L i g h t

1. Light Chapters 16 and 18

2. Light is an electromagnetic wave • Electric field waves are perpendicular to magnetic field waves. • Both are perpendicular to the direction the wave is traveling. • This makes light a transverse wave. • All electromagnetic waves are caused by vibrating charges. • Electromagnetic waves can travel through a vacuum

3. 1. Radio and TV 2. Microwaves 3. Infrared 4. Visible Light 5. Ultraviolet 6. X-rays 7. Gamma Rays Electromagnetic Spectrum(longest wavelength to shortest wavelength)

4. Different wavelengths of light have different colors. • Violet light has a wavelength of about 400 nm • Red light has a wavelength of about 700 nm

5. 1. Red 2. Orange 3. Yellow 4. Green 5. Blue 6. Indigo 7. Violet Color Spectrum(from longest to shortest wavelengths)ROYGBIV

6. 10 Electromagnetic waves • can travel through a vacuum • need a medium to travel through

7. 10 Which of these electromagnetic waves has the shortest wavelength? • Radio • Infrared • X-rays • Ultraviolet • Light

8. 10 Which color of light has the highest frequency? • Blue • Green • Orange • Yellow

9. Pathway of Light • Light travels in a straight line in a vacuum or other uniform medium. • The straight-line path of light has led to the ray model of light. • A ray is a straight line that represents the path of a narrow beam of light.

10. Galileo was first to try to measure speed of light. • Before Galileo, everyone thought light had no speed, but instead traveled instantaneously. • Galileo was the first to hypothesize that light had a finite speed. • Used lanterns with shutters as first experiment. • Decided light was too fast to measure.

11. Olaf Roemer was the second to try • Used time it took Jupiter’s moon Io to eclipse Jupiter. • Recorded the time it took Io to emerge from behind Jupiter. • As the Earth moved further and further away from Jupiter, the longer it took. • He calculated that light took 22 minutes to cross the diameter of the Earth (speed of 220 million meters /sec)

12. First American Nobel prize winner: Albert Michelson • Developed Earth based techniques to measure the speed of light • In 1926, successfully measured speed of light. • Results were within .001 % of currently accepted speed of light

13. Michelson’s Technique • His technique was to use two mountain peaks in California 35 km apart, then time how long it took light to bounce back. • Used an interferometer for timing device

14. The speed of light is a set value C = 300,000,000 m/s = 3.00 x 108 m/s = 300,000 km/s = 186,000 mi./s

15. Speed of light, wavelength of light, frequency of light relationship • c = f • c = speed of light (m/s) •  = wavelength of light (m) • f = frequency of light (Hz)

16. 10 Compared to the velocity of radio waves, the velocity of visible light waves is • slower • faster • the same

17. 10 The main difference between a radio wave and light waves is its • Speed • Wavelength • Both 1 and 2 • None of the above

18. 10 The main difference between a radio wave and a sound wave is its • frequency • wavelength • energy • amplitude • basic nature

19. 10 Which of the following was not one of Galileo’s assumptions about the speed of light? • The speed of light can’t be measured. • Light is very fast. • Light travels instantaneously. • None of the above.

20. 10 Light takes 1.28 s to travel from the Moon to Earth. What is the distance between them? • 2.3 x 108 m • 3.8 x 108 m • 7.7 x 108 m • 440 m

21. Luminous • Objects which create light are said to be Luminous • Examples: Sun, stars, light bulbs

22. ILLUMINATED • Objects which reflect light are said to be illuminated • These objects do not create their own light • examples: Moon, planets, desk, whiteboard, reflectors, people...

23. Luminous Flux • Luminous flux (P) is the rate at which light is emitted from a source. • The unit of luminous flux is the lumen (lm) • A typical 100 W light bulb emits approximately 1750 lm.

24. Illuminance • Often, we are more likely interested in the amount of illumination an object provides. • The illumination of a surface is called the illuminance (E). • Illuminance is the rate at which light falls on a surface. • Illuminance is measured in lumens per square meter (lm/m2) or lux (lx). • If the distance of a surface from the point of light is doubled, the illumination is reduced by a factor of 4.

