Properties of waves including light and sound

Transcription

1 Unit 1 Properties of waves including light and sound Wherever you live in Namibia you will have seen ripples in a river or oshana perhaps when you threw a stone in the water. If you have visited the coast you will have seen the waves in the sea. Do you know what a wave is? Look at this picture of a beach. Can you see the waves hitting the beach? In a river or oshana when birds swim, the water moves up and down causing the ripples to move onwards. At the coast, you might have noticed how the waves crash against the sea wall or the beach. To us, waves look like hills of water travelling towards the shore. But in fact, the water itself is not travelling at all. You may have noticed that seaweed is not carried along a wave. It just bobs up and down because the water moves up and down. The only thing that travels with the wave is energy. This unit is divided into two sections: A Waves in a spring B Waves in water When you have studied this unit you should be able to: define a pulse as a single disturbance in a substance define oscillations or simple harmonic motion as any repeated to-and-fro motion of a fluid or elastic solid, e.g. tuning-fork, pendulum or stretched string describe what is meant by wave motion (propagation) as illustrated by vibrations in ropes, springs and by experiments using water waves distinguish between transverse and longitudinal waves (in terms of pulse movement in relation to wave propagation and movement in a medium of different density) define speed, frequency, period, wavelength and amplitude

2 2 NSSC Physical Science remember and use the equation that contains v =f define the term wave front in wave motion as the adjacent surface points that are in the same phase explain and graphically represent the term wave front use the term wave front to describe what happens to water waves in a ripple tank undergoing: i reflection at a plane surface ii refraction due to a change of speed (use change in depth as change in medium) iii diffraction A wave allows energy to be transferred from one point to another without any particles of the medium travelling between the two points. For example, if a small weight is suspended by a string, repeatedly shaking the other end of the string up and down through a small distance will provide energy to move the weight. Waves, which carry energy, then travel along the string from the top to the bottom. Similarly, water waves may spread along the surface from point A to point B, where an object floating on the water will be disturbed by the wave. This disturbance will cause the object to pulse. No particles of water at A actually travelled to B in the process. When the bob of a pendulum moves to-and-fro through a small angle, the bob is said to be moving with simple harmonic motion. The prongs of a sounding tuning fork, and the layers of air near it, are moving to-and-fro with simple harmonic motion. In all cases wave motion is regular and repetitive. This movement is called oscillation or oscillations A Waves in a spring Some of the terms that we are going to use in this section might already be familiar to you. We are going to look at a rope or spring and carry out experiments to illustrate the terms: wave front, speed, frequency and amplitude. Waves A wave transports energy without displacing the medium or material through which a wave travels. The shape of a wave does not change. Only the size changes. We can show wave motion with a slinky spring. Activity 1 shows wave motion in a slinky spring.

3 Module 2 Unit 1 3 ACTIVITY 1 Spend about 10 minutes on this activity. This activity will show that there are two different kinds of wave. Apparatus spring or slinky (a coiled spring), fixed at one end (or you could do this with another person holding the end). Procedure Take a slinky spring like the one in the diagram and stretch it out along a bench top. Figure 1.1 A stretched slinky spring Hold the free end of the slinky spring and make waves by moving the free end to-and-fro as shown in figure 1.2. Your movements should look like those shown in figure 1.2 below. Figure 1.2 Using a slinky spring to make waves These waves are called transverse waves. Now you can make a different kind of wave. Hold the free end of the spring and make waves by pushing and pulling the spring. Figure 1.3 A longitudinal wave Your movements should look like those shown in figure 1.3. This shows a longitudinal wave. You will learn about this type of wave after we ve looked at the properties of waves.

