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Properties of Light Lab 5 Basic Properties of Rays of Light • Light rays travel in a straight line • Angle of the incident ray of light is always equal to the angle of the reflected ray of light • Light rays bend or refract when they pass from one medium to another • Refraction is caused by the change in speed experienced by a wave when it changes medium • Light can either slow down while crossing the boundary and refract towards the normal or speed up while crossing the boundary and refract away from the normal Refraction of Light • Regardless of what the two media are, there is a definite relationship between the sines of the angles of incidence and refraction – Snell’s Law • Applet of Snell’s Law – http://lectureonline.cl.msu.edu/~mmp/kap2 5/Snell/app.htm Concave and Convex Lenses Focal Point • The focal point of a concave lens is the point where light rays parallel to the axis seem to diverge from after passing through the lens • The distance from the lens to this point is called the focal length of the lens Convex and Concave Lenses • http://microscopy.fsu.edu/primer/java/com ponents/perfectlens/ • http://www.phys.hawaii.edu/~teb/optics/jav a/clens/ • http://www.mtholyoke.edu/~mpeterso/clas ses/phys301/geomopti/lenses.html Parallax • Parallax is the apparent shift of an object against a background due to a change in observer position • By observing parallax, measuring angles, and using geometry, one can determine the distance to various objects • Distance measurement by parallax is a special case of the principle of triangulation, where one can solve for all the sides and angles in a network of triangles if, in addition to all the angles in the network, the length of only one side has been measured Parallax triangulation • In parallax, the triangle is extremely long and narrow, and by measuring both its shortest side and the small top angle α, the long sides can be determined and are assumed to be approximately equal to each other How it works….. • b covers an angle α 2π r covers an angle 360° • so 2π r = (360°/ α b) • r = (360°/2 π α) b • If we know b, we can deduce r • For example, if we know that α = 5.73°, then 2 π α = 36° and r = 10 b Use it to estimate distances … • Stretch your arm forward and extend your thumb, thumb facing your eyes • Close one eye and move your thumb so that, looking with your open eye , you see your thumbnail covering the landmark A • Then open the eye you had closed and close the one with which you looked before, without moving your thumb • It will now appear that your thumb has moved and is no longer in front of landmark, but in front of some other point at the same distance • The distance to the landmark is 10 times the distance between the 2 places your thumb appeared to be • The angle between the lines from the eyes to the outstretched thumb is about 6°, for which the ratio is 1:10 • That angle is the parallax of your thumb, viewed from your eyes Stellar parallax • http://instruct1.cit.cornell.edu/courses/astr o101/java/parallax/parallax.html