Transcript for:
Understanding Waves and Electromagnetic Spectrum

[Music] candidates are expected to have a thorough understanding of the syllabus details outlined in the accompanying figure waves waves transfer energy without transferring matter waves are classified into two types transverse waves and longitudinal waves transverse waves in transverse waves the direction of vibration is perpendicular to the direction of propagation or direction of energy transfer or the direction of wave traveled for example water waves seismic secondary waves Slinky waves and electromagnetic waves we can demonstrate the transverse wave by shaking the spring up and down as shown direction of vibration is up and down which is perpendicular to the direction of wave propagation or direction of energy transfer or direction of wave traveled is to the right direction of vibration is up and down around the equilibrium position this is the equilibrium position when the wave vibrates for one cycle it creates one wave that that travels one wavelength the red particle at the red arrow is moving up and down this shows that wave transfers energy without transferring matter highest peak is called a Crest lowest Peak is called a trough longitudinal waves in longitudinal waves the direction of vibration is parallel to the direction of propagation or direction of energy transfer or the direction of wave traveled for example sound waves Slinky waves and seismic primary waves we can demonstrate a longitudinal wave by shaking the spring forward and backward as shown direction of vibration is forward and backward which is parallel to the direction of wave propagation or direction of energy transfer or direction of wave traveled is to the right direction of vibration is forward and backward around the equilibrium position this is the equilibrium position when wave vibrate for one cycle it creates one wave that travels one wavelength where the spring compresses together this is called a compression where the spring extends apart this is called a raction compression is where wave particles are close together and high pressure or high density rarefaction is where wave particles are far apart and low pressure or low density describing waves here is the graphs of displacement time graph of a traveling wave here is the displacement distance traveled graph of a traveling wave amplitude a of a wave is the distance from equilibrium position to the peak of the wave it represents energy carried by the wave so a wave with a high amplitude carries a lot of energy wavelength Lambda of a wave is the distance between consecutive Peaks or between consecutive identical points it is measured in meters M frequency F of a wave is the number of vibrations per second or numbers of waves that travel past a point per second it is measured in hertz HZ period T of a wave is the time taken for one vibration to completely or for one wave to travel past a point it is measured in seconds s if a water wave travels n waves in t seconds its frequency is n to divide by T its period is T to divide by n so frequency is 1 to divide by period speed V of a wave is the distance traveled by wavefront per unit time it is measured in meters per second if the distance traveled is one wavelength then the time taken is one period period since 1 over period is equal to frequency then speed is equal to wavelength times frequency wave fronts of wave are the lines along Peaks compressions or rare factions of the wave are aligned demonstrating the wave fronts of the water wave in a ripple tank a ripple tank is a shallow tray of water with a light source shining down through it the light illuminates the wave fronts making them visible a straight Dipper can be used to create straight wave fronts in a ripple tank when the Dipper is vibrated up and down it creates a series of parallel wave fronts the screen below the Ripple tank is used to observe the wave fronts of the water wave the stroboscope or video camera is used to make the slow motion of the wave fronts of water wave on the screen a small spere Dipper can be used to create circular wave fronts in a ripple tank when the Dipper is dropped into the water it creates a circular wavefront that expands outward as shown the side view of the water wave in the Ripple tank the top view of the water wave in the Ripple tank we see the wave fronts are straight along crests or troughs of the wave so the distance between consecutive wave fronts is wavelengths because of the wavelength is the distance between two consecutive crests or troughs of the wave the direction of propagation or direction of energy transfer or direction of wave traveled is always perpendicular to the wave fronts reflection of waves when waves hit an obstacle they are reflected the direction of the wave propagation changes but the speed wavelength and frequency remain constant demonstration of wave reflection using a ripple tank the incident wave front hits the obstacle creating the reflected wave front as shown when the series of incident wave fronts hit the obstacle creating the series of reflected wavefronts as shown candidates should be able to draw the diagram of this reflection draw the incident wave fronts and the reflected wave fronts the angle between the incident wave front and the surface of obstacle is equal to the angle between the reflected