Waves : 06/12/2010 Waves W Richards
The Weald School
Some definitions… : 06/12/2010 Some definitions… 1) Amplitude – this is “how high” the wave is: 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 by every second and is measured in Hertz (Hz)
Some definitions… : 06/12/2010 Some definitions… Transverse waves are when the displacement is at right angles to the direction of the wave… Longitudinal waves are when the displacement is parallel to the direction of the wave…
The Wave Equation : 06/12/2010 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
Some example wave equation questions : 06/12/2010 Some example wave equation questions A water wave travels through a pond with a speed of 1m/s and a frequency of 5Hz. How far apart are the waves?
The speed of sound is 330m/s (in air). When Ricky hears this sound his ear vibrates 660 times a second. What was the wavelength of the sound?
Purple light has a wavelength of around 6x10-7m. If its frequency is 5x1014 Hz what is the speed of light?
Red light travels at the same speed. Work out its frequency if its wavelength is about 4x10-7m.
Properties of Light : 06/12/2010 Properties of Light Light travels in straight lines:
Slide 7 : 06/12/2010 Light travels VERY FAST – around 300,000 kilometres per second. At this speed it can go around the world 8 times in one second.
Slide 8 : 06/12/2010 Light travels much faster than sound. For example: Thunder and lightning start at the same time, but we will see the lightning first. 2) When a starting pistol is fired we see the smoke first and then hear the bang.
Slide 9 : 06/12/2010 We see things because they reflect light into our eyes: Homework
Slide 10 : 06/12/2010 Luminous and non-luminous objects A luminous object is one that produces light.
A non-luminous object is one that reflects light. Luminous objects Reflectors
Slide 11 : 06/12/2010 Shadows Shadows are places where light is “blocked”:
Reflection : 06/12/2010 Reflection Incident ray Normal Reflected ray Angle of incidence Angle of reflection Mirror Angle of incidence = Angle of reflection
Using mirrors : 06/12/2010 Using mirrors Two examples: 1) A periscope 2) A car headlight
Slide 14 : 06/12/2010 Refraction through a glass block: Wave slows down but is not bent, due to entering along the normal
Slide 15 : 06/12/2010
Refraction : 06/12/2010 Refraction Refraction is when waves ____ __ or slow down due to travelling in a different _________. A medium is something that waves will travel through. 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 _______. Words – speed up, water, air, bent, medium
Finding the Critical Angle… : 06/12/2010 Finding the Critical Angle… 1) Ray gets refracted 4) Ray gets internally reflected 3) Ray still gets refracted (just!) 2) Ray still gets refracted
Uses of Total Internal Reflection : 06/12/2010 Uses of Total Internal Reflection Optical fibres:
An optical fibre is a long, thin, _______ rod made of glass or plastic. Light 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 : 06/12/2010 Other uses of total internal reflection 1) Endoscopes (a medical device used to see inside the body): 2) Binoculars and periscopes (using “reflecting prisms”)
Wave diagrams : 06/12/2010 Wave diagrams 1) Reflection 4) Diffraction 3) Refraction 2) Refraction
Diffraction : 06/12/2010 Diffraction More diffraction if the size of the gap is similar to the wavelength More diffraction if wavelength is increased (or frequency decreased)
Sound can also be diffracted… : 06/12/2010 Sound can also be diffracted… The explosion can’t be seen over the hill, but it can be heard. We know sound travels as waves because sound can be refracted, reflected (echo) and diffracted.
Diffraction depends on frequency… : 06/12/2010 Diffraction depends on frequency… A high frequency (short wavelength) wave doesn’t get diffracted much – the house won’t be able to receive it…
Diffraction depends on frequency… : 06/12/2010 Diffraction depends on frequency… A low frequency (long wavelength) wave will get diffracted more, so the house can receive it…
Deviation of Light : 06/12/2010 Deviation of Light This happens because the light is refracted:
The electromagnetic spectrum : 06/12/2010 The electromagnetic spectrum Each type of radiation shown in the electromagnetic spectrum has a different wavelength and a different frequency: Each of these types travels at the same speed through a _______ (300,000,000m/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
Using an oscilloscope : 06/12/2010 Using an oscilloscope 1) Quiet sound, low frequency: 2) Quiet sound, high frequency: 3) Loud sound, low frequency: 4) Loud sound, high frequency:
Ultrasound : 06/12/2010 Pre-natal scanning
Cleaning – it can be used to dislodge dirt
Detecting flaws or cracks
Medical treatment Ultrasound Ultrasound is the region of sound above 20,000Hz – it can’t be heard by humans. There are a number of uses for ultrasound:
How does ultrasound work? : 06/12/2010 How does ultrasound 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
Transmitting information : 06/12/2010 Transmitting information As we said before, different types of electromagnetic radiation can be used to send different types of information, e.g. an optical fibre: Optical fibres have two main advantages: they can send more information compared to electrical cables of the same diameter and with less signal weakening. Another example is radio waves: The longer wavelength waves are transmitted by being reflected off the ionosphere (an electrically charged layer in the Earth’s atmosphere).
Transmitting information : 06/12/2010 Transmitting information Sounds can be transmitted long distances by converting them into electrical signals first: Radio waves can also be used as “carrier waves”: Words – signal, frequency, modulated, carrier, sound, amplified
Analogue vs. Digital : 06/12/2010 Analogue vs. Digital Analogue signals (like talking or music) continually vary in amplitude and/or frequency Digital signals, however, are either off or on, and the information is sent in a series of pulses There are two main advantages of digital:
More information can be sent down the same cable
Better quality, because a digital signal can be amplified without amplifying the extra noise:
The Structure of the Earth : 06/12/2010 The Structure of the Earth How do we know this? These facts have all been discovered by examining seismic waves (earthquakes)
Seismic waves : 06/12/2010 Seismic waves Earthquakes travel as waves through the Earth – we call them SEISMIC WAVES. There are two types: P waves:
They are longitudinal so they cause the ground to move up and down
They can pass through solids and liquids
They go faster through more dense material S waves:
They are transverse so they cause the ground to move from right to left
They ONLY pass through solids
They are slower than P waves
They go faster through more dense material
Seismic waves : 06/12/2010 Seismic waves 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.