25. Increasing Illumination • There are two ways to increase the illumination: 1) Use a brighter bulb (increase luminous flux) 2) Move the surface closer to the bulb (decrease the distance) E = P / 4d2 • E = illumination (lx) • P = luminous flux (lm) • d = distance from light source (in meters)

26. Luminous Intensity • Luminous intensity is measured in units of candelas (cd) • One candela = # of lumens/4π

27. Transparent Objects • Allow light to pass through them undisturbed. • No trouble identifying objects behind transparent objects. • Examples: glass, transparencies, clear liquids

28. Translucent Objects • Light can pass through, but not clearly. • Reflect some light, but also allow some light to pass through (transmit) • Examples: tissue paper, lampshades, frosted light bulbs...

29. Opaque Objects • Materials which do not allow light to pass through. • Only reflect light. • Examples: bricks, doors, people...

30. GIVING LIGHT DIRECTION: POLARIZATION

31. Polarization of Light • Unpolarized light vibrates in all directions in the xyz plane. • In this illustration the electric field (E) is vibrating on the y-axis, and the Magnetic field (B) is vibrating on the z-axis. The wave is traveling along the x-axis.

32. Polarization (cont.) • Polarizers are made of long strands of molecules that are all aligned parallel to each other. • Look at the blue areas in the illustration.

33. Polarizers will only transmit light that is vibrating parallel to the direction in which the polarizer is lined up. Polarizers will not allow light to pass through if the light is vibrating perpendicular to it.

34. Polarization (continued) • Light that does not pass through the polarizing filter is absorbed. • Since only part of the total amplitude of the wave passes through the filter, the intensity of the light is reduced.

35. Polarization by Reflection • Light reflected off surfaces is also partially polarized. • The light is usually polarized in the same direction as the surface. • Light reflecting off the road or the surface of a lake is polarized in the horizontal direction. • Therefore, sunglasses consisting of a vertical polarizing material will help reduce glare produced from this type of reflection.

36. Polarization Analysis • Suppose light is polarized by passing it through a filter. What happens if a second filter is placed in the path of the polarized light? • If the polarizing axis of the second filter is parallel to the first, the light will pass through. • If the polarizing axis of the second filter is perpendicular to the first, no light will pass through. • To determine the intensity of the light that will pass through a second filter, Malus’s Law is used. • Malus’s Law states that the intensity of the light coming out of a second filter is equal to the intensity of the light coming out of the first filter multiplied by the cosine squared of the angle between the two filters. • Example: If light passes through two filters at 45o angles to each other, the light coming out of the second filter will have 50% of the intensity of the light that came out of the first. I2 = I1cos2θ I2 = 100cos2(45o) = 50

37. Polarized Light and 3-D Viewing • Vision in 3 dimensions depends on the fact that both eyes give impressions simultaneously, each eye viewing a scene from a slightly different angle. • The combination of views in the eye-brain system gives depth. • A movie can be seen in 3-D when the left eye sees only the left view and the right eye sees only the right view. • This is accomplished by projecting the pair of views through polarization filters at right angles to each other. The overlapping pictures look blurry to the naked eye. • To see in 3-D, the viewer wears polarizing eyeglasses with the lens axes also at right angles. In this way, each eye sees a separate picture just as in real life. • The brain interprets the two pictures as a single picture with a feeling of depth.

38. 3-D Viewing

42. 10 Light reflected from a lake surface is polarized ____. • vertically • horizontally • randomly

43. 10 In order for sunglasses to be effective in reducing glare produced from a road, the glasses should be polarized ___. • vertically • horizontally • Both vertically and horizontally

44. 10 Glasses used for 3-D viewing are polarized _____. • vertically • horizontally • Both vertically and horizontally

45. 10 If two Polaroid filters are held with their polarization axes at right angles to each other, the amount of light transmitted compared to when their axes are parallel is ___. • twice as much • the same • half as much • zero

46. 10 The amount of light that gets through Polaroid filters at 25o, compared to the amount that gets through the 45o Polaroids is • less • more • the same

48. Making Colors by Addition of Light

49. Primary Light Colors • There are three primary light colors • Red, blue, green • All three colors added together make white light • Combinations of any of these two produce secondary colors.

50. Light Color by Addition(secondary colors of light) • Red + Blue = Magenta • Blue+Green=Cyan • Red+Green=Yellow