wave fronts and the surface of obstacle draw the incident Ray that is perpendicular to the incident wave fronts and its direction is the direction of wave propagation direction of energy transfer draw the normal line at right angle to the surface of the obstacle the angle of incidence I is between the incident Ray and normal line draw the reflective Ray that is perpendicular to the reflected wave fronts and its direction is the direction of wave propagation direction of energy transfer the angle of refraction R is between the reflective Ray and normal line space between consecutive wave fronts is wavelength which are equal before and after reflection rules of the reflection the angle of incidence I is equal to the angle of reflection R the incident Ray reflective Ray and normal lie lie on the same plane circular wave fronts can be reflected at a flat obstacle as shown the circular wave fronts is Created from Source s they reflect at the flat obstacle the image of the source R is created behind the obstacle which the reflected waves appear to come refraction of waves when waves travel through one medium to another they are refracted this is because the speed of wave is changed causing its wavelength also change while its frequency remain constant demonstration of wave refraction using a ripple tank this area is shallow water and this area is deep water in a shallow water place the glass block in the water the water waves travel slow in shallow water due to the friction increase so its wavelength also decreases while constant frequency the water travel fast in deep water due to the friction decrease so its wavelength increases while constant frequency the incident wave fronts travel from deep water to shallow water the wave speed decreases to cause the wave fronts Bend toward the normal line and they are close together this causes the wave length to decrease candidates should able to draw the diagram of the refraction of wav this is the boundary between shallow and deep water this region is deep water this region is shallow water draw the incident wave fronts of wave in deep water draw the refractive wave fronts of wave in shallow water draw the direction of incident Ray at perpendicular to the incident wave fronts draw the normal line at perpendicular to the surface of the boundary draw the refractive Ray at perpendicular to the refracted wave fronts the angle of incidence I is between the incident Ray and normal line the angle of refraction R is between the refractive Ray and normal line when the water wave travel from deep water to shallow water at perpendicular to the surface of boundary as shown this region is shallow water this region is deep water the wave fronts are not Bend but the wave fronts are closed together this shows that wavelength decreases due to the wave speed decreases when travel from Deep to shallow water draw the diagram to show the refraction of wave of this situation this region is deep water this is region is shallow water draw the incident wave fronts of wave in deep water draw the refractive wave fronts of wave in shallow water draw the direction of incident Ray at perpendicular to the incident wave fronts draw the normal line at perpendicular to the surface of the boundary draw the refractive Ray at perpendicular to the refracted wave fronts the angle of incidence is between the incident Ray and normal line to be zero the angle of refraction is between the refractive Ray and normal line to be zero therefore the refractive Ray is not Bend when the incident Ray travels along the normal or perpendicular to the boundary this is because the angle of incidence and angle of refractive are equal to zero Doppler effect when a stationary ambulance car emits sound waves the wave fronts spread out symmetrically forming circles around the source if the wave Source moves the waves can become compressed in front of it and stretched behind it this movement changes the wavelength and frequency of the waves in front of the source the wavelength decreases and the frequency increases person a hears sound that has a higher pitch than when the car was stationary behind the source the wavelength increases and the frequency decreases so person B hears a sound with a lower pitch than when the car was stationary these apparent changes in frequency which occur when a SCE of waves is moving is called the Doppler effect and is a property of all [Music] waves candidates are expected to have a thorough understanding of the syllabus details outlined in the accompanying figure electromagnetic spectrum all electromagnetic waves have the following properties they are all transverse waves they can all travel through a vacuum they all travel at the same speed in a vacuum its speed in a vacuum is 3 * 10 the^ of 8 m/s and is approximately the same in air the electromagnetic spectrum is arranged in a specific order based on the wavelengths or frequencies the main groupings of the continuous electromagnetic spectrum are radio waves microwaves infrared visible light red orange yellow green blue indigo violet ultraviolet x-rays gamma rays this order is shown in the diagram from longest wavelength to shortest wavelength lowest frequency to highest frequency lowest energy to highest energy size of wavelength of radio wave is approximately to the size of the building microwave is