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# WAVES LIGHT - PowerPoint PPT Presentation

UNIT 5 WAVES & LIGHT What is a wave? A wave is a disturbance that transmits energy through matter or space. A medium is the matter through which a wave travels. Waves carry energy because they do work. The bigger the wave, the more energy it carries.

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### WAVES & LIGHT

• A wave is a disturbance that transmits energy through matter or space.

• A medium is the matter through which a wave travels.

• Waves carry energy because they do work.

• The bigger the wave, the more energy it carries.

• Energy from waves spread out as the wave travels.

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Supplies –small bowl of water, pencil, timer, towel

• Place a bowl of water on a towel. Using the back of your pencil, gently tap the top of the water in the center of the bowl once.

• Allow the water to stop moving. Tap the water harder than the first time. What happened? What part of the wave changed when you used more energy?

• When the water stops moving tap it with your pencil once every 5 seconds. What happens? The frequency of a wave is a measure of how many waves pass a given spot in a certain amount of time.

• When the water stops moving try to increase the frequency by tapping the water once every second. What happens?

Try this: Ocean waves seem bigger on windy days because they are generated by wind blowing across the sea surface. Try to make waves by blowing across your bowl of water. Add a rock to the pan and see how waves bend (refract) around objects that block their path and interfere with each other as they move back and forth.

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• Waves are related to vibrations.

• Most waves are caused by a vibrating object.

• Vibrations involve transformations of energy.

• Termites choose food by vibrations.

• Tacoma Narrows Bridge collapse

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• Waves, such as water or sound waves, are a periodic disturbance of the medium through which they travel.

• Most of the information we collect through our senses travels in the form of a wave. Sound travels to our ears in a longitudinal wave. Light travels to our eyes in an electromagnetic wave. Signals that reach our televisions and radios travel in the form of electromagnetic waves, and are converted into picture or sound once reaching the device.

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• Examples of different waves

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• There are three types of waves:

• Mechanical waves require a material medium to travel (air, water, ropes). These waves are divided into three different types.

• Transverse waves cause the medium to move perpendicular to the direction of the wave.

• Longitudinal waves cause the medium to move parallel to the direction of the wave.

• Surface waves are both transverse waves and longitudinal waves mixed in one medium.

• Electromagnetic waves do not require a medium to travel (light, radio).

• Matter waves are produced by electrons and particles.

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• Travel without a medium

• Consist of changing electric and magnetic fields in space

• Light waves –travel at the speed of light

• Stars emit all of the electromagnetic waves

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• Radio waves, television waves, and microwaves are all types of electromagnetic waves. They differ from each other in wavelength. Velocity is the same.

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• Radio waves –can’t hear, all around

• AM –amplitude modulated, kilohertz

• FM –frequency modulated, megahertz

• Microwaves –high frequency radio waves

• Communication

• Cooking food by matching the vibrations of H2O and creating constructive interference. Transfers energy as heat.

• Infrared waves –heat

• Humans emit and can sense with skin receptors

• Used by mosquitos to locate prey

• Visible light –part of electromagnetic spectrum we see

• White light is divided into the colors by wave length

• Ultraviolet waves –emitted by the sun

• Causes cataracts and vision damage, kills skin cells

• Used to sterilize medical equipment

• Breaks down paint and plastics

• X-rays –denser atoms absorb more of these rays

• Bones are denser than organs so they show up when special film is used

• Can cause DNA shifts in sex cells and can lead to birth defects – this is why x-rays are not usually taken of babies or the reproductive cell of people

• Gamma rays –emitted by radioactive decay

• Pass through the human body, are stopped by several feet of lead or concrete

• Can cause DNA mutations, kills cancer cells, sterilize insects for biological control

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More About the Uses of Electromagnetic Waves

• Microwaves –atomic and molecular research; microwave ovens

• Infrared waves (IR)–infrared photography; optical microscopes and telescopes

• Visible light–visible-light photography; optical microscopes and telescopes

• Ultraviolet light (UV)–sterilizing medical instruments; identifying florescent minerals

• X-rays –medical examination of bones, teeth, and organs; cancer treatment

• Gamma Rays–food irradiation; studies of structural flaws in thick materials

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• Example to try

• Try changing these waves

• Experiment with a slinky and a friend. Each of you hold one end of the slinky and stretch it. Shake the slinky up and down. This makes a mechanical transverse wave.

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• Particles in a medium can vibrate either up and down or back and forth.

• Waves are classified by the direction that the particles in a medium move as the wave passes by.

• Transverse –wave causes particles in the medium to vibrate perpendicularly to the direction of the wave

• Longitudinal –wave causes particles in the medium to vibrate parallel to the direction of the wave

• Surface wave –occur at the boundary between two different mediums. Particles move both perpendicularly and parallel to the direction the wave travels (circular).

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• For longitudinal waves, the medium is displaced in the direction of travel.

