1. Waves 2
Sound and Light

2. Describing Sound Waves
Sound is a mechanical (longitudinal) wave.
A ringing bell is in a bell jar that has been evacuated with a vacuum pump-why can’t you hear the bell?
Mechanical waves require a medium-they can’t travel through a vacuum.

3. Sound waves are longitudinal.
Often drawn as a pressure wave.
Don’t confuse the pressure-time graph with a transverse wave - sound is a longitudinal wave.

4. How do ears detect sound?
Ears detect fluctuations in air pressure
Sound waves travel fastest in solids - particles are closer together
The human ear can hear 20 Hz to 20,000 Hz frequencies

5. Examples: tuning fork, opera singer and a glass, drum in band room
What is resonance?
One object vibrating at the same natural frequency of a second object forces the second object to vibrate
Examples: tuning fork, opera singer and a glass, drum in band room

6. How can the Doppler Effect be used to explain shock waves?
Source moves at same speed or faster than the wave (speed of sound)
When the source of the sound moves faster than the sound, the wave fronts bunch up and form a cone
A sonic boom occurs when the wave fronts compress and pile up along the edge of the cone

8. What can happen to waves at boundaries?
Reflection
Transmitted to new medium, some may be absorbed.
Refraction

9. What is an echo?
Reflected sound wave reaches ear more than 0.1 seconds after original sound

10. How long will it take for an echo to be heard in a 1200 meter well?

11. What is diffraction? How does it influence sound waves?
Spreading out of waves as they pass through an opening or around a barrier
You can hear someone shouting in the hallway even if they aren’t right in front of the door
Long wavelength owl sounds diffract around trees, while high tweets of small birds don’t diffract much

12. What is light? Light is an electromagnetic disturbance
Accelerating charged particles (electrons) move to a lower energy level and emit energy
Remember…no medium is necessary
Luminated objects generate their own light, while illuminated objects reflect light to our eyes from other sources

13. What is the range of frequencies for visible light in a vacuum?
See reference table
3.84 x1014 Hz (red) to 7.69 x1014 Hz (violet)
Violet - _________  __________ f
Red - _________  __________ f
What is ROY G BV?
Colors of visible light
small
high
large
low

14. How can we represent how light travels?
Travels in a straight line
Represented by rays (straight lines with arrows used to show path of a narrow beam of light)
What is the speed of light in space (vacuum)?
C = 3.00 x 108 m/s = f

15. In a medium other than a vacuum, how are v and c related?
v < c
Remember - v depends on the nature of the medium
Velocity of light in air and in a vacuum are the same within three significant digits….use the same value

16. What are dispersive materials, and how are they different from nondispersive materials?
Some materials allow waves of different frequencies to pass through at different speeds - these are dispersive
Colors are separated, violet is refracted the most

17. What is refraction?
Refraction is the change in direction of light, and it occurs when the wave passes into a new medium. The wave changes speed (and wavelength). The amount the speed changes depends on the medium.

19. Water has a greater optical density, so light slows down
Diagram:
v = c
v < c
i = incident angle
r = refracted angle
Water has a greater optical density, so light slows down

20. What happens when…… speed DECREASES in the new medium?
(from less optically dense to more optically dense medium)
Light bends toward the normal (i > r)
Speed INCREASES in the new medium?
(from more optically dense to less optically dense medium)
Light bends away from the normal (i < r)

21. What happens when…… The incident ray is along the normal?
Speed changes, but the wave doesn’t bend (i = r = 0)
Light must enter a medium obliquely to bend (at an angle, NOT along the normal)

23. Absolute Index of Refraction (Refractive Index)
Measure of optical density
Speed in a new medium depends on its optical density
Found in reference table
n = index of refraction
c = speed of light in a vacuum (or air)
v = speed of light in medium

24. Example: What is the speed of light in diamond?

25. Snell’s Law
Equation:

26. Example: Light in water is incident on diamond
Example: Light in water is incident on diamond. The angle of incidence is 45°. What is the angle of refraction?
n1 = 1 = 45°
n2 = 2 = ?

27. Which has a higher index of refraction?
As n increases (more optically dense), light bends more and the refracted angle decreases

28. Critical Angle
Critical angle is the angle of incidence where the angle of refraction is 90°
The refractive medium must have a lower refractive index than the incident medium (light must speed up in the refractive medium).
When critical angle is exceeded you get “total internal reflection” - light doesn’t leave the medium

29. Light blue - refraction
Red - critical angle
Dark blue - total internal reflection

30. Example: Find the critical angle of light in crown glass if the refracted medium is air.

31. Total Internal Reflection

33. Law of Reflection

35. How can we describe images in a plane mirror?
Image is:
the same distance behind the mirror
the same size
erect (upright)
laterally reversed

36. Types of reflection:
Regular reflection - polished surfaces - also called specular reflection
All rays reflected back are parallel
Examples: mirror, pool of water

37. Diffuse reflection - scattering of light - caused by reflection from irregular surfaces - normals are not parallel
Examples: paper, blue sky

38. Wave Nature of Light

39. DIP Diffraction Interference Polarization Light as a wave
Christian Huygens - explained light as a series of wave fronts whose direction could be represented by a light ray
3 pieces of evidence light behaves as a wave:
Diffraction
Interference
Polarization
Remember with the word….
DIP

40. What is polarization?
Evidence that light is a transverse wave rather than a longitudinal one
Separates light beams by plane of vibration

42. Why is the intensity of light reduced when it passes through polarizing material?
Waves not in the plane of vibration of the polarizing material do not pass through.
How can two polarizing filters be used so that no light will pass through?
Place the filters perpendicular to each other.

43. What is diffraction?
Diffraction is the spreading of waves in all directions behind an obstacle, shown below as light passes through a narrow slit.

44. Slit smaller than wavelength:

45. Slit larger than wavelength

47. Diffraction around a barrier

49. Young’s Double Slit Apparatus
When light passes through a double slit arrangement, what happens when a wave crest meets a wave crest on the screen?
Crest meets crest - constructive interference - bright areas
Crest meets trough - destructive interference - dark areas

51. What happens when more slits are added?
More bands of interference
What is a monochromatic light source?
Light of only one wavelength is emitted (one color)

52. Coherent coherent not coherent
Sources that produce waves with a constant phase relationship are …..
Coherent
coherent
not coherent
Coherent waves are “in step” with each other

53. Lasers
Lasers produce a beam of light with a very narrow band of frequencies in which all the waves remain in phase for a longer time than ordinary sources

54. What are absorption spectra, and how are they different from line spectra?
Absorption spectra - certain wavelengths are absorbed by certain materials
Low temperature gases absorb certain wavelengths
Line (emission) spectra - certain wavelengths are emitted
Produced by high temperature gases at low pressure