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Electromagnetic waves option g review


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Electromagnetic waves option g review

  1. 1. Electromagnetic Waves- Transverse waves- Travel at the speed of light- Can travel in a vacuum-Created by alternating electric and magnetic fields.-James Maxwell created radio waves by passing analternating current through a wire.
  2. 2. Visible range – from approx 400 nm to 700 nm- red has the lowest frequency and highest wavelength
  3. 3. Properties of EM WavesDispersion – white light is separated into the visible spectrumRed is dispersed the least, violet the most The lowest wavelength refracts the most.
  4. 4. Depending on the frequency of the EM wave and whatsubstance it is hitting, the wave will be transmitted orabsorbed.Transmission – the wave will pass through the substance.Ex. X-rays will pass through most solids Light will pass through airAbsorption – the wave will be absorbedand excite the substanceEx. Microwaves are absorbed by watermolecules UV is absorbed by the ozone White light hitting a blue filter.layer Some light is reflected, blue light is transmitted while the other colors are absorbed.The intensity (brightness) of light is indirectly related tothe distance to the source squared.
  5. 5. Scattering - light will be scattered by particles in theairRed scatters least, blue scatters most Example – Blue sky, red sunsets
  6. 6. LASERLight Amplification by the Stimulated Emission ofRadiationMonochromatic (one color) coherent (same frequency, inphase) lightPopulation inversion
  7. 7. Interference and diffractionLight falling on two slits will produce aninterference pattern(Young’s two-slit experiment)
  8. 8. DThe equations sof interference d npatterns: sin   d x n  D d x   (n  1) D d d sin   n
  9. 9. Path difference
  10. 10. s = D/ds = how far apart maximaareD = distance from slits toscreend = distance between slits
  11. 11. A source of light of unknown wavelength isused to illuminate two very narrow slits adistance 0.15 mm apart. On a screen at adistance of 1.30 m from the slits, brightspots are observed separated by a distanceof 4.95 mm. What is the wavelength of thelight being used?571 nm
  12. 12. Two narrow slits 0.01 mm apart areilluminated by a laser of wavelength 600nm. Calculate the fringe spacing on ascreen 1.5 m from the slits.
  13. 13. Diffraction gratingd sin  = n Light of wavelength 680 nm falls on a diffractiongrating that has 600 lines per mm. What is theangle separating the central maximum (n=0)from the next (n=1)? = 24.1o
  14. 14. Red light (wavelength = 700 nm) is shonethrough a grating with 300 lines/mm.a) Calculate the diffraction angle of the first red line.b)Calculate the diffraction angle of the second red line.
  15. 15. Optical instrumentsConverging lensA lens that is thicker in the middle than at the edges. Light isrefracted toward the principal axis (the straight line that goesthrough the center at right angles to the lens surface).
  16. 16. Parallel rays will refract to a pointon the principal axis called thefocal point. The distance from thelens to the focal point is the focallength.
  17. 17. Optometrists describe the power of a lens as P= 1/ fExample: A lens with a focal length of 25 cm would havea power of 4.0
  18. 18. Real image – an image that is created by rays of lightpassing through a lens - can be projected on a screen - invertedVirtual image – rays do not actually meet, only theirmathematical extensions do - cannot be projected on a screen - upright (erect)
  19. 19. Ray diagrams – pictures used to identify what kind of image will be created by a lens.Rays to draw1) Parallel rays refract through the focus2) Rays through the center continue straight thru3) Rays thru the focus refract parallelWhere these rays meet describe the image. Only two rays are necessary.
  20. 20. Using graph paper, draw a ray diagram for an objecta) at 2F, b) between F and 2F, c) at F and d) inside F
  21. 21. This can also be done mathematically with the thin lensformula.1/ = 1/v + 1/u f u = object distance v = image distanceImage distance : (+) means real image, (-) meansvirtualAlso the magnification can be calculated.m = hi/ho = -v/ u
  22. 22. A converging lens has a focal length of 15 cm.An object of height 2 cm is placed 60 cm fromthe lens. Determine the image.
  23. 23. An object is placed 15 cm in front of a converginglens of focal length 20 cm. Determine the image.
  24. 24. A diverging lens causes light to refract AWAYfrom the focus.What kind of image does a diverging lenscreate?Virtual only
  25. 25. The TelescopeThe refracting telescope works by bending light with twolenses. The objective lens makes a small real image of theobject while the eyepiece lens acts as a magnifying glass. The focal points of the two lenses should be at the same point to produce a focused image.
  26. 26. Draw a ray diagram for a telescope. Note wherethe objective lens focuses the light from a star andwhere the final image is located and in whatdirection.Step 1: Draw the lenses and axis (no foci)Step 2: Draw a ray passing through the center ofthe objective hitting the eyepiece halfway downStep 3: Draw two more rays entering the objectiveat the same angle as the first. The top ray shouldhit the bottom of the eyepiece.
  27. 27. Step 4: The bottom ray will cross where the othertwo intersect. This is the focus. Draw in the firstimage.Step 5: The rays emerge from the eyepieceparallel. To find the angle draw a dotted line fromthe top of the image through the center of theeyepiece
  28. 28. The angular magnification can be worked out bythe simple formula:  M i owhere i and o are small angles in radians.•The angle i is the angle subtended by the objectto the unaided eye.•The angle o is the angle subtended by the imageto the eye.The magnification can also be shown to be relatedto the focal lengths of the lenses by: fo M fe
  29. 29. In a telescope the eyepiece has a focal length of 2cm and the objective has a focal length of 220cm. What is the magnification?Magnification = fo/fe = 220/2 = 110If the moon subtends an angle of 8.8 x 10-3 rad tothe naked eye, what would the angle be for theimage of the moon observed through the telescope?Angle subtended by the Moon = 8.8 x 10-3 rad x 110 =0.97 rad
  30. 30. A telescope is constructed from two lenses: an objective lens of focal length 100 cm and an eyepiece of focal length 10 cm. The telescope is used in normal adjustment.a) Calculate the angular magnificationb) What is the distance between the lenses?
  31. 31. Microscope – similar to a telescope except thata) The focal points of the two lenses are NOT at the same pointb) The image from the objective lens is formed inside the focal length of the eye piece.c) The object is located very close to the objective lens
  32. 32. Step 1: Draw the lenses and an axis.Step 2: Draw a ray through the center of theobjective to a point halfway down the eyepiece.Draw an object a short distance from theobjective.Step 3: Draw a parallel ray from the object to theobjective. Continue this ray to the bottom of theeyepiece. Mark Fo at the point where this raycrosses the axis. Construct an image at the placewhere the two rays intersect.
  33. 33. Step 4: Draw a dotted line from the top of the firstimage through the middle of the eyepiece.Choose a point on this line beyond the objective anddraw the rays coming from this point.
  34. 34. Aberrations – there are two flaws inherent in all lensesthat must be corrected for perfect images.Spherical aberrationRays hitting the very edge of a lens do not focus to theexact same spot as most of the light.
  35. 35. Correcting: Using an aperature to block thelight hitting the edges of the lensCameras, eyeballs
  36. 36. Chromatic aberrationBecause different wavelengths of light(colors) refract slightly differently, not all ofthe colors focus at the some exact point.
  37. 37. Solution: making a lens out of twodifferent materials so that one materialfixes the other. This is called anachromatic doublet.