Introduction to Waves

Transcription

1 Introduction to Waves 1

2 What s a wave? A wave is a disturbance that transfers energy from one place to another. The direction of energy transfer is the direction of propagation of the wave. 2

3 Transverse and Longitudinal Waves A transverse wave is a wave in which the disturbance is at right angles to the direction of propagation of the wave. Ex. A wave set up on a guitar string. Ex. Electromagnetic waves are transverse waves. A longitudinal wave in a wave in which the disturbance is along the direction of propagation of the wave. