WAVES & EM SPECTRUM. Chapters 10 & 15

Transcription

1 WAVES & EM SPECTRUM Chapters 10 & 15

2 What s a wave? repeating disturbance transfers energy through matter or space Oscillation back & forth movement carries energy w/o transporting matter can travel through a medium not all waves need a medium: light & radio waves can travel through space

3 Mechanical Waves travel through matter 2 types of mechanical waves: transverse compressional

4 Transverse Waves matter in the medium moves back & forth at right angles to the direction the wave travels Ex. a water wave travels horizontally as the water moves vertically up & down.

5 Compressional Waves matter in the medium moves back & forth in the same direction the wave travels Energy, not matter, is carried forward Ex. Slinky

6 The Parts of a Transverse Wave high points are crests low points are troughs Midway point = equilibrium line

7 The Parts of a Compressional Wave a compression is a region where the coils are close together less-dense region is called a rarefaction

8 Wavelength distance between one point on a wave & the next point just like it crest to crest or trough to trough compression to compression or rarefaction to rarefaction

9 Frequency & Period Frequency (ϝ) = # of wavelengths that pass a fixed point each second unit = hertz (Hz) Period = amt. of time it takes one wavelength to pass a point units = seconds (s) As ϝ increases, λ decreases, & period decreases

10 Frequency & Energy How does the frequency of the wave determine the amount of energy transfer?

11 Period & Frequency The amount of time needed to complete one cycle is called a period

12 Wavelength is Related to Frequency speed depends on the medium Sound waves usually travel faster in liquids & solids than gases. Light waves travel more slowly in liquid & solids than in gases or in empty space Sound waves usually travel faster if the temperature of the material is increased

13 Calculating Wave Speed

14 Amplitude & Energy How does the amplitude of the wave determine the amount of energy transfer?

15 amplitude of a wave was an indicator of how much energy was transferred every second by the wave

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17 Amplitude of Compressional Waves how compact the medium is at compressions denser the compressions = larger amplitude & more energy

18 Reflection wave strikes an object & bounces off of it all types of waves can be reflected How does the reflection of light allow you to see yourself in the mirror? light strikes your face & bounces off light reflected off your face strikes the mirror & is reflected to your eyes

19 Echoes Sound waves form then travel through the air to your ears & other objects. Sometimes they hit an object & reflect off it & come back to you Your ears hear the sound again

20 The Law of Reflection beam striking the mirror = incident beam beam that bounces off the mirror = reflected beam line drawn perpendicular to the surface of the mirror is the normal

21 The Law of Reflection formed by the incident beam & the normal is the angle of incidence formed by the reflected beam & the normal is the angle of reflection incidence = reflection All reflected waves obey this law.

22 Refraction bending of a wave caused by a change in speed as changes media

23 Refraction of Light in Water Light waves slow down in water Causing them to bend toward the normal Light waves speed up in air Causing them to bend away from the normal

24 Diffraction the apparent bending of waves around small obstacles & the spreading out of waves past small openings

25 Diffraction and Wavelength Amt. of diffraction depends on size of the obstacle/opening compared to λ If an obstacle is smaller than the λ, the waves bend around it If the obstacle is larger than the λ, the waves do not diffract as much (or at all)

26 Interference waves overlap & & combine to form a new wave 2 kinds Constructive Destructive

27 Constructive Interference waves add together crests arrive at the same place at the same time & overlap Amplitude of the new wave = the sum of the amplitudes of the original waves

28 Destructive Interference waves subtract from each other as they overlap crests of a wave meet the troughs of another Amplitude of the new wave is the difference of the amplitudes of the overlapping waves

29 Standing Waves form when waves equal in wavelength & amplitude traveling in opposite directions continuously interfere with each other The places where the two waves always cancel are called nodes Produced in musical instruments

30 Resonance process by which an object is made to vibrate by absorbing energy at its natural frequencies If enough energy is absorbed, the object can vibrate so strongly that it breaks apart.

31 Matching Natural Frequencies A good example of this phenomenon is the collapse of the Tacoma Narrows Bridge in the state of Washington. Soon after it opened, the bridge began to sway. The wind coming through the canyon struck the bridge in gusts that arrived at intervals matching the natural frequency of the bridge. This caused an increase in the amplitude of the motion of the bridge, eventually leading to its collapse on November 7, This is an example of an engineering disaster that could have been avoided by better understanding natural frequencies.

32 Waves and Particles a wave is a disturbance that carries energy a particle is a piece of matter Einstein - EM waves can behave as a particle, a photon, whose energy depends on the frequency of the waves

33 Particles as Waves Can matter behave as a wave? If a beam of electrons were sprayed at two tiny slits, you might expect that the electrons would strike only the area behind the slits, like the spray paint.

34 Particles as Waves Instead, it was found that the electrons formed an interference pattern This type of pattern is produced by waves when they pass through two slits and interfere with each other.

35 Particles as Waves Water waves produce an interference pattern after passing through two openings. It is now known that all particles, not only electrons, can behave like waves.

36 Electromagnetic Waves made by vibrating electric charges can travel through space (no matter) EM waves travel by transferring energy btwn vibrating electric & magnetic fields All objects emit EM waves energy carried by EM waves is radiant energy

37 Electric and Magnetic Fields Magnetic fields exist around magnets even if the space around the magnet contains no matter. electric charges are surrounded by electric fields even if the space around it contains no matter. charges exert forces on each other even when they are far apart.

38 Magnetic Fields and Moving Charges Electric charges can be surrounded by magnetic fields. An electric current flowing through a wire is surrounded by a magnetic field. An electric current in a wire is the flow of electrons in a single direction. the motion of the electrons creates the magnetic field around the wire.

39 Changing Electric & Magnetic Fields A changing magnetic field creates a changing electric field. A changing electric field creates a changing magnetic field.

40 Making Electromagnetic Waves EM waves are produced when an electric charge moves back & forth, causing the electric field around it to change Because the electric charge is in motion, it also has a magnetic field around it. This magnetic field also changes as the charge vibrates. The vibrating electric charge is surrounded by changing electric & magnetic fields.

41 Making Electromagnetic Waves A vibrating electric charge creates an EM wave that travels outward in all directions Because the electric & magnetic fields vibrate at right angles to the direction the wave travels, an EM wave is a transverse wave The wave in only one direction is shown here.

42 Wave Speed All EM waves travel at 300,000 km/s (speed of light) in the vacuum of space Nothing travels faster than the speed of light. In matter, the speed of EM waves depends on the material they travel through. hese waves travel at the same speed through space; 300 million m/s ( miles/second). hat means that radiation from the sun, which is 150 billion meters (93 million miles) away, takes bout 8 minutes to get to Earth.

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44 What is Electromagnetic Radiation? EM radiation is the formal name for electromagnetic waves. It is a form of energy that is produced from the disturbances of atoms or disturbances within atoms. When an atom is disturbed and undergoes a change, it can release energy in the form of an EM wave. The nature of these disturbances determines the frequency of the EM waves (EM radiation) produced. There are many different types of atoms, and many different ways that each atom can be disturbed. So it makes sense that there are many different types of EM waves produced by atoms.

45 frequencies range from below 60 cycles per second up to trillions of cycles per second. as the frequency of the wave increases, so does the energy it carries.

46 Radio & Microwaves Radio waves λ 1 mm (longest waves) Microwaves = λ 1 mm

47 Radio Waves having the longest wavelengths used mostly for communication wavelengths up to thousands of meters includes radio, short wave radio, and television wave frequency is measured in Hertz AM radio signals are kilohertz FM radio signals are in megahertz TV stations broadcast at higher frequencies The Federal Communications Commission reserves specific frequencies for various organizations like police and fire companies

48 Microwaves wavelength longer than infrared, their applications are diverse We are most familiar with microwaves as a cooking method. In a microwave oven, water molecules readily absorb microwaves. The absorbed energy of the microwaves is transformed into random kinetic energy of the molecules. The temperature of the food then increases and the food cooks as though it had been on a standard stove. How can we be sure the microwaves will stay inside the oven and not escape into the kitchen? There are holes in the screen that are big enough to let the tiny wavelength visible waves through, but block the much longer wavelength microwaves. Microwaves are 12.4 cm long and are reflected by the screen back into the oven to be absorbed by the food.

49 Radar - Radio Detecting And Ranging radio waves are transmitted toward an object By measuring the time required for the waves to bounce off the object & return to a receiving antenna, the object can be found located

50 Magnetic Resonance Imaging (MRI) uses radio waves to help diagnose illness Protons in H atoms in bones & soft tissue behave like magnets & align with the strong magnetic field Energy from radio waves causes some of the protons to flip their alignment. As they flip, they release radiant energy

51 Magnetic Resonance Imaging (MRI) A radio receiver detects this released energy and uses it to create a map of the different tissues

52 Infrared Waves thermal energy transmitted by infrared waves λ 1 mm to 750 billionths of a meter Every object emits infrared waves

53 Infrared (IR) Waves The name means below red because the wavelengths are longer than the longest visible light waves. This means they vibrate at a lower frequency than visible light. infrared frequencies are most often associated with heat energy since warm objects emit IR radiation as a way to cool.

54 Visible Light range of EM waves detected by your eyes λ 750 billionths to 400 billionths of a meter

55 Visible Light Your eyes contain substances that react differently to various wavelengths of visible light, so you see different colors. These colors range from short-wavelength blue to long wavelength red. If all the colors are present, you see the light as white.

56 Ultraviolet Waves λ 400 billionths to 10 billionths of a meter Overexposure to UVA & UVB can cause skin damage & cancer longer-wavelength UVA rays shorter-wavelength UVB UV light entering cells damage protein & DNA For some single-celled organisms, damage can mean death, which can be a benefit to human health (ex: goggle cabinet)

57 Ultraviolet (UV) Waves beyond violet λ 400 billionths to 10 billionths of a meter We cannot see UV, our eyes are not sensitive to it Many other organisms, such as bees and deer, can see in this range. This is the reason that deer can detect the UV brighteners found on clothing that was washed with modern detergents. UV is divided into sub categories based on wavelength UV-A waves are the longest wavelength followed by the UV-B waves. Both UV-A and UV-B waves deliver energy to your skin that can produce sunburn damage or worse. UV-C waves have the shortest wavelength (highest frequency) of all the UV waves. Most UV-C waves coming from the Sun do not reach the earth because they are absorbed by ozone molecules in the upper atmosphere. On the surface of the Earth, we create and use UV-C waves to kill bacteria and sterilize things such as the goggles that you wear in science class.

58 Useful UVs UV waves are useful because they make some materials fluoresce Fluorescent materials absorb ultraviolet waves & reemit the energy as visible light Police sometimes use fluorescent powder to show fingerprints etc. when solving crimes

59 X Rays λ ten billionths to ten trillionths of a meter Doctors use low doses of X rays to form images of internal organs. used in radiation therapy to kill diseased cells in the human body

60 X-Ray Waves Named because William Roentgen, their discoverer, didn t know exactly what they were. X-rays have many applications because these high energy waves can penetrate materials that lower frequency waves cannot. The medical field has benefited greatly from the use of X-rays to look inside of the human body. Special machines are used to generate these high energy waves and receptors that are sensitive to these waves are used to detect them. Most X-ray waves simply pass through the muscles and organs of human body, but they can not pass through bones. On an X- ray image, the bones show up as white images giving the doctor the ability to see inside the human body without making any incisions.

61 Gamma Rays λ shorter than about 10 trillionths of a meter These are the highest-energy EM waves can penetrate through several cm of lead produced by processes that occur in atomic nuclei used in radiation therapy to kill diseased cells in the human body

62 Gamma Waves highest frequency EM radiation extremely high energy radiation and is highly toxic to organisms. In carefully controlled doses the energy of gamma waves can be put to use killing cancer cells without killing the patient. also used to sterilize medical instruments and bandages. Over exposure can cause immediate death, but even low doses of gamma radiation is also associated with cell damage that causes cancer. People who work with substances that emit gamma radiation monitor their exposure very carefully.