L E A R N I N G O U T C O M E What are waves? Why are waves formed? Waves Y E A R 1 0, C H A P T E R 8 G J Z A H R A, B. E D ( H O N S ) Why does a pool filled with water look shallower than it really is? Are all waves the same or are there different waves? Why cannot sound be heard in outer space? What happens when waves hit a wall or other surface? What happens to light waves when they pass from air to water? What happens to waves when they pass through a gap? How can waves be produced and studied in a school laboratory? What do terms like frequency, wavelength and amplitude mean? Name and Surname: Class:
Practice Exercise 8A P A G E 2 8.1 What are Waves? There are many different waves around us: water waves, light waves, sound waves, ultraviolet (UV) waves, radio waves, etc. However, all waves involve a transfer of energy from one place to another. A wave is the transfer of energy from one place to another. A Mexican wave moves across a whole stadium, but the people all remain in their seating position. If you hold a slinky coil, there are two ways in which you can transfer energy from one place to another: moving it vertically or horizontally. There are two types of waves, transverse and longitudinal. In a transverse wave, particles oscillate at 90 to the direction of wave travel In a longitudinal wave, particles oscillate in the same direction of wave travel Longitudinal waves involve the oscillation of particles parallel to the direction of the transfer of energy. A medium is always required for longitudinal waves. Transverse waves involve the movement of the wave perpendicular to the direction of transfer of energy. Some transverse waves travel through a medium, such as water waves, while others do not require a medium, such as light waves. When a wave moves through a medium, the particles do not change position but oscillate in the same position. A pulse is one disturbance moving through a medium. Fill in the table below Longitudinal Transverse Examples Movement of particles with respect to the direction energy transfer
P A G E 3 8.2 Wave Properties Longitudinal waves have parts in which the particles are close together followed by parts where particles are far apart. These regions are called compressions and rarefactions respectively. On the other hand, transverse waves have high parts called crests and low parts called troughs. Every wave, whether longitudinal or transverse, has some basic features like: Earthquakes produce both transverse and longitudinal waves. Information about how such waves travel through a planet, gives astronomers details about a planet s composition. The amplitude, A, of a wave is the maximum distance between the part of the wave where there is zero particle displacement to the part where there is maximum particle displacement. In a transverse wave this is simply the distance between a crest and the centre position. It is measured in metres, m. The wavelength, λ, of a wave is the horizontal distance over which the wave s shape repeats itself. It is the length of one wave. It is measured in metres, m. The frequency, f, of a wave is the number of waves per second. It is measured in hertz, Hz. The periodic time, T, is the time taken for one single wavelength. It is measured in seconds, s. In the ground, seismic longitudinal waves travel faster than the deadlier transverse waves. These give a warning of a few seconds or minutes prior to the earthquake s full devastating power. The higher the amplitude of the transverse wave, the more devastating the earthquake will be. The velocity, v, of a wave is the speed and direction of the wave in metres per second, m/s. In this diagram label a compression, a rarefaction, a crest, a trough and an amplitude. For both waves mark and label the wavelength
Practice Exercise 8B P A G E 4 8.3 Wave Equations The frequency, f, is the number of waves per second. This means that mathematically the frequency can be defined as: f number _ of _ waves time f = number of waves per second On the other hand, periodic time, T, is the time taken by one complete oscillation. This means that: f = 1 / T T time number_ of _ waves T 1 f v = f λ The velocity is the distance travelled per unit time. Since the time taken for a wave to ravel a distance of one wavelength is the periodic time, the equation can be changed into: v s t T By substituting T with 1/f, the common wave equation is obtained: v f 1. Right now there are approximately 6 x 10 14 light waves entering our eyes every second. What is the frequency of light waves? Calculate the periodic time. 2. A boat passes near a buoy which oscillates 30 times in 10 seconds. Calculate the frequency of the wave. 3. If the distance from one crest to the next in the previous question is 1.2m, calculate the speed of the wave. 4. A sound wave travels through a liquid with a wavelength of 1.5m and a frequency of 300Hz. What speed is the wave travelling at? 5. A sound wave travels through a solid with a wavelength of 210cm and a frequency of 300Hz. What speed is the wave travelling at? 6. A sound wave has a speed of 340m/s and a frequency of 2kHz. What is the wavelength of the wave? 7. Mobile phones operate at a frequency of around 950MHz. The waves have a wavelength of 31.6cm. Find the velocity of the wave. Hence determine the time taken for the wave to travel once round the earth (distance = 4.0x10 7 m). 8. In a water wave, the horizontal distance between a crest and a through was observed to be 45cm and took 0.75s to pass through a given point. Calculate the wavelength and periodic time of the wave. Hence calculate the velocity of the wave.
