Reflection and refraction L 31 Light and Optics [2] at a surface • Measurements of the speed of light (c) √ Index of refraction n = c/v > 1 • Index of refraction vmedium = c/n – the bending of light – refraction √ Incident Normal line – total internal reflection √ Light ray reflected • Color (wavelength and frequency, c = λf ) Light ray • Dispersion • rainbows • Atmospheric scattering • blue sky and red sunsets • Law of reflection – mirrors – Image formation refracted Light ray 1 2 Seeing through the window Seeing through a window When the angle of incidence is small, most of the A light ray is offset slightly incident light passes thru the glass, only a small When it passes thru a pane amount is reflected. Of glass. The thinner the Glass, the smaller the offset. r ef lec ted ted smit tran ent incid outside 3 inside 4 Windows behaving as mirrors VISIBLE LIGHT When the angle of incidence is r e f large (grazing l e transmitted c incidence) more t e d light is reflected, the window is like a mirror. t n e Color Î WAVELENGTH OR FREQUENCY d i c n i Wavelength × Frequency = c (speed of light) IN OUT 5 = 3 x 108 m/s 6 1 The index of refraction (n) depends Different colors are refracted (bent) by of the color (wavelength) of the light different amounts this effect is called DISPERSION color Wavelength (nm) n Red 660 1.520 orange 610 1.522 yellow 580 1.523 green 550 1.526 White light red blue 470 1.531 violet 410 1.538 violet contains all Glass prism wavelengths –9 1 nanometer (nm) = 1 × 10 m 7 (colors) 8 The rainbow Why is it a rain BOW ? • Rainbows are caused by dispersion of sunlight The drops must be from water droplets which act as tiny prisms At just the right Angle (42 degrees) Between your eyes And the sun to see The rainbow. This Angle is maintained Along the arc of a Circle. 9 10 Atmospheric scattering Atmospheric scattering: blue sky • Short wavelengths are • Why is the sky blue and sunsets red? scattered more than long • It is due to the way that sunlight is wavelengths scattered by the atmosphere (N and O ) • Blue light (short) is 2 2 scattered 10 times more • ScatteringÆ atoms absorb light energy than red light and re-emit it, but not at the same • The light that we see in wavelength the sky when not looking directly at the sun is • Sunlight contains a full range of scattered blue light wavelengths in the visible region 11 12 2 Atmospheric scattering: red sunset Why are clouds white? • Clouds consist of water droplets and very tiny ice • At sunset, the sun particles is low on the • The water droplets and ice scatter the sunlight horizon • Scattering by water and ice (particles) is very • When looking at the different from scattering by molecules sun it appears red • The atoms are smaller than the wavelength of because much of light, but the ice and water particles are larger the blue light is • Scattering by particles does not favor any scattered out particular wavelength so the white light from the leaving only the red sun is scattered equally Æ clouds are white! 13 14 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: Sharp image mirror Fuzzy or no image 15 16 You only need a mirror half as tall as image formation by plane mirrors 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. 17 18 3 The image of your right Spherical or curved mirrors hand is your left hand Concave mirror Focus AMBULANCE is painted parallel light rays are focused to one point backward so that you see it correctly in your real- view mirror 19 20 Where is the light bulb? convex mirror image of light bulb light bulb F focus f f A concave mirror will form a real image of an object placed at twice its focal length at a distance of twice the focal length. It will be inverted parallel rays diverge from a focus behind and the same size as the object. the mirror 21 22 Dish antennas Magnifying mirrors signal from Homer’s satellite image Homer detector at when something placed within the focus of a concave the focal mirror, an enlarged, upright image is formed. point of the dish this principle is used in a shaving or makeup mirror 23 24 4 Convex mirrors: wide angle view Object Image A convex lens provides a wide angle view. Since it sees more, the images are reduced in size. Passenger side mirrors are often of this type with the warning: “objects appear further than they actually are". Because they appear smaller they look further away. 25 5.