Скачать презентацию Demonstrate understanding of aspects of wave behaviour Physics

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Demonstrate understanding of aspects of wave behaviour Physics A. S. 1. 4 2013

Waves • Waves are a means of transferring energy through a medium without transferring matter • Light and Sound travel as waves, in straight lines • Speed of light 300, 000 Km/sec • Speed of sound 300 m/sec • Electromagnetic waves form a large spectrum from radio to gamma rays

Electromagnetic Spectrum • The EMR energy increases as the wavelength gets shorter • Low energy = communication, High energy = hazard to life

Waves • There are many types of wave, but they are all either: • Longitudinal – particles vibrate/oscillate parallel to the direction of the wave (eg Sound) • Transverse – particles vibrate/oscillate at 90 o to the direction of the wave (eg water) • The involvement of particles means these are mechanical waves • EMR waves e. g light are transverse waves that do not need a medium (no particles are involved)

Understanding waves Transverse waves Longitudinal waves compression and rarefaction of particles

Some definitions… Crest 1) Amplitude – measured from mid position to crest or trough Trough 2) Wavelength ( ) – this is the distance between two corresponding points on the wave and is measured in metres: 3) Frequency – this is how many waves pass a point every second and is measured in Hertz (Hz)

Terminology • • • Peak Trough Amplitude Wavelength (λ) Period (T) – time taken for the wave to travel 1 cycle OR time for a wave to pass a point, measured in seconds 1 cycle TIME

Sound We hear things when they vibrate. If something vibrates with a high frequency (vibrates very ______) we say it has a _____ pitch. If something vibrates with a low frequency (vibrates ______) we say it has a ____ pitch. Words – slowly, low, high, quickly Higher frequency = Higher pitch

Frequency This sound wave has a high _____ frequency: This sound wave has a low ___ _frequency: What is the relationship between frequency and wavelength? Lower frequency = higher wavelength

Amplitude This sound wave has a high _____ amplitude (loud): Amplitude = energy (volume, brightness etc) This sound wave has a low _____ amplitude (quiet): Sound video

Draw these waves: 1) Quiet sound, low frequency (i. e. high wavelength): 2) Quiet sound, high frequency (i. e. low wavelength): 3) Loud sound, low frequency: 4) Loud sound, high frequency:

The Wave Equation The wave equation relates the speed of the wave to its frequency and wavelength: Wave speed (v) = frequency (f) x wavelength ( ) in m/s in Hz in m V f

Questions • Sound travels at 330 m/s. If a sound note is played at 484 Hz find its wavelength. • 0. 68 m (2 -3 sig fig) • The speed of light is 3. 0 x 108 m/s and the wavelength of red light is 650 x 10 -9 m. Find the frequency of red light. • 4. 6 x 1014 Hz • 4 complete water waves pass a point every 2 secs. The distance between 6 crests is 12 m. Find the speed of the wave. • 4. 8 m/s (draw the waves out)

Period and Frequency • Period (T) –time for a wave to pass a point, measured in seconds • Frequency – how many waves pass a point every second • Period and frequency are related by the equation: frequency (f) = 1 / period (T) in Hz in s • F=1/T • F is frequency (Hz) • T is period (s) 1 f T

Questions • What is the period of a 330 Hz wave? • 0. 003 s • What is the frequency of a wave with period of 0. 4 s? • 2. 5 Hz • Complete the questions pg 113, 114 ESA study guide • Video – properties of waves

Reflection • All waves can reflect. • In a plane (flat) mirror images are reflected about the normal. (an imaginary line 90 o to the mirror) • Plane mirrors also laterally invert images. • Plane mirrors produce virtual images (shown by dashed line).

Reflection Angle of incidence = Angle of reflection Normal Reflected ray Incident ray Angle of incidence Angle of reflection Mirror

Lateral Inversion Images in plane mirrors appear the same size and as far behind as in front but they are switched from “side to side” L R Light from each hand reflects from the mirror into our eyes. The eyes register the image in direct line of sight. The persons left hand appears to be the right. Why is the wave behind the mirror dashed? Light is blocked by the mirror so the image is virtual.

