Unit 9 – Waves & Optics

1. Unit 9 – Waves & Optics Chapters 14-16

2. Electromagnetic waves is a transverse wave consisting of oscillating electric and magnetic fields at right angles to each other. 14-1 Characteristics of Light

3. The EM spectrum includes more than visible light. • EM waves vary depending on frequency & wavelength.

4. Electromagnetic Spectrum

5. Radio • Radio waves generally are utilized by antennas of appropriate size (according to the principle of resonance), with wavelengths ranging from hundreds of meters to about one millimeter. • Lowest frequencies allow communication with submarines • High frequencies are used for FM radio broadcasts, wireless LAN, radar and radio astronomy

6. AM – KHz • Amplitude modulation • FM – MHz • Frequency modulation

7. Microwaves • EM radiation may also cause certain molecules to absorb energy and thus to heat up; this is used in microwave ovens. • Garage door openers • Radar • Atomic and molecular research • Aircraft navigation • Microwave communication

8. Infrared • Infrared photography • Pit vipers • Infrared therapy • Military applications • Analyze art/relics • Weather forecasting

9. Visible Light • Above infrared in frequency • This is the range in which the sun and stars emit most of their radiation. • Visible light (and near-infrared light) is typically absorbed and emitted by electrons in molecules and atoms that move from one energy level to another.

10. Ultraviolet • Causes sunburn/cancer. • The Sun emits a large amount of UV radiation, but most of it is absorbed by the atmosphere's ozone layer before reaching the surface. • Used for: • Sterilization of medical instruments • Identification of fluorescent minerals • Detect forged documents • Forensics

11. X-rays • Detect stars around black holes • Medicine and industry • Border control/airports

12. Gamma Rays • Food treatment • Often used to kill living organisms, in a process called irradiation. • Sterilizing medical equipment (as an alternative to autoclaves or chemical means) • Treatment for cancers

13. All forms of EM travel at the speed of light. • c = f c = speed of light in a vacuum (3.0 x 108 m/s) l = wavelength (meters) f = frequency (Hertz)

14. Brightness • Brightness decreases the further you get from the source of the light. • Brightness decreases by the square of the distance from the light source. • Ex. Twice as far, 1/4 times as bright

15. 14-4 What is color? • Visible light is a small slice of the electromagnetic spectrum. • The colors are distinguished by their frequencies.

16. What about black & white? • When all of the colors of light are combined, you get white light. • White is therefore not technically a color, it is a combination of all of them. • If you remove all of the colors of light, you get black. • Black is not a color, it is the absence of color.

17. Perception of color • We have receptor cells in the retina of our eyes: rods and cones • Rods are responsible for detecting light intensity. • Animals with good night-vision have lots of rods in their eyes. • More sensitive than cones. • Cones are responsible for detecting colors • There are three types: red, green, and blue • Rods & Cones

18. Cones and color vision • Each type is sensitive to a certain range of frequencies • As light strikes the cone cell, a chemical reaction sends an electrical signal to the brain. Your brain converts these to what you see. • Notice that we are most sensitive to light in the green to yellow range. • What do we see in the dark?

19. Colorblindness • About 7 percent of the male population has some degree of colorblindness. • This does not mean that they don’t see colors, they just see them differently and have difficulty distinguishing some colors. • It can disqualify people from some careers. • It is a recessive trait on the X chromosome • Females can be colorblind, but it would require 2 defective X chromosomes, very rare. • Colorblind Test

25. How colorblind people see the world

26. Color by addition • If our eyes only really see red, green, and blue, then we should be able to make any color from combinations of those. • By mixing these three colors of light, we can make any image. • Component video inputs for televisions use these: RGB

27. Primary Additive Color • When you combine all 3 additive primary colors, you produce white light • When you combine 2 additive primary colors, you produce a secondary color. • Red + Green = Yellow • Red + Blue = Magenta • Green + Blue = Cyan • Additive Color Simulator

28. Subtractive Color Process • We use this to mix paints and print pictures. • Yellow reflects red and green and absorbs blue. • Magenta absorbs green. • Cyan absorbs red. • mag + cyan = blue • mag + yellow = red • cyan + yellow = green • mag + yellow + cyan = black

29. Complements • Complementary colors - any two colors of light which when mixed together in equal intensities produce white. • The complementary color of red light is cyan light. • This is reasonable since cyan light is the combination of blue and green light • And blue and green light when added to red light will produce white light.

30. Color by reflection • The color of an object occurs when that object absorbs all colors except the color it appears to be. • If a leaf is green, that means that it absorbs all colors but green. • The leaf reflects green light. • Dyes and pigments work on this principle

31. Unit 9 - Day 2 Reflection – 14.2 & 14.3 Flat Mirrors & Curved Mirrors

32. 14.2 – Flat mirrors • Reflection – the turning back of a wave at a surface. • Most substances absorb a little incoming light and reflect the rest. • A good mirror reflects 90% of the incoming light. • You see objects and light rays because of reflected light.

33. The texture of a surface affects how it reflects light. Light reflected from a rough, textured surface is called diffuse reflection. Light reflected from a smooth, shiny surface is called specular reflection.

34. Incoming and reflected angles are equal. • The angle of incidence for a wave reflected from a surface is equal to the angle of reflection.

35. Angle of incidence – the angle between the ray that strikes a surface and the normal to that surface. Angle of reflection – angle formed by the line normal to a surface and the direction in which a reflected ray moves. These angles are measured from the normal.

36. Flat mirrors create a virtual image. • A virtual image is one that is formed by light rays that only appear to intersect. • In flat mirrors, the right & left sides are reversed. The image is just as far behind the mirror as the object is in front of the mirror.

37. 14.3 – Curved Mirrors • Concave spherical mirrors focus light to form real images. • A real image is formed when light rays actually intersect at one point. • These mirrors produce images that can be smaller than, larger than, or the same size as the original object.

38. Convex Spherical Mirrors • For a convex mirror, the image of the object is upright, reduced in size and located behind the mirror • Why would we use them?

39. Mirror equations • 1 + 1 = 1 • di do f • di = distance of image • do = distance of the object • f = focal length • Radius = r = 2 f or ½ r = f

40. Magnification (M) • hi = height of image • ho = height of object • If image is larger than the object, then M > 1 • If image is smaller than the object, then M < 1 • M = hi = -di • ho do

41. Signs • do = positive = object in front of mirror • di = positive = real image • di = negative = virtual image • f = positive = concave mirror • f = negative = convex mirror • M = positive = upright & virtual • M = negative = inverted & real

42. Ray Diagrams • Rules for drawing rays • From the top of the object • Parallel to the axis & reflect back through f • Through f & reflect back parallel • Through the center of curvature • Image where all three lines intersect.

43. Unit 9 - Day 3 Refraction – 15.1

44. Refraction - light BENDS when it goes from one kind of substance to another (because of the change in average speed).

45. The average speedof the light wave changes due to change in media, but THE FREQUENCY DOES NOT CHANGE! Using v = f, we see that the wavelength has to change.

46. Index of refraction – ratio of the speed of light in a vacuum to its speed in a transparent medium • High index of refraction (dense) – slower, so bends toward the normal • Low index of refraction (less dense) – faster, so bends away from the normal

47. Different media result in different amounts of refraction.

48. Example: If light travels at 1.94 x 108 m/s through salt, what is the index of refraction?

49. Snell’s Law • Example: A light ray of wavelength 589 nm traveling through the air strikes a smooth flat slab of crown glass at an angle of 30o to the normal. Find the angle of refraction.

50. Unit 9 - Day 4 Lenses – 15.2