Properties of Waves Including light and sound Longitudinal and Transverse Waves The dark areas are compressions The light areas are rarefactions The highest points on the wave are peaks The lowest points on the wave are troughs Wave Motion A wave is a transfer of energy from one point to another. Wave Diagrams and Terminology Wavelength ( ) one whole cycle of a wave Amplitude maximum displacement of wave from its rest position Frequency Number of waves passing per second unit hertz [Hz] The time it takes for one wave to pass is called the period unit second [s] period = 1/f A wavefront represents part of a wave. The distance between them is the wavelength. Wavefront direction - the direction that the waves are moving in The Wave Equation v = f v is the speed of the wave (m/s) is the wavelength in meters (m) f is the frequency in Hertz (cycle/s) Reflection Waves are reflected from a surface at the same angle they hit it. The direction of the wave is changed The speed of the wave is not changed Reflection Law of reflection: angle of incidence = angle of reflection Normal The incident ray is the incoming ray of light. The reflected ray is the reflected ray of light The normal line is at right angle to the mirror The angles of incidence and reflection are measured from the ray to the normal Refraction When waves go from deep water to shallow water they become slower. The frequency of the oscillations remains the same but the wavelength decreases due to the depth change and so the speed decreases. If the barrier between the shallow water and the deep water is at an angle, the wave will change direction Diffraction As a wave approaches move through a gap, they bend around the corners. This is diffraction If the gap is the same size as the wavelength, the diffraction is greater. Image in a plane mirror Image is laterally inverted (back to front) Image is virtual it is formed at a point behind the mirror The image is the same size as the object The image is the same distance behind the mirror as the object is in front A line joining equivalent points on the object and image passes through the mirror at right angles Reflected ray Virtual image Object Incident ray Finding the position of an image in a plane mirror 1. Put a mirror on a bit of paper, put a pin in front of it and mark the position of the pin in the mirror. 2. Line up one edge of a ruler with the image of the pin. Draw a line along the edge to mark its position then repeat with the ruler in a different position 3. Take away the mirror, pin and ruler and extend the two lines to find out where they meet. This is the position of the image 4. Put the first pin back in and put another one in where the lines meet. Put the mirror back and the image and the second pin should line up 1.Draw an incident ray from object to mirror 2.Draw in a normal 3.Draw a reflected ray (remember angle of reflection=angle of incidence). 4.Repeat steps 1-3 with a second incident ray. 5.Extend the two rays (blue lines) until they meet. This is the position of the image. Finding an image position by construction [1] mirror Object Image Finding an image position by construction [2] mirror 1.Draw a long line from the object, through the mirror at a right angle. 2.Measure the distance from the object to the mirror. 3.At an equal distance behind the mirror mark a pint on the extended line point I. This is the image position Object Image Refraction of light When light travels from one medium, to a medium of different density, light will be refracted. normal Incident ray Refracted ray Ray emerges parallel to incident ray air glass Angle of incidence Angle of refraction The diagram shows a ray of light travelling through the air, hitting a glass block and being refracted. Refraction calculations Refractive index the amount that light is bent by a medium refractive index = speed of light in a vacuum speed of light in medium Or using Snells Law refractive index = sin i sin r Where i is the angle of incidence and r is the angle of refraction Dispersion of Light When white light is refracted it splits into the colours of the spectrum. This is called dispersion. As with a block that has parallel sides, the light will come out parallel to the incident ray, when light is refracted by a prism it is deviated Total internal reflection The inside surfaces of transparent materials can act like a mirror. When light shines on these surface some is reflected and some refracted out of the material. Each material has a critical angle (c): if the incident angle is greater than this critical angle the result will be total internal reflection i refracted ray reflected ray air glass c no refraction Optical fibres Thin strands of glass that make use of total internal reflection. Digital signals in the form of pulses of light are transmitted along optical fibres. Uses include television and telephone signals to your house and they can be used as cameras for medical examinations Lenses 3.2 (c) Thin converging lens Describe the action of a thin converging lens on a beam of light Use the term principal focus and focal length Draw ray diagrams to illustrate the formation of a real image by a single lens Draw ray diagrams to illustrate the formation of a virtual image by a single lens Use and describe the use of a single lens as a magnifying glass 3.2 (d) Dispersion of light Give a qualitative account of the dispersion of light as shown by the action on light of a glass prism Converging Lens Parallel light rays passing though a converging lens converge on the other side Diverging light rays will emerge parallel to