How to draw reflection diagrams • Draw the position of the reflected candle. (same size, as far behind as in front) • Draw 2 rays from the image to the eye (line of sight) • Image must be virtual (dashed lines) • Use the law of reflection to show light travels from the candle to the mirror. (normal line) • Show the direction the rays move. (arrows in the centre)

Refraction through a glass block: How does the brightness of a refracted ray differ from a reflected one? Why? Set up this experiment and complete the diagrams Wave slows down and bends towards the normal due to entering a more dense medium Wave speeds up and bends away from the normal due to entering a less dense medium Wave slows down but is not bent, due to entering along the normal

Refraction Rules 1. Describe the optical densities of the media 2. Frequency remains the same. 3. State the change of speed (faster or slower) 4. Therefore the wavelength changes (increase or decrease) 5. So the refracted ray bends (towards or away from the normal) LESS MORE

Refraction • When going from less optical density to more it bends towards the normal. • When going from more optical density to less it bends away from the normal. • Refraction always involves a change in speed, direction and wavelength. Optical density changes: Air < water < glass < diamond

Refraction is the bending of waves. It happens when waves ____ __, wavelength and direction due to entering a _____ (substance) of different optical density. When a pen is placed in water it looks like this: In this case the light rays are slowed down by the water and are _____, causing the pen to look odd. The two mediums in this example are ______ and _______. Draw a ray diagram to show why the pen appears bent. Words – change speed, water, air, bent, medium http: //www. bcscience. com/bc 8/pgs/quiz_section 4. 2. htm

How to draw refraction diagrams • Draw the image • Draw 2 rays from the image to the viewer (line of sight) • Draw 2 rays from the object to the points of refracted rays at the interface. • Show the direction the rays move. (arrows in the centre) • Draw a normal to check refraction is correct • wiki

Question • Explain how the prism creates a spectrum. • White light travelling through air enters a more optically dense medium (plastic) and is refracted towards the normal because it changes its speed, direction and wavelength, as it exits the prism it enters a less optically dense medium (air) and again is refracted this time away from the normal. The shorter wavelength blue light travels faster and is bent more than the longer wavelengths so a spectrum is formed.

Finding the Critical Angle… 1) Ray gets refracted 3) Ray still gets refracted (just!) THE CRITICAL ANGLE 2) Ray still gets refracted 4) Ray gets internally reflected

Total Internal Reflection • If light goes from a more optically dense medium to a less optically dense one it is refracted away from the normal. Θi < Θr • When the angle of refraction reaches 90 o the angle of incidence is known as the critical angle. • Any further increase in the angle of incidence will result in the light being reflected rather than refracted. Rules • More optically dense to less • Θi = Θc when refraction is at 90 o • Θi > Θc therefore reflection

Mirages • The mirage is a reflection of the sky. Explain using the ideas of total internal reflection how the mirage occurs. In your answer consider how the optical density of the air changes as it gets hotter. • Air near the road surface is heated and less optically dense. Light from the sky therefore travels from a more to a less optically dense medium and bends away from the normal. The Θi = Θc when refraction is at 90 o. In this case the Θi > Θc therefore reflection of the sky occurs and the brain misinterprets it as water.

Uses of Total Internal Reflection Optical fibres: An optical fibre is a long, thin, _______ rod made of glass or plastic and sheathed in a low optical density material. When light hits the boundary the angle of incidence is greater than the critical angle and so is _______ reflected from one end to the other, making it possible to send ____ chunks of information Optical fibres can be used for _____ by sending electrical signals through the cable. The main advantage of this is a reduced ______ loss. Words – communications, internally, large, transparent, signal

Other uses of total internal reflection 1) Endoscopes (a medical device used to see inside the body): 2) Binoculars and periscopes (using “reflecting prisms”) Complete All questions in activity 8 B pg 130 ESA study guide

Rainbows • In order to see a rainbow the observer must be looking towards the rain with the sun behind them.

Refraction and Dispersion • White light can be refracted and dispersed into the visible spectrum. • When light enters a more optically dense medium it refract, slows down, decreases wavelength and bends towards the normal. • Red light slows less than violet and decreases its wavelength by less therefore it bends less towards the normal. • When travelling into a less optically dense media the opposite occurs. In a prism light bends twice in the same direction (down and Down)

Wave diagrams 1) Reflection 2) Refraction 3) Refraction 4) Diffraction

Diffraction is the ability of waves to spread from a gap or get around a barrier. (don’t use the term bending) More diffraction if the size of the gap is similar to the wavelength More diffraction if wavelength is increased (or frequency decreased)

Low frequency sounds diffract more http: //hyperphysics. phy-astr. gsu. edu/hbase/sound/imgsou/difr. gif

Diffraction over or around the hill • If a man and woman are at the source whose voice would the child hear? • Man has low pitch voice • Low frequency, high wavelength because V=f • So mans voice diffracts more and is heard CHILD http: //www. pa. op. dlr. de/acoustics/essay 1/beugung_en. html