approximately to the size of baseball infrared is approximately to the size of pinpoint visible light is approximately to size of the bacterial ultraviolet is approximately to size of virus xrays is approximately to size of atom gamma rays is approximately to size of subatomic particles the higher the frequency the higher the energy of the electromagnetic waves electromagnetic wave with higher energy is highly ionizing harmful to cells and tissues causing cancer such as UV X-rays and gamma rays electromagnetic wav with lower energy is less ionizing less harmful to humans such as radio waves microwaves infrared and visible light useful for communications uses of electromagnetic waves electromagnetic waves have a variety of uses and applications radio waves communication radio waves are used to transmit the signal of radio and television they are also used in cellular networks GPS and Wi-Fi radio frequency identification uses radio waves to identify people or objects they are used in a variety of applications such as inventory tracking access control and payment systems Bluetooth is used to communicate between two Bluetooth compatible devices astronomy radio telescopes are used to observe the naturally occurring radio waves that come from stars planets galaxies and other astronomical objects microwaves communication microwaves are used to transmit the signal of satellites television and mobile phones they are also used in radar systems cooking microwaves are used to heat food in microwave ovens they work by causing the water molecules in food to vibrate which creates heat infrared is emitted by warm objects electric grills some electric grills use infrared heat to cook food remote controls remote controls use infrared light to send signals to devices such as TV Intruder alarms are the motion sensor thermal imaging infrared cameras can be used to create images of objects that emit infrared radiation this can be used for medical imaging security and Industrial applications Optical fibers Optical fibers are used to transmit the signal in the communication which is more efficiently than visible light visible light seeing visible light is the part of the electromagnetic spectrum that can be detected by the human eye it is used for vision photography and videography communication visible light can be used to transmit data in Optical fibers ultraviolet fluorescence some substances glow when they are exposed to ultraviolet light this can be used to create security markings and detect fake Bank notes tanning ultraviolet light from the sun can cause tanning it can also cause skin cancer sterilizing ation ultraviolet light can be used to sterilize surfaces and kill bacteria it is used in hospitals Laboratories and food processing plants x-rays Medical Imaging x-rays are used to create images of the inside of the body they are used to diagnose diseases and injuries security scanning x-rays are used to scan luggage and people at airports and other security checkpoints gamma rays sterilizing food and medical equipment gamma rays can be used to bacteria and living things cancer treatment gamma rays are used to kill cancer cells harmful effects of electromagnetic waves on people the higher the frequency of an electromagnetic wave the higher its energy electromagnetic wave with higher energy is more ionizing meaning that it can remove electrons from atoms or molecules this can be harmful to cells and tissues and can even cause cancer electromagnetic wave with lower energy is less ionizing and is less harmful to humans it can still be harmful however if it is absorbed in large amounts beyond the visible part of the spectrum the energy becomes large enough to ionize atoms the main risks associated with electromagnetic w waves are summarized as radio waves no known danger microwave possible heat damage to internal organs when the water molecules in the body absorb microwav strongly infrared inframed can cause heating effects in tissues meaning it can also cause skin burns but it is less likely to cause internal damage than microwaves visible light very bright light can cause eye damage this is because the retina of the eye is sensitive to light ultraviolet if eyes are exposed to high levels of UV it can cause severe eye damage skin cancer ultraviolet is ionizing meaning it can kill cells or cause them to malfunction resulting in premature aging and diseases such as skin cancer x-rays x-rays are the most ionizing radiation meaning they are able to penetrate the body and cause internal damage so they are caused to kill cells mutation of genes and cancer gamma rays the harmful effects of gamma rays like as x-rays because they are the most ionizing radiation so they are caused to kill cells mutation of genes and cancer it is important to note that the harm ful effects of electromagnetic waves depend on the amount of radiation that is absorbed a small amount of radiation may not be harmful but a large amount of radiation can be very harmful the risk of harm also depends on the type of tissue that is exposed to the radiation for example the retina of the eye is more sensitive to light than other tissues so it is more likely to be damaged by exposure to visible light the best way to protect yourself from the harmful effects of electromagnetic waves is to limit your exposure this means avoiding using electronic