G J Z A H R A, B. E D ( H O N S ), S T A L B E R T T H E G R E A T C O L L E G E Properties of Waves P A G E 5 Name and Surname Class Look at the waves below and answer the following questions EX 8C 1. Which of the waves has the largest: a. Amplitude? b. Wavelength? c. Frequency? (3marks) 2. Are the waves transverse or longitudinal? Explain your answer. (3 marks) 3. The above waves cannot be (cross out the incorrect option): sound waves / light waves / water waves / seismic (earth quake) waves. (1 mark) 4. On wave (a) label a crest and a trough (2 marks) 5. Look at wave (b): a. How many waves are present? (1 mark) b. If the time is 2.25s, find the frequency of the wave. (2 marks) c. Find the periodic time for the wave (2 marks) d. Calculate the wavelength if the speed of wave (b) is 4 m/s (2 marks)
P A G E 6 8.4 Reflection, Refraction and Diffraction Typically, there is a large number of waves in a system. Also, drawing all waves can look rather confusing. In physics we use wavefronts to avoid drawing each and every wave. Wavefronts also result in simpler diagrams. A wavefront is an imaginary line joining points on different waves which are in phase (for example, joining the crests of different water waves as shown in the diagram). The distance between two wavefronts is the wavelength of the wave. When waves hit a surface, they are usually reflected backwards. In reflection, the angle with which the waves hit the surface, called the angle of incidence, is equal to the angle with which waves leave the surface, called the angle of reflection. The angle of incidence and the angle of reflection are measured from a line called the normal which is an imaginary line at 90 to the surface. Reflection of water waves is easy to see whenever waves hit a surface. Reflection of light waves is essential for us to see things around us. Not all surfaces are perfectly straight. When waves hit a converging surface, they converge (come together). On the other hand, if they hit a diverging surface, they diverge (spread apart).
P A G E 7 Water waves are refracted if they move from deep to shallow water or vice versa. In shallow water, the wavelength of water waves is shorter than that in deep water. Refraction is when a wave changes the medium in which it is travelling. When a wave changes its medium, its wavelength and velocity (both speed and direction) change. For example, when light waves travelling in air enter water, they change direction. Their wavelength decreases as the new medium (water) is said to be optically denser than air. The velocity increases. In refraction, the frequency never changes. As a result of refraction of light in water, objects in the water appear closer than they really are. When a wave passes through an opening, some of the waves change direction and the wavefronts can be seen to lose their perfect linearity. If a gap is small compared to the wavelength of the wave, diffraction will occur. Diffraction is when waves spread after passing through a gap. The speed, frequency and wavelength are not affected by diffraction, but the waves change their direction of travel. Since sound waves have a wavelength of about 1m, while light waves have a wavelength of about 480 750 nm, sound waves are diffracted through door ways while light waves are not. This explains why we can hear someone talking behind a doorway while we cannot see him/her.
P A G E 8 Reflection, refraction and Diffraction of Waves E X E R C I S E 8 D G J Z A H R A, B. E D ( H O N S ), S T A L B E R T T H E G R E A T C O L L E G E 1. Copy and complete the diagrams then answer the following related questions A B C D E F 2. WHICH OF THE DIAG RAMS SHOW a. Reflection? b. Diffraction? c. Refraction? 3. T H E SPEED AND WAV ELENGT H CHANGE ONLY IN ONE OF TH E A B O V E SIT U A T I O N S a. In which situation do the speed and wavelength vary? b. If the wavelength decreases, but the frequency remains constant, how does the velocity change? 4. A P A R T I C U L A R S O U N D H A V E OF FREQUEN C Y 500Hz T R A V ELS A T 3 3 0 m / s a. Calculate the wavelength of the wave b. Calculate the periodic time of the wave
Diffraction in a Ripple Tank Refraction in a Ripple Tank Reflection in a Ripple Tank P A G E 9 8.5 The Ripple Tank The ripple tank consists of a glass, shallow tank filled with some water as shown in the diagram. Straight waves are created by an oscillating paddle which dips in and out of the water. Circular waves can be created by attaching a circular object, called a dipper, to the motor. A source of illumination is placed on top of the tank so that the waves are more clearly viewed on a screen below the tank. The screen below the tank shows wavefronts, and so the wavelength of the waves can be measured using a ruler. The frequency of the waves is equal to that of the oscillating paddle, which is given by the motor. Therefore the frequency of the waves can be changed by changing the speed of the motor. Objects can be placed in the ripple tank to act as surfaces along which the water is reflected or diffracted. Other objects can be immersed to made shallow areas where the water will be refracted.
P A G E 10 Practice Exercise 8E I N S T R U C T I O N S : Answer all questions 1a. In the adjacent picture, which letter marks the: Show all your working Do not write on this sheet i. wavelength? ii. amplitude? 1b. Copy part of the diagram and mark a crest and a trough. Use of calculators is allowed E Q U A T I O N S : v = fλ 2a. Draw a longitudinal wave and on the diagram mark the following: i. A compression ii. A rarefaction iii. The wavelength v = s/t 3a. What is the difference between a transverse and a longitudinal wave? 3b. Give an example of each type of wave. 4a. Fill in the following table about three different waves: M A R K S : Question 1 1+1+2 Question 2 1*3 Question 3 2+2 Question 4 (3*2)+1+2 /4 /3 /4 /9 Wave Velocity, m/s Frequency, Hz Wavelength, m A 3 0.15 B 200 5 C 3x10 8 3 4b. Which of the waves above will not be diffracted through a door 1.5m wide? 4c. Wave C is a radio wave. If the wave is transmitted from a radio station 12km away, how long will it take to reach its destination? 5. Copy and complete the diagrams below to show what happens when the waves hit the barrier or pass through the gap Question 5 2*4 /8 Question 6 2 /2 Total /30 6. Draw a diagram to show refraction of water waves when they travel from deep to shallow water.