4 4 NSSC Physical Science Properties of waves From Activity 1 we learned that there are two types of waves. One of them is called the transverse wave. Figure 1.2 illustrates a transverse wave. In transverse waves the oscillations (movements) are at right angles to the direction of energy (wave) movement. The movement of the slinky spring itself is from side to side. Transverse waves can be recognised by the pattern of the waves. The wave rises to the highest point and falls to the lowest point. The highest point on each wave is called a crest. The lowest point between the crest is called a trough. Look at figure 1.6 in Activity 2 on page 5. In that figure the points marked P, Q and R are crests. The points marked S, T and U are troughs. Some examples of transverse waves are light waves and water ripples on the surface of a pond. Other examples of transverse waves are radio waves, infrared waves and electromagnetic waves. Longitudinal waves Figure 1.4 shows a longitudinal wave. The wave is made up of compressions and expansions, which move along the slinky spring. A longitudinal wave is a wave in which the oscillations are along the direction of the wave movement. Sound waves are longitudinal waves. Figure 1.4 A longitudinal wave Describing waves You can see the terms we use to describe the waves on this displacement-distance graph. Figure 1.5 Properties of waves Wavelength Wavelength is the distance between two successive identical points on a wave and is represented by the Greek letter (lambda).

5 Module 2 Unit 1 5 Amplitude Amplitude is the strength of a wave. It is one half the distance from the bottom of the trough to the top of a crest. It is represented by the letter a. Frequency Frequency is the number of oscillations that pass a given point in one second, frequency is measured in hertz (Hz) and is represented by f. One hertz is one wave per second. The higher the frequency, the more energy you will get from waves. Speed/velocity The speed of a wave is the distance moved by one complete wave in one second and is denoted by v. The shape and amplitude of a wave do not affect its speed. But waves that travel through different materials travel at different speeds. Velocity, wavelength and frequency are related by the formula: Velocity = wavelength frequency v= f ACTIVITY 2 Spend about 10 minutes on this activity. Figure 1.6 A wave 1 a What type of wave is shown in figure 1.6? b Give two examples of each of the following: i Transverse waves. ii Longitudinal waves. c Distinguish between longitudinal and transverse waves: You have finished examining the types of waves on a spring. Now we will look at the behaviour of waves in water.

6 6 NSSC Physical Science B Waves in water Have you ever played with a small ball such as a tennis ball? If you bounce the ball on the floor, it comes back to you. If you throw it against a wall, it bounces and comes back to you. I hope you are familiar with this game. Waves bounce off barriers in the same way as a ball does. This is called reflection. In this section you are going to study some more properties of waves. We talked earlier about ripples, and about the small circular waves that move from the position where a stone is dropped. We are going to learn more about these ripples in this section. Properties of water waves If you are able to watch waves at the sea shore you might notice many things happening to them. For example, as plane waves (waves with a straight wave front) aproach the shore, their wavelength gets smaller. When the waves strike the shore straight on, they are reflected (turned back). The reflected waves will bounce back in the direction they came from. If the wave strikes the shore at an angle, it bounces off at the same angle in the opposite direction. We are describing a wavefront. This is any line or section taken through an advancing wave that joins all points that are in the same positions as their oscillations. Wavefronts can have any shape, e.g. circular or straight, and are at right angles to the direction of the waves. For example, a wave stretching along a beach is a wavefront. Another example is the circular ripples that spread out when a stone is dropped into a pond. When waves enter shallow water, they slow down. The first part of each wave to reach shallow water will slow down before the rest of the wave. This causes the wavefront to change direction. The bending of waves caused by a change in wave speed is called refraction. Some of the properties of water waves such as reflection, refraction and diffraction can be shown by using a ripple tank. The following practical activity will help you to understand this phenomenon. ACTIVITY 3 How to show reflection and refraction in water waves Spend about 20 minutes on this activity. Read through the activity and try to do the experiment if you have the apparatus. Apparatus a ripple (a shallow tray with a glass bottom and shelving) a lamp a metal or plastic rod (about 3 cm) a vibrator water a pencil a thick sheet of glass sides (these reduce reflections from the edges).

