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# Light

Light. The most famous and accurate. 1880. Michelson ’ s results. C = 186,282.3960 miles per second, plus or minus 3.6 feet per sec. C = 299,792.4562 kilometers per second, plus or minus 1.1 meters per second. The Speed of Light.

## Light

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1. Light

2. Michelson’s results C = 186,282.3960 miles per second, plus or minus 3.6 feet per sec. C = 299,792.4562 kilometers per second, plus or minus 1.1 meters per second

3. The Speed of Light • The speed of light in a vacuum is a universal constant • Accepted values 3.00 x 108 m/s or 186,000 mi/s • A beam of light could travel around the earth, it would make 7.5 trips in one second. • The distance light travels in one year is called a light-year.

4. Electromagnetic Waves • Light is energy that is emitted by accelerating electric charges—often electrons in atoms. • This energy travels in a wave that is partly electric and partly magnetic. • Such a wave is an electromagnetic wave. • Light is a small portion of the broad family of electromagnetic waves

5. Electromagnetic Waves • This list includes in order of increasing (f) : • radio waves, microwaves, infrared waves, visible light waves, ultraviolet waves, X rays, and gamma waves

6. Electromagnetic WavesPOP QUIZ • QUESTION: • Is it correct to say that a radio wave is a low-frequency light wave? Is a radio wave also a sound wave?ANSWER: • Both a radio wave and light wave are electromagnetic waves originating from the vibrations of electrons. Radio waves have lower frequencies of vibration than a light wave, so a radio wave may be considered to be a low-frequency light wave. A sound wave is a mechanical vibration of matter and is not electromagnetic. A radio wave is not a sound wave.

7. Electromagnetic Waves • The range of electromagnetic waves, or the electromagnetic spectrum • The lowest frequency of light we see in our eyes appears red • The highest frequency of light we see in our eyes appears violet • Electromagnetic waves of frequencies lower than the red of visible light are called infrared • Electromagnetic waves of frequencies higher than those of violet are called ultraviolet

8. Light and Transparent Materials • Light is energy carried in an electromagnetic wave • Light incident upon matter, forces electrons to vibrate • How a receiving material responds when light is incident upon it depends on the frequency of light and the natural frequency of electrons in the material

9. Propagation The natural vibration frequencies of an electron depend on how strongly it is attached to a nearby nucleus Different materials have different electric “spring strengths.”

10. Light and Transparent Materials • The energy of vibrating electrons is reemitted as transmitted light. • The (f) of the reemitted light = (f) incident light • A slight time delay occurs between absorption and reemission. • There is lower average speed of light through a transparent material.

11. Light and Transparent Materials The Secret of the Archer Fish • Light travels at different average speeds through different materials. • Light travels slightly more slowly in water than in the atmosphere, but its speed there is usually rounded off to c. • When light emerges from these materials into the air, it travels at its original speed, c. • Glass is transparent to visible light, but not to ultraviolet and infrared light. Light is not changed because of the perpendicular

12. Opaque Materials • Most materials absorb light without reemission and thus allow no light through them; they are opaque. • In opaque materials, any coordinated vibrations given by light to the atoms and molecules are turned into random kinetic energy—that is, heat

13. Why are metals Shiny? • Metals are also opaque. • When light shines on metal and sets these free electrons into vibration, their energy does not “spring” from atom to atom in the material, but is reemitted as visible light. • This reemitted light is seen as a reflection and that is why metals are shiny.

14. Opaque Materials • Our atmosphere is transparent to visible light and some infrared, but almost opaque to high-frequency ultraviolet waves. • The small amount of ultraviolet that does get through is responsible for sunburns.

15. Polarization Unpolarized light A light wave which is vibrating in more than one plane Polarized light are light waves in which the vibrations occur in a single plane EX: Light emitted by the sun, by a lamp in the classroom, or by a candle flame

16. Polarization • Polarization explains why light waves are transverse and not longitudinal • A single vibrating electron emits an electromagnetic wave that is polarized. • A vertically vibrating electron emits light that is vertically polarized, • A horizontally vibrating electron emits light that is horizontally polarized.

