Light Waves

1. What we call light is a small portion of the electromagnetic spectrum All the different colors are electromagnetic waves with different wave lengths Wave speed = frequency x wavelength EM radiation is alternating electric and magnetic fields Light Waves

2. EM Spectrum long  low f low energy short  high f high energy

3. Changing magnetic/electric field induces electric/magnetic field Electromagnetic Waves

4. Remember, the lower the frequency the longer the wavelength and vice versa Wave speed of light is 300,000,000 m/s Electromagnetic Spectrum

5. R O Y G. B I V red orange yellow green blue indigo violet Visible Light • Visible Light • small part of the spectrum we can see • ROY G. BIV - colors in order of increasing energy

6. Lowest frequencies of visible light are red Highest frequencies are violet The order is red, orange, yellow, green, blue, indigo, violet Colors

7. Colored objects are a result of selective reflection and absorption of light Sunlight is a mixture of colors We call this mixture white light When white light strikes a red object, the red frequencies are reflected and the other frequencies are absorbed Colors

8. Sunlight

9. White objects reflect all frequencies Black objects absorb all frequencies Objects can only reflect the colors of light that shine on them If you shine blue light on a red object it will look black Colors

10. We can also consider shining light through semi-transparent objects Filters used on the lights in a theater are materials that absorb some frequencies and let other frequencies pass through Red filters transmit red light through the material while other frequencies are absorbed Colors

11. Our eyes and brains are very complicated color processing sensors and computers Artists have learned how to mix colors to make pleasing combinations and to create a huge array of different colors All based on what the sensors in our eyes respond to Mixing Colors

12. Visual Response

13. Mixing Colors • Primary light colors • red, green, blue • additive colors • combine to form white light • EX: computer RGBs

14. Mixing Colors • Filter • transparent material that absorbs all light colors except the filter color

15. Mixing Colors • Pigment • colored material that absorbs and reflects different colors • Primary pigment colors • cyan, magenta, yellow • subtractive colors • combine to form black • EX: color ink cartridges

16. Mixing Colors Light Pigment When mixing pigments, the color of the mixture is the color of light that both pigments reflect.

17. Light and Matter • Opaque • absorbs or reflects all light • Transparent • allows light to pass through completely • Translucent • allows some light to pass through

18. This is why the sky is blue! The higher frequencies are scattered more by the molecules in the atmosphere Reds and oranges just pass through, so the sky appears blue Selective Scattering

19. Molecules in atmosphere scatter light rays. • Blue Sky & Red Sunsets • NOON • less atmosphere • less scattering • blue sky, yellow sun • Shorter wavelengths (blue, violet) are scattered more easily. • SUNSET • more atmosphere • more scattering • orange-red sky & sun

20. Diffraction Waves bend when they encounter an object Stand in the water and watch the wake from a boat hit you Look behind you and you will see that after a short distance, the wake continues on It filled in the hole by bending around you This bending is called diffraction

21. Diffraction • Diffraction • bending of waves around a barrier • longer wavelengths (red) bend more - opposite of refraction

22. Diffraction Diffraction is the third way to bend light The other two are reflection and refraction The amount of bending that occurs depends on the relative sizes of the object and the wavelength of the wave Longer wavelengths bend easier than short ones

23. Diffraction

24. Interference • Interference • constructive  brighter light • destructive  dimmer light

25. Interference When the waves are hitting the edges of something, the new bending waves tend to interfere with each other and we get some new patterns Recall the principle of superposition We simply add the amplitudes

26. Interference

27. Interference Wave nature of light was demonstrated by Young

28. Interference The bright and dark areas result from differences in path lengths from the slits to the screen This changes the where the peaks and troughs appear Remember the principle of superposition

29. Interference

30. Interference

31. Can get interference from a single slit Waves coming through one side of the slit interfere with waves coming through the other side Extend the idea to three, four… slits Make something with hundreds of slits Called a diffraction grating Interference

32. Used in spectrometers (devices to separate light into colors It spreads the spectrum Since different colors have different wavelengths, the constructive interference occurs at different locations Diffraction Gratings

33. Diffraction Gratings

34. Diffraction Gratings • glass or plastic made up of many tiny parallel slits • may also be reflective • spectroscopes, reflective rainbow stickers, CD surfaces

35. Look at an oil slick floating on the surface of a pond The slick has a rainbow of colors depending on the angle of viewing This results from interference in the very thin film of oil Called iridescence Thin Films

36. Thin Films

37. Thin Films - Bubbles & Oil Slicks • interference results from double reflection

38. A phenomenon that occurs in transverse waves only Polarization

39. These waves are plane-polarized All the motion is confined to a plane Polarization

40. Shake an electron up and down and you create an electromagnetic wave that is plane-polarized in the vertical direction Shake an electron side-to-side and you create an EM wave that is plane-polarized in the horizontal direction Polarization

41. A standard incandescent bulb emits light that is unpolarized The electrons are shaking in random directions So, the light has its electric field shaking in different directions for different waves Polarization

42. Polarization

43. Some transparent crystalline materials have a remarkable property These materials have their atoms arranged in non-cubic structures These crystals effectively divide the light into two beams that are plane-polarized at right angles to each other Polarization

44. Polarization Any polarization direction can be split into a horizontal and vertical component. Like adding two separate beams together.

45. Some crystals strongly absorb one of these beams while letting the other beam pass right through These materials are called polarizers Take a thin sheet of such a material and imbed it between two sheets of glass or cellulose, and you have a Polaroid filter Polarization

46. Polarization

47. What happens if we work with two polarizers? If we place them so they are aligned, then the light passing through the first polarizer, will also pass through the second one But if the polarizers are at right angles, the second one will absorb all the light Polarization

48. Polarization

49. Most of the light reflected from non-metallic surfaces becomes polarized Consider the glare from glass or water Reflected wave has more vibrations parallel to the surface Analogous to skipping a rock across a pond Polarization

50. Sunglasses are polarized to block rays reflecting from horizontal surfaces light the road or a lake This means the direction of polarization of the sunglasses is vertical so it blocks horizontally polarized rays Polarization