1. Chapter 18 The Electromagnetic Spectrum and Light
18.1 Electromagnetic Waves
18.2 The Electromagnetic Spectrum
18.3 Behavior of Light

2. 18.1 Electromagnetic Waves
What are Electromagnetic Waves?
Def.-transverse waves consisting of changing electric fields and changing magnetic fields
Differ from mechanical waves in how they are produced and how they travel

3. 18.1 Electromagnetic Waves
What are Electromagnetic Waves?: How They Are Produced
Produced by constantly changing fields
Electric field-a field in a region of space that exerts electric forces on charged particles
Electric fields are produced by electrically charged particles and by changing magnetic fields

4. 18.1 Electromagnetic Waves
What are Electromagnetic Waves?: How They Are Produced
Magnetic Field-a field in a region of space that exerts magnetic forces
Magnetic fields are produced by magnets, changing electric fields, and by vibrating charges
Key Concept: Electromagnetic waves are produced when an electric charge vibrates or accelerates.

5. 18.1 Electromagnetic Waves
What are Electromagnetic Waves?: How They Travel
Changing electric fields create changing magnetic fields and vice versa.
They regenerate each other; as they regenerate, their energy travels in the form of a wave
Electromagnetic waves do not need a medium (mechanical waves do)

6. Electromagnetic Waves
Figure 2
Electromagnetic Waves

7. 18.1 Electromagnetic Waves
What are Electromagnetic Waves?: How They Travel
Key Concept: Electromagnetic waves can travel through a vacuum, or empty space, as well as through matter.
Electromagnetic radiation-the transfer of energy by electromagnetic waves traveling through matter or across a space

8. 18.1 Electromagnetic Waves
The Speed of Electromagnetic Waves: Michelson’s Experiment and The Speed of Light
Light travels faster than sound (lightening occurs before thunder)
Albert Michelson ( )-physicist-calculated the speed of light

9. 18.1 Electromagnetic Waves
The Speed of Electromagnetic Waves: Michelson’s Experiment
8-sided rotating mirror on one mountain; 1 mirror on another mountain opposite of 1st mountain
Shined light at one face of the rotating mirror; light was reflected to stationary mirror on opposite mountain.
Knowing how fast mirror rotated and how far the light traveled from mountain to mountain and back again=speed of light calculated

10. 18.1 Electromagnetic Waves
The Speed of Electromagnetic Waves: Michelson’s Experiment and The Speed of Light
Light and all electromagnetic waves travel at the same speed when in a vacuum regardless of observer’s motion
Key Concept: The speed of light in a vacuum, c, is 3.00 X 108 m/s

11. 18.1 Electromagnetic Waves
Wavelength and Frequency
Key Concept: Electromagnetic waves vary in wavelength and frequency.
Wavelength is inversely proportional to the frequency b/c the speed of electromagnetic waves in a vacuum is constant.
Speed=wavelength x frequency

12. Section 18.1

13. 18.1 Electromagnetic Waves
Wave or Particle?
Electromagnetic radiation can also behave as a stream of particles.
Newton was first to propose a particle explanation.
Evidence: light travels in a straight line and it casts as shadow

14. 18.1 Electromagnetic Waves
Wave or Particle?
Is light a wave or a particle? It is both.
Key Concept: Electromagnetic radiation behaves sometimes like a wave and sometimes like a stream of particles.

15. 18.1 Electromagnetic Waves
Wave or Particle?: Evidence for the Wave Model
Thomas Young-physicist-showed that light behaves like a wave.
Passed beam of light through a single slit then a double slit (bright and dark bands)
Bands showed that light had produced an interference pattern (bright=constructive; dark=destructive)

16. Interference (Wave Model of Light)
Figure 5
Interference (Wave Model of Light)

17. 18.1 Electromagnetic Waves
Wave or Particle? Evidence for the Particle Model
(Einstein) Blue light causes electrons to be emitted; red light does not
Photoelectric effect-the emission of electrons from a metal caused by light striking the metal
Einstein proposed that light and all electromagnetic radiation consists of packets of energy (photons)