Radio wave diffraction • FM radio is high frequency • v = fx • Therefore short wavelength which can’t diffract much so the house doesn’t get reception • AM radio is low frequency • Therefore long wavelength which diffracts more so the house gets reception

Sound refraction • Sound travels faster in warm air. • Lower part of wave front gets ahead of upper part, so front turns upward. http: //sol. sci. uop. edu/~jfalward/physics 17/chapter 10. html

ripple tank demo

Sound reflections (echos) • When sound waves hit a surface they are reflected. • They are travelling in the same medium so they will have the same frequency, speed and wavelength BUT the direction will change. • The amplitude of reflected wave is less as some of the energy is “lost”. • V = d/t V= speed (m/s) D = distance (m) T = time (s)

Sonar can also be used to tell the different layers beneath the sea floor. http: //www. coml. org/edu/tech/count/acoustic-tech-obs/bottom-penetrating-sounders/2 -dositsreflec. gif

Application - Echolocation in mammals • Some animals use ultrasound to find out about their surroundings, navigate and find food and mates. • High pitch, low energy clicks reflect off close objects. • The animal detects and determines how far away an object is based on the time difference. • Advantages – – High frequency, can’t be heard by predators Short wavelength, more likely to reflect off rather than diffract Doesn’t require much energy to produce the clicks Energy dissipates quickly, less interference of signal • Disadvantages – Energy dissipates quickly, no good for far away objects

How does ultrasound work? Ultrasound is the region of sound above 20, 000 Hz – it can’t be heard by humans. It can be used in pre-natal scanning: How does it work? Ultrasonic waves are partly _____ at the boundary as they pass from one _______ to another. The time taken for these reflections can be used to measure the _______ of the reflecting surface and this information is used to build up a _____ of the object. Words – depth, reflected, picture, medium

Other uses of ultrasound 1) Echo sounding The ultrasound is reflected from the sea floor. 2) Breaking down kidney stones Ultrasonic waves break kidney stones into much smaller pieces 3) Cleaning (including teeth) Ultrasound causes dirt to vibrate dirt off without damaging the object

The Structure of the Earth A thin crust 10 -100 km thick A mantle – has the properties of a solid but it can also flow A core – made of molten nickel and iron. Outer part is liquid and inner part is solid How do we know this? These facts have all been discovered by examining seismic waves (earthquakes)

Seismic waves Earthquakes travel as waves through the Earth – we call them SEISMIC WAVES. There are two types: P waves: 1) They are longitudinal so they cause the ground to move up and down 2) They can pass through solids and liquids 3) They go faster through more dense material S waves: 1) They are transverse so they cause the ground to move from right to left 2) They ONLY pass through solids 3) They are slower than P waves 4) They go faster through more dense material

Seismic waves S waves will only travel through a solid P waves travel through the Earth and are refracted when they pass through a medium The paths of these waves are all curved because density is gradually changing These observations tell us 3 things about the Earth: 1) It has a thin crust, 2) it has a semi-fluid mantle where density increases with depth, 3) a core with a liquid outer part and a solid inner part.

The electromagnetic spectrum Each type of radiation shown in the electromagnetic spectrum has a different wavelength and a different frequency: High frequency, _____ wavelength Gamma rays X-rays Low frequency, _____ (high) wavelength Ultra violet Visible light Infra red Microwaves Radio/TV γ Each of these types travels at the same speed through a _______ (300, 000 m/s), and different wavelengths are absorbed by different surfaces (e. g. infra red is absorbed very well by ______ surfaces). This absorption may heat the material up (like infra red and _______) or cause an alternating current (like in a __ _______). Words – black, microwaves, long, short, TV aerial, vacuum

How Microwaves and Infra-red work Microwaves are absorbed by water molecules up to a depth of a few centimetres. The heat then reaches the centre of the food by conduction. Infra-red waves are absorbed by the surface of the material and the energy is then passed to the centre of the food by conduction. The higher the frequency of the wave, the greater its energy

X-rays and gamma (γ) rays X-rays are absorbed by ____ parts of the body, like ____. Unfortunately, over-exposure to x-rays will damage cells. Gamma rays can be used to treat _______. A gamma ray source is placed outside the body and rotated around the outside of the tumour. Doing this can ___ the cancerous cells without the need for ______ but it may damage other cells and cause sickness. Tracers can also be used – these are small amounts of ______ material that can be put into a body to see how well an organ or ______ is working. Words – radioactive, gland, cancer, hard, bones, kill, surgery