one another from a converging lens The point at which they meet is called the focal point The distance between the centre of the lens and the focal point is the focal length Formation of a real image by a single lens The image is inverted The image is real If the object is moved further away (beyond 2F) from the lens the image will be smaller If the object is closer to the lens (between F and 2F) the image will be larger If an object is closer to a converging lens than its principal focus, the rays never converge They form a virtual image behind the lens Converging lens as a magnifying glass The Electromagnetic Spectrum Electromagnetic radiation is a group of types of radiation. It travels as a transverse wave and in a vacuum all wavelengths travel at the same speed: the speed of light 3x10 8 m/s Higher frequency radiation has more energy and is more harmful Summary of Electromagnetic Radiation Frequency Type of electromagnetic radiation Typical useWavelength highestgamma radiationkilling cancer cellsshortest X-rays medical images of bones ultraviolet radiationsunbeds visible lightseeing infrared radiation optical fibre communication microwavescooking lowestradio wavestelevision signalslongest Sound Waves Core Describe the production of sound by vibrating sources Describe the longitudinal nature of sound waves State the approximate range of audible frequencies Show an understanding that a medium is needed to transmit sound waves Describe an experiment to determine the speed of sound in air Relate the loudness and pitch of sound waves to amplitude and frequency Describe how the reflection of sound may produce an echo Supplement Describe compression and rarefaction State the order of magnitude of the speed of sound in air, liquids and solids Sound Waves Sound waves are longitudinal waves. Sound waves are produced when objects vibrate. The vibrations travel in the same direction as the wave travels. At some points the air molecules are pushed together increasing pressure (compression). At other points the air molecules are further apart decreasing pressure (rarefaction). As a result the sound wave travels through the air, with the air particles vibrating backwards and forwards. c c c c c r r r r wavelength Sound in a Solid, Liquid and Gas Sound waves need a medium to travel through - a solids, liquids and gases. The particles making up solids are very close together, so they can transfer sound energy from particle to particle very quickly. In liquids the particles are relatively close together, so they can also transfer sound waves from particle to particle fairly quickly. However, in gases the particles are widely spaced, therefore it takes longer for the sound waves to pass from particle to particle. MediumSpeed (m/s) gas (air)340 liquid (water)1500 solid5000 The more dense the medium, the faster the sound waves travel through it. Measuring the Speed of Sound Use a data logger to measure the time it takes for the sound to get from one microphone to the other. Then use the speed equation. Sound and Amplitude When a wave travels through any material, it causes the particles making up that material, to move from their position of rest. The maximum movement away from the rest position is known as the amplitude. The greater the amplitude, the louder the sound, and the more energy it has. Sound and Frequency The faster that an object vibrates, the higher the pitch of the sound produced. The more frequent the vibrations, the greater the frequency. The frequency range for normal human hearing is between 20Hz and 20,000Hz. Practice calculations If a sound wave travels 1 m in 0.003s, what is its speed? If a sound wave has a speed of 340 ms -1, how far does it travel in 2s? For homework, find out what the speed of sound is in a solid a liquid and a gas Question A man stands in front of a wall and shouts. It takes 0.2s for the sound go to the wall and to get back to him (the echo time), or echo. What is his distance from the wall? (Using speed of sound in air 330ms -1 ) Hint work out TOTAL distance first! Echoes When a sound wave reflects off a hard surface and we hear it again this is an echo. The distance between a sound and an object can be found by using the wave equation but remember, it will be double the distance between the sound and the hard surface s = 2d/t Where s is the speed of sound in air (or whatever medium it is travelling through), d is distance and t is time. This method is called echo-sounding and has many applications Mapping the ocean floor You work aboard a scientific survey ship. Your job is to map the profile of the sea floor in the locations that you ship visits. The following is echo-sounding data between two points of the journey. Complete the table and calculate the distance from the bottom of the ship the sea floor. Use graph paper to plot a profile of this part of the sea floor. Distance along path (km) Echo time Distance What is this machine showing? Can you recall the wave equation? Making Waves Adjust the equipment for each of the following. Draw the waveform and describe how frequency and amplitude have been changed. 1.Loud sound with a high pitch 2.Loud sound with a low pitch 3.Quiet sound with a high pitch 4.Quiet sound with a low pitch Practice Calculations 1.Calculate the frequency of a wave with a speed of 1200 ms-1 and a wavelength of 3m 2.Calculate the wavelength of a wave with a speed of 200ms-1 and a frequency of 120Hz 3.Calculate the speed of a wave with a frequency of 200Hz and a wavelength of 2m. 4.Calculate the speed of a wave with a time period of 2s and a wavelength of 3m