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WAVES What are waves? a wave is a disturbance that transfers energy from place to place. ◦ Light energy moving from the computer screen to your eye moves as light waves. ◦ Sound energy moving from a radio to your ear moves as sound waves. What carries waves? A medium, a medium is the material through which a wave travels. ◦ Waves can move along ropes, strings or across the surface of water & even some can travel through space Wave which move energy from place to place are called progressive waves Waves are everywhere in nature Examples of waves ◦ Sound waves, ◦ visible light waves, ◦ radio waves, ◦ microwaves, ◦ water waves, ◦ sine waves, ◦ telephone chord waves, ◦ stadium waves, ◦ earthquake waves, ◦ waves on a string, What causes waves? Waves are created when a source of energy causes a medium to vibrate. A vibration is a repeated back and forth or up and down motion. So, vibrating object is a source of waves Wave Fronts Wave fronts are the lines or surfaces connecting the particles moving at the same phase and are at the same distance from a wave source. · Wave fronts are always perpendicular to the direction of propagation. TYPES OF WAVES There are two (2) types of waves: transverse and longitudinal waves Transverse waves the vibrations are at a 90 degree angle to the direction of travel of the wave. 1. Vibrations Direction n.a rasih/waves Examples = electromagnetic waves, ripples on water, waves on a rope, sine wave, n.a rasih/waves Parts of a transverse wave n.a rasih/waves 2. Longitudinal waves - In Longitudinal waves the vibrations are along the same direction as the wave. -Particles of the medium vibrate parallel to the direction of propagation of the wave - Example: sound waves & waves on a spring waves n.a rasih/waves Parts of a longitudinal wave A longitudinal wave, made up of compressions and rarefactions - Compressions are areas where particles are close together - Rarefactions - areas where particles are spread out. The particles move in a direction that is parallel to the direction of wave propagation. parts of a transverse waves Wavelength () Amplitude (A) Frequency (f) Period (T) Wavelength Distance between two neighbouring peaks (crests) or two neighbouring troughs on a distance graph Distance of two neighbouring points which are vibrating together in same way (in phase) Distance moved by the wave during one oscillation. Waveform showing wavelength and amplitude Displacement & Amplitude Displacement of a particle on a wave = its distance from its rest/equilibrium position ◦ Vector quantity (+ve or –ve) Amplitude = maximum displacement of a particle in the wave Represents the energy being transferred by the wave. Period & Frequency The period of a wave is the time for a particle in the wave to make one complete cycle/vibration. The frequency of a wave is the number of complete vibrations (cycles) per unit time or in one second. Frequency is measured in Hertz (Hz). 1 Hz = 1 cycle per second. Period (T) & Frequency (f) (cont) 1 f T The lower the frequency is the longer the time period will be. What is the formula for wave speed? For any set of waves, the wave speed (v) can be calculated from the frequency (f) and wavelength () using this formula: wave speed = frequency x wavelength v = f x What are the units of speed, frequency and wavelength? Wave speed is measured in metres per second (m/s). Frequency is measured in hertz (Hz). Wavelength is measured in metres (m). Can I use a formula triangle? A formula triangle helps you to rearrange a formula. The formula triangle for wave speed (v), frequency (f) and wavelength () is shown below. Cover the quantity that you are trying to work out, which gives the rearranged formula needed for the calculation. …which gives the formula… So to find frequency (f), cover up f… x f = v Examples: 1. A wave has frequency of 50 Hz and a wavelength of 10 m. What is the speed of the wave? solution : v = f λ = ( 50x10) = 500m/s 2. A wave has frequency of 5 Hz and a speed of 25 m/s. What is the wavelength of the wave? solutions: λ = v /f = (25/5) = 5m Everyday life applications of waves. Communication e.g Radio and TV waves. You can cook with waves. e.g Microwaves. Eels and snakes use transverse body waves to push against the water or ground to help them move. Ultrasonic or high-frequency sound waves have been used to clean jewelry and teeth. Burglar alarms can use the lazer light to detect motion in a room. Exercise: 1. A wave with a frequency of 14 Hz has a wavelength of 3 meters. At what speed will this wave travel? 2. The speed of a wave is 65 m/sec. If the wavelength of the wave is 0.8 meters, what is the frequency of the wave? 3. A wave has a frequency of 46 Hz and a wavelength of 1.7 meters. What is the speed of this wave 4.If 120 waves are produced per minute ,find (a)frequency (b) the period Homework Questions 1. A wave has a wavelength of 15 cm and has a frequency of 10 waves/second. What is the speed of the wave? 2. The speed of a wave on a rope is 50cm/s and it’s wavelength is 10cm. What is it’s frequency? 3. A wave is traveling with a velocity of 125m/s and has a frequency of 20 waves/second. What is the length of the wave? Dangers of waves{tsunami 2004} WAVE EFFEECTS - Waves can undergo reflection, refraction and diffraction. The properties of waves can be investigated using Ripple tanks n.a rasih/waves n.a rasih/waves Waves transfer energy without transferring matter. Frequency= waves/time 1. Reflection - - Reflection is the bouncing back of waves as they strike a barrier. The angle at which the waves strike the barrier is equal to the angle at which they leave the barrier. n.a rasih/waves 2. Refraction - As waves move from one medium to another, their speed and wavelength changes. - Frequency remains the same. - Refraction is the bending of a wave as it moves from one medium to another as a result of change in wave speed. n.a rasih/waves Refraction of water waves As water waves move from shallow to deep waters: a. b. c. Wave speed increases, Wavelength reduces Frequency remains unchanged. As water waves move from deep to shallow waters: a. Wave speed reduces b. Wavelength increases c. Frequency remains unchanged. Diffraction of water waves Diffraction is the ability of waves to bend round the sides of an obstacle or spread out as they pass through a gap. When waves pass a barrier they curve around it slightly. When they pass through a small opening, they spread out almost as if they had come from a point source. These effects happen for any type of wave: water; sound; light; seismic waves, etc.