • Nearsightedness iris is the colored part • Farsightedness • the lens is filled with a jelly-like substance; the ciliary muscle can change the shape of Æby changing the focal the lens and thus length, (accommodation) the lens change its focal is able to focus light onto the length retina for objects located at various distances the physics of the human eye The relaxed eye can easily focus on distant objects. To focus on close objects the lens is squeezed to When a nearsighted person views a distant object, the lens shorten it’s focal length, making it possible to converge the cannot relax enough to focus at the retina. The rays converge rays onto the retina. The near point is the distance at which too quickly. The remedy is to place a diverging lens in front the closest object can be seen clearly. It recedes with age. of the eye to first diverge the 5 Pencil in lucite block Æ the top half of the pencil is glued exactly at the position where the image of the bottom half is formed in the block Ædue to refraction at the front surface the bottom of the pencil (its image ) appears closer to the front surface of the block Æthe bottom half of the pencil cannot be seen from the sides of the block because any ray from the bottom of the pencil suffers total internal reflection on the sides of the block. When a farsighted person tries to focus on a close object the lens cannot be squeezed enough to focus on the retina. The focusW point is behind the retina. The remedy is to place e a convergingh lens in front of the eye to converge the rays before they enter the eye. top view 6.
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