17. Polarization

18. More Polarization

19. Polarization Quiz What is the result of shining light through two polarizing filters whose transmission axes are parallel to each other The first filter will polarize the light, blocking half of its vibrations. The second filter will have no effect. Which of the three pairs of sunglasses to the right would be best for blocking road glare? A. The light coming from the road will have a slight vertical polarization

20. Lumination • Luminous objects • are objects which generate their own light

21. Illumination • Illuminated objects • are objects which are capable of reflecting light to our eyes • None of us are light-generating objects • It is only by reflection that we, see

22. Reflection • Reflection: when a wave reaches a boundary, some or all of it bounces back into the first medium • Totally Reflected waves are reflected back rather than transmitted into another object • Partially Reflected some energy is transmitted into a new medium

23. Reflection • Luminous objects emit light in a variety of directions • your eye only sees the very small portion of rays coming towards it • Ray = a narrow beam of light

24. The Law of Reflection • angle of incidence = The angle made by the incident ray and the normal • angle of reflection = The angle made by the reflected ray and the normal • Normal = imaginary line drawn perpendicular to the surface

25. The Law of Reflection • angle of incidence = angle of reflection

26. The Law of Reflection • Reflected waves travel back to the medium from whence they came • Incident rays and reflected rays make equal angles with the normal

27. The Law of Reflection

28. Do Now Problems 1. Differentiate between the angle of incidence and the angle of reflection 2. What is meant by the normal to the surface? 3. What is the law of reflection .

29. Pop Quiz 1.  Consider the diagram at the right. Which one of the angles (A, B, C, or D) is the angle of incidence? Which one of the angles is the angle of reflection? 2. A ray of light is incident towards a plane mirror at an angle of 30-degrees with the mirror surface. What will be the angle of reflection? 60 degrees. Note the angle of incidence is not 30 degrees!

30. The Law of Reflection is Always Observed (regardless of the orientation of the surface) Mirrors • light reflects off surfaces in a very predictable manner - law of reflection • Rays of light are reflected from mirrors in all directions

31. Mirrors • Virtual images are images which are formed in locations where light does not actually reach • Virtual Image are formed through reflection that can be seen by an observer but can not be projected on a screen • light from the object does not actually come to a focus

32. Mirrors • Your eyes cannot differentiate between a real image and a virtual one Your image in a plane mirror is? ALWAYS VIRTUAL !!!

33. Refraction of Light • The bending of the path of light • Refractionoccurs as light passes across the boundarybetween two medium • A synonym for refraction is "bending”

34. Refraction • The change in speed that occurs at an interface or boundarybetween two different media

35. Refraction of Light • The speed of a light wave is dependent upon the optical density of the material through which it moves

36. Refraction • The optical density of a material relates to the tendency of the atoms of a material to maintain the absorbed energy of an electromagnetic wave • One indicator of the optical density of a material is the index of refraction value

37. Refraction of Light • When light rays enter a medium in which their speed increases (less dense) the rays bendaway from the normal.

38. SFA = Slow to Fast, Away From Normal If a ray of light passes across the boundary from a material in which it travels slowly into a material in which travels faster, then the light ray will bend away from the normal line

39. Refraction of Light • When light rays enter a medium in which their speed decreases (more dense) the rays bendtoward the normal

40. FST = Fast to Slow, Towards Normal If a ray of light passes across the boundary from a material in which it travels fast into a material in which travels slower, then the light ray will bend towards the normal

41. Refraction of Light • How does refraction causes the bottom of a clear lake or pool to appear? CLOSER !!!!

42. Refraction of Light • Which medium is more dense?

43. Refraction of Light • This is caused by the change in speed of light as it passes from one medium to another How many times is the light refracted in the picture?

44. Refraction of Light • The wave speed is always greatest in the least dense medium • The wavelength is always greatest in the least dense medium

45. Refraction of Light • Thefrequencyof a wave is not altered by crossing a boundary • However, both the wave speed and the wavelength are changed

46. Refraction • INDEX OF REFRACTION (n) • Defined as the ratio of the speed of light in vacuum to the speed of light in the medium • index of refraction : n = c / v • The speed of light in a given material is related to this quantity Every substance has its own specific index of refraction. The next slide has a few examples

47. Material Index of Refraction Vacuum 1.0000 <--lowest optical density Air 1.0003 Ice 1.31 Water 1.333 Ethyl Alcohol 1.36 Plexiglas 1.51 Crown Glass 1.52 Light Flint Glass 1.58 Dense Flint Glass 1.66 Zircon 1.923 Diamond 2.417 Rutile 2.907 Gallium phosphide 3.50 <--highest optical density

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