• For example, air is compressed and expanded in the same direction that a sound wave travels.

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• For transverse waves, the medium is displaced perpendicular to the direction of travel. Ripples on the surface of a pond are an example of a transverse wave: the water is displaced vertically while the wave itself travels horizontally.

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• Occur at the boundary between two different mediums. Particles move both perpendicularly and parallel to the direction the wave travels (circular).copied from 3 slides earlier change

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• Medium and trough

• Crest

• Wavelength

• Amplitude

• Period

• Frequency and wave speed

• Wave interactions

• Reflection, refraction, diffraction

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• The shortest distance between peaks, the highest points, and troughs, the lowest points, is the wavelength.

• Distance measurement, SI unit –m.

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• The shortest time that a point takes to return to the initial position (one vibration) is called period, T.

• It is a time measurement and is expressed in SI unit of seconds (s).

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• The number of vibrations per second is called frequency (f) and is measured in hertz (Hz). Here's the equation for frequency:f = 1 / T

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• The amplitude of a wave is the distance from a crest to where the wave is at equilibrium. The amplitude is used to measure the energy transferred by the wave. The bigger the distance, the greater the energy transferred.

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• By knowing the frequency of a wave and its wavelength, we can find its velocity. Here is the equation for the velocity of a wave:

• Wave speed (v) m/s = frequency (Hz) 1/s x wavelength ( ) m

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• However, the velocity of a wave is only affected by the properties of the medium. It is not possible to increase the speed of a wave by increasing its wavelength. By doing this, the number of vibrations per second decreases and therefore the velocity remains the same.

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• The arrangement of particles in a medium determines how well waves travel through it.

• Gas (Air) – waves don’t travel fast because the molecules are far apart in a gas and move about randomly.

• Liquid (Water) – molecules are closer together so vibrations are transferred quickly from one molecule to another.

• Solid – vibrations travel very quickly because the molecules in a solid are tightly bound to each other so as one starts to vibrate all the others start to vibrate.

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• In a given medium the speed of waves remains constant.

• Frequency can change.

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• When waves move through a medium or space they move in:

• Straight lines –waves in an ocean

• Spread out in circles –ripples on a pond

• Spread out in spheres –sound waves in air

• When a wave meets another object, another wave or when a wave passes into another medium, something happens.

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• Reflection is the bouncing back of a wave as it meets a surface or boundary

• At a free boundary the reflected wave is exactly like the original wave

• At fixed boundary the reflected wave is like the original wave, but it is turned upside down and travels in the opposite direction.

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• Diffraction is the bending of a wave as it passes an edge or an opening.

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• Refraction is the bending of waves caused by a change in speed when they move from one medium to another.

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• Interference is the combination of two or more waves that exist in the same place at the same time.

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Interference of Light Waves

• Interference of light waves produce colorful displays.

• Ex: The iridescent colors of soap bubbles are caused by interfering light waves and are determined by the thickness of the film. They are not the same as rainbow colors but are the same as the colors in an oil slick on a wet road.

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• Constructive –crest of one wave overlaps crest of another and they are reinforced = new wave whose amplitude is the sum of the amplitudes of the two individual waves.

• Destructive –crest of one wave meets the trough of another = new wave has an amplitude that is the difference of the amplitude of the individual waves.

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• Don’t move through medium.

• Caused by interference.

• Cause the medium to vibrate in a loop or in a series of loops.

• Shows some regions of no vibration (nodes) and other regions of maximum vibration (antinodes).

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160 mL dishwashing soap

1 gallon water

35 mL glycerin

100 g sugar

40 mL salt

1.4 L water (distilled is better)

150 mL dish washing detergent

12 mL glycerin

1 part of washing-up detergent

2 parts of glycerin

3 parts of water

For long living bubbles:

1 part commercial bubble solution

1 part water

1 part cup glycerin

For no-tears soap bubbles:

60 mL baby shampoo

200 mL water

45 mL corn syrup

Soap bubbler

WAVES & LIGHT

• Consists of longitudinal waves

• Spreads in all directions away from the source.

• Travel faster through liquids and solids than gases.

• Echoes are reflected sound waves

• Explanation

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• Sound is determined by:

• Frequency

• Pitch –how high or low sound is determined by the frequency at which the sound waves strike the eardrum in your ear.

• Amplitude

• Intensity –determines loudness.

Picture of a sound wave

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• Loudness is a logarithmic scale

• The relative intensity of sounds is found by comparing the intensity of the quietest sound a person can hear; the threshold of hearing. Measured in decibels (dB).

• 0 dB is the quietest sound humans can hear

• 30 dB is a whisper

• 50 dB is a normal conversational level

• 90 dB over a long period of time causes hearing loss

• 120 dB is at the threshold of pain

• 150 dB is the sound of a jet

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Picture of a jet breaking the sound barrier.