devices for long periods of time staying away from sources of strong radiation and wearing protective clothing when necessary [Music] candidates are expected to have a thorough understanding of the syllabus details outlined in the accompanying figure General properties of light waves light waves are the transverse waves and electromagnetic waves so light can transfer energy through through the vacuum speed of light in the vacuum is 3 * 10 ^ of 8 m/s which is approximately equal to speed of light in air light is travel in the straight line light wave exhibits the refraction and refraction reflection of light on plain mirror when a ray of light strikes a plain flat mirror it is reflected so that the angle of incidence I is equal to the angle of reflection r mirrors are often used to change the direction of a ray of light while its speed frequency and wavelength remain constant the normal is a line at right angles to the mirror the angle of incidence is the angle between the incident Ray and the normal the angle of reflection is the angle between the reflected ray and the normal therefore the rule of reflection are angle of incidence I equals angle of reflection R the incident Ray reflective Ray and normal line lie on the same plane Periscope consists of two plain mirrors it is one example that uses two mirrors to change the direction of rays of light rays from the object strike the first mirror at an angle of 45° to the normal the rays are reflected at 45° to the normal and so are turned through an angle of 90° by the mirror at the second mirror the rays are again turned through 90° changing the direction of rays of light in this way allows an observer to use a periscope to see over or around objects refraction of light light will refract when it travels through the transparent materials such as glass angle of incidence is denoted by I angle of refraction is denoted by R as the incident light Ray enters the glass block it slows down and bends towards the normal as the light Ray leaves the glass block it speeds up and bends away from the normal the frequency of light is unchanged as it travels from one medium to another the wavelength of light changes directly with the speed of light as the lights speed up the wavelength increases as the light slows down the wavelength decreases when light travels from an optically less dense medium to a more dense medium it slows down and so bends towards the normal when light travels from a more dense to less dense medium it speeds up and bends away from the normal if the angle of incidence is 0° the ray enters along the normal to the surface of the glass block the light slows down but does not change direction because the the angle of refraction is also zero as it leaves it speeds up but does not change direction refractive index different materials can bend rays of Light by different amounts we describe this by using a number called the refractive index n refractive index n of a material medium is the ratio of sin I and sin r we can use the equation below to calculate the refractive index of a material n equals sin I over sin R where I is the angle of incidence R is the angle of refraction n is the refractive index it have no unit the refractive index in air is approximately equal to one in figure as shown we can find the refractive index of medium a using the following equation as shown substitute I = 50° n r equal 40° the results of n is 1.2 we can find the refractive index of medium B using the same following equation as shown substitute I equal 50° n r equal 30° the results of n is 1.5 from the results of example we can see that light light bends in medium are less than the medium B so light refracts in the medium are less than in the medium B and the refractive index of medium a is less than the medium B this shows that the density of medium a also less than the density of medium B an experiment to investigate the refractive index of glass place a glass block on a piece of paper and trace a around the block with a pencil shine a ray of light in air on the glass block place two pins along the incident Ray with a separation of more than 5 cm to ensure accuracy when drawing the line place two pins along the emerging Ray from other side of glass block with a separation of more than 5 cm remove the glass block and pins then Mark at the pins holes turn off the light box use ruler to draw the incident Ray and the emerging Ray then connect the entry and exit points to show the path of Light Within the glass block draw the normal line at the point of incidence measure the angle of incidence I and the angle of refraction R calculate the refractive index of the glass using n equal sign I over sign the critical angle and the total internal reflection in a medium as the ray shine on the semicircle glass block the light slows down but does not change direction this is because the angle of incidence is 0° the ray enters along the normal to the surface of the glass block then the angle of refraction is also zero as the ray reach the point a some Ray reflect and some Ray refract when a ray of light travels from a denser medium glass to a less dense medium air it bends away from the normal as the angle of incidence I increases the angle of refraction increases the intensity of refractive Ray decreases while intensity of reflective Ray increases when