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Refraction PowerPoint Presentation

Refraction

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Refraction

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  1. Refraction

  2. Optical Density • Inverse measure of speed of light through transparent medium • Light travels slower in more dense media • Partial reflection occurs at boundary with more dense medium • If incident angle not 90 degrees, refraction occurs

  3. Optical Refraction • Bending of light rays as they pass obliquely from one medium to another of different optical density • Angle of refraction measured to normal from refracted ray • Passing from lower to higher density, light refracted towards normal; high to low, away from normal

  4. Index of Refraction • Ratio of speed of light in a vacuum (c) to its speed in a substance • n =c/v • Measured by refractometer, used to test purity of substance

  5. Snell’s Law • Relates index of refraction to the angle of refraction • Between any two media ni(sin qi) = nr(sin qr) • Since nair = 1.00, for light passing from air into another transparent medium, n = sin qi/ sin qr

  6. Atmospheric Refraction • Causes gradual curve of light from stars and sun • Creates mirages that look like wet spots on roads • Makes sun visible 2-3 min. before sunrise and after sunset

  7. Mirage Formation

  8. Highway Mirage

  9. Laws of Refraction • Incident ray, refracted ray & normal line all lie in same plane • Index of refraction for homogeneous medium is constant, independent of incident angle • Oblique ray passing from low to high optical density is bent towards normal and vice versa

  10. Dispersion • Transparent media react differently to different wavelengths, slowing short waves more than long waves • Different wavelengths are refracted to a different degree, violet more than red • Causes spreading of the light according to wavelength (frequency) - rainbow

  11. Dispersion • Prisms, water drops readily disperse light due to non-parallel surfaces • Rainbows created by refraction through many drops • Each color produced by a set of drops at a certain angle from the eye

  12. Dispersion in Raindrops

  13. Rainbow Physics

  14. Total Reflection • At media boundary, light from denser medium refracted back into it, rather than exiting into less dense medium • Critical angle: incident angle that produces refracted angle of 90 degrees • At critical angle, refracted ray parallel to media boundary

  15. Total Reflection • From Snell’s law: n = sin 90o/sin icso sin ic = 1/n • Critical angle for water is 48.5 deg., for diamond it is 24 deg. • If incident angle > critical angle, total reflection occurs • Causes diamond’s sparkle, fiber optics

  16. Total Internal Reflection

  17. Fiber Optics

  18. Lenses • Transparent object with nonparallel surfaces, at least one of which is curved • Usually glass or plastic but can be water, air, other transparent solid, liquid or gas • Converging: thicker in middle, converges (focuses) rays • Diverging: thinner in middle, diverges (spreads) rays

  19. Lens Terms • Each side of lens has center of curvature and focus • Real focus (converging lens) where light rays pass through • Real image forms on same side of lens as real focus, opposite side of object

  20. Lens Terms • Virtual focus (diverging or converging) where light rays appear to have originated • Virtual image forms on same side of lens as virtual focus and object • Focal length: distance from center of lens to focal point; depends on curvature and index of refraction of lens

  21. Mirrors & Lenses: Differences • Secondary axes pass through center of lens • Principal focus usually near C; use 2F instead of C in ray diagrams • Real images on opposite side of lens as object, virtual images on same side • Convex lenses are like concave mirrors, concave lenses like convex mirrors

  22. Images of Converging Lenses • Object at infinite distance forms point image at F on opposite side • Object at finite distance > 2F forms real, reduced image between F and 2F on opposite side • Object at 2F forms real, same size image at 2F on opposite side

  23. Images of Converging Lenses • Object between F and 2F forms real, magnified image beyond 2F on opposite side • Object at F forms no image, rays are parallel • Object between F and lens forms enlarged, virtual image on same side (magnifying glass)

  24. Images of Diverging Lenses • Always virtual, erect, reduced size • Often used to neutralize or minimize effect of converging lens (glasses)

  25. Lens Equations • 1/f = 1/do + 1/di • hi / ho = di / do • For simple magnifier, magnification M = hi / ho = di / do for normal vision, di = 25 cm, so M = 25 cm/f (f - focal length)

  26. f-numbers • Ratio of focal length to aperture (effective diameter), used to rate camera lenses • Determines light gathering power of lens • “Fast” lenses have low f-numbers, gather more light, need shorter exposure times • Since area of lens is prop. to square of diameter, f-2 lens is 4 times faster than f-4, 16 times faster than f-8

  27. The Microscope • Objective lens forms enlarged, real image • Eyepiece magnifies image of objective producing greatly magnified, inverted, virtual image • Objective power = tube length/focal length • Total magnification M=25length/fe fo( all in cm)

  28. Telescopes • Reflectors have one converging mirror and a converging eyepiece lens • Refracting telescopes have large objective lens instead of a mirror • Object at great distance means small, real image is produced by objective mirror or lens

  29. Telescopes • Eyepiece lens enlarges objective image producing magnified, inverted, virtual image • Large telescopes are reflectors due to size and expense of large lens • Binoculars, terrestrial telescopes use extra lens or prism to invert image to upright position

  30. The Eye • Cornea and lens work together to focus light on retina producing inverted, small image • Brain circuitry inverts image so it seems right side up

  31. Vision Correction • Nearsighted means light focuses in front of retina—corrected with diverging lens • Farsighted means light would focus behind retina—corrected with converging lens

  32. Cameras • Cameras focus light on the focal plane where the film is located • Produce real, inverted, smaller image, like the eye • Some cameras use a diverging lens for a viewfinder

  33. Lens Aberrations • Spherical aberration: like mirrors, light passing through edges not focused at same point as through center - correct with lens combination • Chromatic aberration: different colors refracted differently, focus at different points - correct with lens coatings, lenses of different materials