• Humans can hear sounds from 20 Hz to 20,000 Hz.

• Infrasound is any sound below the range of human hearing.

• Ultrasound is any sound above the range of human hearing.

• Works well with sonar. Sonar uses reflected sound waves to determine the distance to and location of objects. Used underwater.

• Used by doctors to view organs inside the body (sonograms).

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• Produces beats or a series of loud and soft sounds.

• Music

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• Rely on standing waves

• Harmonics give every instrument a unique sound –whole number multiples of natural frequencies

• Amplification creates resonance. Instruments use a resonance to amplify sound

• Resonance is an effect in which the vibration of one object causes the vibration of another object at a natural frequency.

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• The ear

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How the Ear Works

• Sound is collected by the visible part of the ear (pinna) and directed through the outer ear canal.

• The sound makes the eardrum vibrate, which in turn causes a series of three tiny bones (the hammer, the anvil, and the stirrup) in the middle ear to vibrate.

• The vibration is transferred to the snail-shaped cochlea on the inner ear ; the cochlea is lined with sensitive hairs which trigger the generation of nerve signals that are sent to the brain.

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• The eye and How we see color

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• Light enters the eye through the cornea (clear tissue). The cornea is responsible for 70% of the refraction of light in the eye.

• From the cornea, light passes through the pupil to the lens. The curvature of the lens determines how much further the light is refracted. Muscles adjust the curvature.

• The image is focused on the retina where the brain interprets the image.

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• What you see depends on the amount of light and the color of objects as they reflect or absorb light.

• Intensity is the brightness of light.

• There are three types of objects.

• Opaque –absorbs or reflects all light and can’t see through it.

• Transparent –light passes through it freely and you can see through it.

• Translucent –allows some light to pass through it but you can’t see clearly through it.

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• Modeled as an electromagnetic wave or a stream of particles called photons

• Energy of a photon is proportional to the frequency of the light wave

• Comes in a wide range of frequencies and wavelengths.

• Visible light – 4.3x1014Hz to 7.5x1014Hz

• Differences in the frequency in visible light account for the different colors we see

• Electromagnetic waves also exist at other frequencies we can’t see

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Light-Electromagnetic spectrum

• Full range of light at different frequencies and wavelengths.

• Explanation

• All electromagnetic waves travel at the speed of light in empty space 3x108m/s.

• Light is slowed when it travels through air or water.

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• Reflection occurs when a wave strikes an object and bounces off.

• Smooth surfaces reflect light in one direction.

• Rough surfaces scatter light.

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• Law of Reflection –the angle of incidence is equal to the angle of reflection.

• Angle of reflection –the angle the light is reflected off the surface

• Angle of incidence –the angle of the light striking the surface

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• Flat mirrors –angle of reflection = angle of incidence so you can see yourself (virtual image forms behind)

• Convex mirrors –rays come in parallel but are reflected out in different directions (bulges out) (real image appears in front)

• Concave mirrors –rays come in parallel but are reflected toward each other (curve in) (virtual image)

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• Refraction makes objects appear to be in different positions because light travels at different speeds in different mediums.

• Refraction in the atmosphere creates mirages because light travels at different speeds in air of different temperatures.

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• Refract light

• Converging lens – bends light inward

• Can create a virtual image or a real image depending on the distance from lens to object

• Diverging lens –bends light outward

• Can only create a virtual image

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• Dispersion is an effect in which white light separates into its component colors.

• A prism causes dispersion. It is a transparent block with a triangular cross section.

• Rainbows are caused by dispersion and internal refraction.

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• Objects reflect the color you can see and absorb all others.

• Longest wavelength, lowest frequency, red

• Shortest wavelength, highest frequency, violet

• Primary colors of light are red, blue, and green.

• You mix them together and get white.

• Primary pigments are magenta, cyan, and yellow.

• You mix them together and get black.

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• Additive primary colors are red, green, and blue.

• Combining the light from any two of these produces secondary colors of magenta, cyan and yellow.

• Combining the light from all three primary colors makes white.

• Combining the light from all three secondary colors makes black.

• Experiment mixing light colors

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• Primary Colors of Pigment

• Primary Colors of Light

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• Materials:unflavored gelatinwaterfood coloringpipettesMix many packets of gelatin with half of gelatin manufacturer recommend amount of water. Cook according to manufacturers directions. Chill in a deep bowl - the more depth and gelatin, the better. After it has set, for at least 12 hours, release from bowl, place upside down on a cookie sheet - it should look like a hill. Fill pipettes with food coloring. Insert pipettes into gelatin mold and squeeze out the coloring. The food coloring can squirt out of the mold so be careful. The gelatin can be reheated and molded again.

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• The change in observed frequency of a wave resulting from the motion of the source.

• Occurs for light and sound waves.

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• Wave review page

• Good review

• Waves

• Traffic waves

• School site –look at links

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