18. 18.1 Electromagnetic Waves
Wave or Particle?: Evidence for the Particle Model
The greater the frequency of and electromagnetic wave, the more energy each of its photons has.
Blue light=higher frequency (photons have more energy so electrons emitted); red light=lower frequency (photons have less energy so no emission of electrons)

19. Photoelectric Effect (Particle Model of Light)
Figure 6
Photoelectric Effect (Particle Model of Light)

20. 18.1 Electromagnetic Waves
Intensity
The closer to light you are, the brighter it is.
The further away you are, the dimmer the light.
Photons from light travel outward in all directions.
In a small area (near the light) light is more intense.

21. 18.1 Electromagnetic Waves
Intensity
Def.-the rate at which a wave’s energy flows through a given unit (ie. brightness)
Key Concept: The intensity of light decreases as photons travel farther from the source.

22. 18.2 The Electromagnetic Spectrum
The Waves of the Spectrum
William Herschel (1800)-used a prism to separate the wavelengths present in sunlight
Colors: red, orange, yellow, green, blue, and violet
Each color’s temperature is different; temp. is higher in area beyond red color band (no color present); there is invisible radiation beyond the red end of the color band (infrared radiation)

23. 18.2 The Electromagnetic Spectrum
The Waves of the Spectrum
Electromagnetic spectrum-the full range of frequencies of electromagnetic radiation
Key Concept: The electromagnetic spectrum includes radio waves, infrared rays, visible light, ultraviolet rays, X-rays, and gamma rays.

24. The Electromagnetic Spectrum
Figure 9
The Electromagnetic Spectrum

25. 18.2 The Electromagnetic Spectrum
Radio Waves
Have the longest wavelengths in the spectrum; have lowest frequency
Key Concept: Radio waves are used in radio and television technologies, as well as in microwave ovens and radar.

26. 18.2 The Electromagnetic Spectrum
Radio Waves: Radio
Two ways radio stations code and transmit information on radio waves
Both are based on a wave of constant frequency and amplitude. Pg. 541
Amplified modulation (AM)-the amplitude of the wave is varied; frequency remains the same

27. 18.2 The Electromagnetic Spectrum
Radio Waves: Radio
Frequency modulation (FM)-the frequency of the wave is varied; the amplitude remains the same
AM signals travel farther than FM signals

28. 18.2 The Electromagnetic Spectrum
Radio Waves: Television
Radio waves carry signals for TV programming.
The process is like transmitting radio signals BUT radio waves carry information for pictures as well as for sound

29. 18.2 The Electromagnetic Spectrum
Radio Waves: Microwaves
Def.-the shortest-wavelength radio waves
Cook and reheat food (water or fat molecules absorb microwaves=increase in thermal energy of the molecules)
Also carry cell phone conversations.

30. 18.2 The Electromagnetic Spectrum
Radio Waves: Radar
Radar technology uses a radio transmitter to send out short bursts of radio waves
Waves reflect off objects encountered, bounce back to where they came from (picked up and interpreted by radio receiver)

31. 18.2 The Electromagnetic Spectrum
Infrared Rays
Have higher frequencies than radio waves; lower frequencies than red light
Key Concept: Infrared rays are used as a source of heat and to discover areas of heat differences.
Can’t be seen; can be felt (warmth)
Thermograms-color-coded pictures that show variations in temperature

32. 18.2 The Electromagnetic Spectrum
Visible Light
The visible part of the electromagnetic spectrum
Key Concept: People use visible light to see, to help keep them safe, and to communicate with one another.