R equals 90° the angle of incidence is equal to the critical angle C therefore the critical angle is the is the angle of incidence in denser medium where the angle of refraction is 90° we can calculate the critical angle using the equation s c equal 1 / n if the angle of incidence is increased beyond the critical angle total internal reflection occurs the total internal reflection occurs when the light Ray travels from denser medium to less dense medium the angle of incidence is more than the critical angle of the medium no refraction of light the angle of incidence is equal to the angle of reflection uses of the total internal reflection in everyday life Periscope consists of the glass prisms which are used in submarines as a light Ray travels through a periscope total internal reflection occurs at Point a and point B a glass right angle prism with an angle 4 5° the critical angle of glass is about to 42° as the light Ray travels from Air to glass at the angle of incidence is zero so the light slows down but does not change direction the light Ray reflects at the point a with the incident angle of 45° which is greater than the critical angle of glass 43° this causes the light Ray to undergo total internal reflection as it leaves the prism it speeds up but does not change direction binocular consists of the prisms as shown as the light Ray travels through a binocular and the total internal reflection occurs at points a b c and d the prisms in this binocular are glass right angle prism 45° as the light Ray travels from Air to glass at the angle of incidence of zero so the light slows down but does not not change direction the light Ray reflect at the point a with the angle of incidence is 45° which is greater than the critical angle of glass this causes the total internal reflection to occur then the light Ray reflect at the point B with the angle of incidence is 45° which is greater the critical angle of glass this causes the total internal reflection to occur again as it leaves from the prism it speeds up but does not change direction rear reflector rear reflectors use total internal reflection to reflect light back to the source they are often used on vehicles to improve their visibility to other Road users Optical fibers an optical fiber is a thin glass cylindrical core coated with a transparent material of lower refractive index cladding the cladding has a lower refractive index than the core so it is less dense this means that total internal reflection will occur for all rays of light that strike the boundary between the core and clouding at an angle greater than the critical angle Optical fibers can be used to communicate signals for example telephone conversations or in medical applications such as endoscopy a method of examining inside the body in the communications Optical fibers are used in the TV internet and phones light or infrared carries the information along the optical fibers they can carry a large amount of information at very high speed in medicine Optical fibers are used in the endoscopes that allow the surgeons to see inside the body of their patients light Ray from the outside is transmitted through the optical fibers and undergo total internal reflection which prevents them from leaving the optical fibers light Ray reflected at the inside the body and return back to the outside along the other Optical fibers this light is sent to the computer monitor describing sound wave sound wave is the longitudinal wave which means that the particles of the medium vibrate in the same direction as the wave is traveling sound waves are produced by vibrating sources when a vibrating Source moves back and forth it creates pressure variations in the surrounding medium sound waves require a medium to transfer their energy the medium can be air water or any other solid or liquid this is why sound cannot travel through a vacuum the sound travels faster in solids than in liquids and faster in liquids than in gases the speed of sound in air is approx approximately to 330 m/s the speed of sound in liquids is approximately to 1,500 m/s the speed of sound in solids is approximately to 300 to 500 m/s the explanation of how a source such as a drum produces sound when you Bang a Drum the skin vibrates the vibrating drum skin causes nearby air molecules to vibrate this is because the air molecules are bumping into each other the vibrating air molecules cause other nearby air molecules to vibrate and so on this creates a series of compressions and rare factions in the air the compressions are regions where the air molecules are closer together and high pressure the RAR factions are regions where the air molecules are farther apart and low pressure the compressions and rare factions travel through the air as sound waves the distance between the consecutive compressions or consecutive rif factions is the wavelength the harder you bang the bigger the vibrations and the louder the sound this causes the air molecules to be closer together in the compressions and farther apart in the rare factions the greater the difference between the air pressure in the compressions and rare factions the louder the sound an experiment to determine the speed of sound in air the speed of sound in air is approximately 330 m/s at 25 cels speed of sound in the hot air is faster than in the cold air this