7 Module 2 Unit 1 7 Procedure 1 pour water into the tray switch on the lamp place a straight metal or plastic rod in the tank so that it reflects the waves (like a mirror) draw a sketch and record your observations. 2 touch the water with the tip of your pencil record your observations place the thick sheet of glass in the tank and adjust the depth so that there is very little water over the glass sheet send waves across the tank and look at the waves when they travel through the shallow water. Figure 1.7 A wave tray Results Figure 1.8 shows what you should have seen. If plane waves strike a straight barrier, reflected plane waves are produced. I am sure you got a similar result. Figure 1.8 shows the waves before and after they are reflected by the straight barrier. The angle of incidence and the angle of reflection are the same. This is one law of reflection. You will study this in the next unit. Figure 1.8 Reflection of waves in a wave tray When circular waves hit a straight barrier, the reflected waves appear to be spreading out from a point behind the barrier. The reflected waves are circular and have a centre that is an equal distance behind the barrier, creating a virtual image of the source. This is a practical demonstration of a wavefront. Figure 1.9 A wavefront

8 8 NSSC Physical Science Do not confuse a wavefront, which occurs in water, with a longitudinal wave in sound (see page 40 in this module). A wavefont in water is any line or section taken through a wave moving forward which joins all the points of this wave that are in the same position as their oscillations. You can think of it as a collection of waves and it can have any shape, for example, circular or straight. Figure 1.9 on page 7 shows a circular wavefront. Refraction and diffraction Refraction Refraction is the change in direction of a wave when it travels into a new medium or density which makes it travel at a different speed. A wave that has been refracted is called a refracted wave. When you are looking at water waves in a wave tray, the medium and the density of the water do not change, but you still see refraction patterns. This is because the change in the depth of the water in the wave tray is the same as a change in medium or density. Diffraction Diffraction is the bending effect which occurs when a wave meets an obstacle or has to move through a small space. The amount by which the wave bends depends on the size of the obstacle or the size of the space. The smaller the obstacle or space the more the wave bends. Refraction of straight waves The frequency of a series of waves depends on the frequency of the source of the waves. Whatever happens to the waves, their frequency cannot change. If the waves enter a region where they travel more slowly, then their wavelength will decrease but there is no change in frequency. When waves enter shallow water, they slow down as shown in figure The waves change direction, bending away from the normal when they pass from shallow to deeper water. This change in direction occurs because the waves speed up when they pass into deeper water. Figure 1.10 Refraction of straight waves Showing diffraction of straight waves using a small gap When waves pass through a gap or hole, they tend to spread out. The larger its wavelength in relation to the size of the hole, the more they spread out. This is called diffraction. This is the bending effect that occurs when a wave meets an obstacle or passes through a gap.

9 Module 2 Unit 1 9 Figure 1.11 Diffraction of waves through a gap ACTIVITY 4 Spend about 15 minutes on this activity. 1 a Complete these sentences. 2 i All waves carry from one place to another. ii There are two types of waves. In a wave, the vibration is at right angles to the direction of propagation. In a wave the vibrations are in the same direction as the direction of propagation of the waves. iii The number of waves per passing any point is the of the waves and is measured in the is the distance between wave fronts a b c Figure 1.12 Moving waves i What happens to the waves in figure 1.12? ii Complete the diagrams. Summary Wave motion is a transfer of energy from a vibrating source. Waves can be of two kinds, transverse and longitudinal. Waves have amplitude (a), wavelength ( ), speed (v) and frequency (f) Frequency and amplitude depend on the energy of the source. Wave speed depends on the properties of the substance or medium in which waves are carried.

10 10 NSSC Physical Science The equation v = f, applies to all kinds of waves. Waves are reflected according to the law: angle of incidence (i) = angle of reflection (r), (i=r). Waves are refracted according to the law of refraction (where, pronounced nu, is a constant called the refractive index). Waves can be diffracted. Waves from many sources (or a single source and its reflected images) pass through each other without loss of energy or change of direction. Check your progress After you have completed this unit, try this little exercise without looking back through the sections. It is a good exercise for the exams. Circle the letter that represents the correct answer. 1 What happens when waves move from deep to shallow water? (1) a b Their wavelength decreases, because their velocity decreases. Their wavelength decreases, because their velocity increases. c Their frequency increases, because their velocity increases. d Their frequency decreases, because their velocity increases. 2 The bending of waves round an obstacle is known as. (1) a diffraction b interference c reflection d refraction. 3 Which row in the table correctly shows the nature of these waves? (1) transverse longitudinal a light radio b radio sound c sound water d water light 4 Which distance on the diagram represents the amplitude of a wave? (1)