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Example of Ultrasonic sound waves
• 1 to 15 million Hz• Passes through most
materials, but reflects at medium boundaries
• Not harmful like x-rays
Used more frequently for other purposes
SONAR
• Sound, Navigation, and RAnging
• Uses acoustic signals and echo returns to locate objects
• Ultrasound waves generally used
Light can be modeled as a wave
• Transverse waves
• Electromagnetic wave (no medium, consist of changing electric and magnetic fields)
• Reflection, refraction, or diffraction
• Interference, produce standing waves
Wave model can not explain some observations
• Some frequencies of light can knock off electrons from a metal plate upon impact
• Some light waves with more energy can not.
• Dim Blue light does, Bright red does not
Light can be modeled as a stream of “particles”
• These particles are called photons
• Do not have mass
• Packets of energy
• Unlike a wave, located in a particular place
• Also can be used with reflection, refraction
Scientist have not determined if light is only a wave or particle
They currently accept either models, depending on the
situation
Energy and Frequency
• The energy of light is proportional to frequency
• Higher frequency more energy
• Blue light has more energy than red light
Speed of light
• Depends on medium
• C = speed of light = 3 x 108 m/s in nothing
• Slows down as the density of material increase
• Visualized by the bending of light (refraction)
Brightness of light
• measured by objects Luminosity
• # of photons or wave-fronts passing through a certain area of space
• Luminosity decreases proportionally with distance
Homework on page 498
Question 1: Identify 2 factors that may affect the speed of sound.
Answer: type of medium, temperature
Homework
Question 2: Explain why sound travels faster in water than air
Answer: In air, molecules are far apart, collide less often
Homework
Question #3: Distinguish between infrasound and ultrasound
Answer: Infrasound are sound waves with a frequency under 20 Hz. Ultrasound are sound waves greater than 20,000 hz.
Homework
Question #4: Determine which of the following must change when the pitch gets higher
Answer: Frequency increases, wavelength decreases
Homework
Question #5, Determine which of the following must change when the sound gets louder
Answer: a,d Amplitude and intensity increase because there is more energy in the sound wave
Homework
Question #9: Why are sonograms made with ultrasound waves instead of audible sound waves?
Answer: Ultrasound waves pass easily through tissue, audible sound waves do not
Quiz question
• Determine which of the following must change when the pitch gets higher?
a) intensity
b) wavelength
c) speed of the wave
d) amplitude
e) frequency
From Friday
• Evidence that light travels as a series of waves
• Evidence that light travels as a stream of particles
• What is a photon?
• How does the amount of energy carried by a light wave relate to its frequency?
Connections between light and sound
• Both light and sound can vary in intensity
light-brightness
sound-loudness
• Both can vary in speed by change of medium– Light slows down with more dense mediums– Sound speed up with more dense mediums
Goals for Today
• Discuss the electromagnetic spectrum in more detail
• Determine why you see things as a certain color
Electromagnetic Spectrum
• Visible light make up just a small part of the range of light waves, just like audible sound.
• Electromagnetic spectrum consists of light at all possible energies, frequencies and wavelengths
Similarities between different parts of the spectrum
• All move at the same speed through a given material
• All do not need a medium to move
• All are transverse waves
Differences in the spectrum
• Different waves have different wavelengths and frequencies
• Different frequencies carry different amount of energy
• Larger frequency, larger amount of energy
Regions of the spectrum
• Gamma and x-rays have the largest frequency (therefore, smallest wavelength)
• Small enough to pass through most parts of body without meeting anything
• In x-rays, those that hit bone are absorbed, and do not hit the photographic plate (dark and light parts)
The good and bad of gamma and x-rays
• High energy
• Interaction may kill living cells or alter them to produce cancerous cells
• Can also be used to kill cancerous cells
a form of “ radiation therapy”
Ultraviolet light
• Smaller frequencies than x-rays but more than visible light
• This is the portion of light spectrum that causes sunburn, even on cloudy days
Infrared light
• Frequencies of light slightly smaller than visible light
• All matter warmer than absolute zero gives out infrared or “radiant heat”
• Can be sensed by some living beings
• Infrared cameras, heat seeking missiles
Microwaves and Radio waves
• Microwave, Radio and TV waves are the largest in wavelength, smallest in frequency
• Boundary between types is arbitrary
Microwaves
• Microwaves are
reflected by metals
pass thru air, glass, paper plastic
absorbed by fat, water and sugar
Energy of wave passes into food, makes molecules vibrate- heat up
Colors that you see
• Different wavelengths (and frequencies) correspond to different colors of light
• An object that reflects a certain wavelength of light has that color
• If multiple wavelengths of light are reflected, secondary colors are created
• For light
• If all visible wavelengths of light are combined in sunlight
• Humans detect yellow-green the easiest
• Black is the absence of reflected light
• Mixing all three primary colors produces white light