is because the air particles in hot air have higher speed than the cold air so the speed of sound in air varies from 330 to 350 m/s investigate the speed of sound between two points two people stand a distance of around 100 m apart the distance between them is measured using a tape measure one person has two wooden blocks which they bang together the second person has a stopwatch which they start when they see the first person banging the blocks together and stops when they hear the sound this is then repeated several times and an average value is taken for the time the speed of sound can then be calculated using the equation speed is equal to the distance traveled by sound to divide by time taken reflection of sound sound waves behave in the same way as any other wave when a sound wave from a turning fork strikes a surface it may be reflected like light waves sound waves are reflected from a surface so that the angle of incidence is equal to the angle of reflection the speed frequency and wavelength of the sound remain constant the reflection of sound causes Echo sonar and the echo sounding can be used to measure depth or to detect objects underwater a sound wave can be transmitted from the surface of the water the sound wave is reflected off the bottom of the ocean the time it takes for the sound wave to return is used to calculate the depth of the water the distance the wave travels is twice the depth of the ocean this is the distance to the ocean floor plus the distance for the wave to return the depth of the water can then be calculated using the equation speed equals the twice of the depth to divide the echo time investigate the speed of sound using the Echoes of sound The Echoes of sound is caused by the reflection of sound a person stands about 50 m away from a wall using a tape measure to measure this distance the person claps two wooden blocks together and listens for the echo a second person has a stopwatch and starts timing when they hear one of the claps and stops timing when they hear the echo the process is then repeated five times and an average time calculated the distance traveled by the sound between each clap and Echo will be 2 * 50 M the speed of sound can be calculated from this distance and the time using the equation speed is equal to the twice of distance to the wall and divide by Echo time the echo time is time that the sound travels forward and backwards refraction of sound waves all waves can be refracted even sound waves for example if some parts of a sound wave are traveling through warm air will travel more quickly than those parts traveling through cooler air as a result the direction of the sound wave will change it will be refracted although it is not possible to see sound waves being refracted we can sometimes hear their effect if a person a is standing at the edge of a large pond or Lakey can sometimes hear sounds from person B on the other side of the water much more clearly than we would expect this is due to refraction infigure explains how this happens most of the sound person B here travels to us in a straight line path B but some sound travels upwards path a if the temperature conditions are right as shown which usually happens in nighttime some parts of a sound wave are traveling through warm air they will travel more quickly than those parts traveling through cooler air then as the sound waves travel through the air they are refracted and follow a curved path downwards path C Persona now receive two sets of sound waves so the sound person B here seems louder and clearer pitch and frequency of sound the frequency of a sound wave is related to its pitch sounds with a low pitch have a low frequency or long wavelength sounds with a high pitch have a high frequency or short wavelength loudness and amplitude of sound the amplitude of a sound wave is related to its volume sounds with a large amplitude have a high volume sounds with a small amplitude have a low volume audible range humans can hear sounds between about 20 htz and 20,000 Hertz in frequency although this range decreases with age the sound with frequency higher than 20 ,000 Herz is called ultrasound which is beyond the range of human hearing the sound with frequency lower than 20 HZ is called the infrasound investigate the speed frequency of sound using the oscope although we cannot see an actual sound wave we can see an image or representation of it by connecting a microphone to a piece of apparatus called an oscilloscope when the sound wave from a source such as a tuning fork enters the microphone the oscilloscope draws the longitudinal sound wave as a transverse wave which allows us to see features such as the waves amplitude and frequency more easily from the trace drawn on the screen we can measure the time for one complete vibration or one complete wave this is called the time period of the Wave t on the screen the distance between Peaks is four divisions and one division is set up to 0.5 seconds then the period is 4 * 0.5 to equal 0.02 seconds we can then find the frequency of the sound f using the equation frequency equals 1/ period so frequency equal 1 / 0.2 which is 50 hertz I hope you found this video helpful if you did I would be grateful if you would subscribe share like and leave a positive comment your support will encourage me to create more content thank you