33. Figure 13

34. 18.2 The Electromagnetic Spectrum
Ultraviolet Rays
Ultraviolet radiation has higher frequencies than violet light.
Key Concept: Ultraviolet rays have applications in health and medicine, and in agriculture.
Help skin produce vitamin D; kill microorganisms; help plants grow
Excessive exposure: sunburn, wrinkles, and eventually skin cancer

35. 18.2 The Electromagnetic Spectrum
X-Rays
Have very short wavelengths; have higher frequencies than UV rays
Key Concept: X-rays are used in medicine, industry, and transportation to make pictures of the inside of solid objects.
Teeth and bones absorb x-rays (white); tissue (dark); too much exposure can kill or damage living tissue

36. 18.2 The Electromagnetic Spectrum
Gamma Rays
Have the shortest wavelengths in the electromagnetic spectrum; have the highest frequencies
Have the most energy and greatest penetrating ability of all the electromagnetic waves

37. 18.2 The Electromagnetic Spectrum
Gamma Rays
Overexposure to gamma rays can be deadly.
Key Concept: Gamma rays are used in the medical field to kill cancer cells and make pictures of the brain, and in industrial situations as an inspection tool.

38. 18.3 Behavior of Light Light and Materials
Nothing is visible without light.
How light behaves when it strikes an object depends on many factors (this includes the material the object is made of)
Key Concept: Materials can be transparent, translucent, or opaque.

39. 18.3 Behavior of Light Light and Materials
Transparent-material transmits light (allows most of the light that strikes it to pass through)
Ex. Windows, water
Translucent-material scatters light (can see through the material, but objects seen through it don’t look clear or distinct)
Ex. Frosted glass, some soaps

40. 18.3 Behavior of Light Light and Materials Most materials are opaque.
Opaque-material either absorbs or reflects all of the light that strikes it
Does not allow any light to pass through
Ex. Wood, metal

41. 18.3 Behavior of Light Interactions of Light
When light encounters matter, some or all of the energy in the light can be transferred to the matter.
Light can affect matter; matter can affect light.
Key Concept: When light strikes a new medium, the light can be reflected, absorbed, or transmitted. When light is transmitted, it can be refracted, polarized, or scattered.

42. Interactions of Light: Reflection
18.3 Behavior of Light
Interactions of Light: Reflection
Image-a copy of an object formed by reflected (or refracted) waves of light **[image in a mirror]
When light reflects from a smooth surface=clear, sharp image
When light reflects from a rough surface=blurred image or no image
Ex. Calm water vs. “moving” water

43. Interactions of Light: Reflection
18.3 Behavior of Light
Interactions of Light: Reflection
Regular reflection- occurs when parallel light waves strike a surface and reflect all in the same direction
Diffuse reflection-occurs when parallel light waves strikes a rough, uneven surface, and reflect in many different directions

44. Interactions of Light: Refraction
18.3 Behavior of Light
Interactions of Light: Refraction
A light wave can refract, or bend, when it passes at an angle from one medium into another.
Ex. Underwater objects appear closer and larger; Objects appear to break at the surface of water
Can cause a mirage-a false or distorted image
Mirage-occurs because light travels faster in hot air than in cooler denser air; refracts more as it enters hotter and hotter air=light follows curved path to the ground; light looks like it was reflected from water

45. Interactions of Light: Polarization
18.3 Behavior of Light
Interactions of Light: Polarization
Polarized light-light with waves that vibrate in only one plane
Polarizing filters transmit this type of light
Unpolarized light vibrates in all directions.
Vertical polarizing filter-stops light vibrating on a horizontal plane (polarized sunglasses block horizontal polarized light from the sun *glare b/c horizontal light reflects more strongly*)
Horizontal polarizing filter-blocks waves vibrating on a vertical plane

46. Figure 20
Polarization

47. Interactions of Light: Scattering
18.3 Behavior of Light
Interactions of Light: Scattering
Earth’s atmosphere has many molecules and other tiny particles= sunlight can be scattered
Def.-light is redirected as it passes through a medium
Small particles in the air scatter shorter-wavelength blue light more than light of longer wavelengths.

48. Interactions of Light: Scattering
18.3 Behavior of Light
Interactions of Light: Scattering
By the time sunlight reaches your eyes, most of the blue and some green and yellow have been scattered.
What’s left that we see? Longer wavelengths of light, orange and red (sunset/sunrise)
Why is the sky blue? The sun scatters blue light in all directions more than other colors of light.
Sky appears blue although air is colorless.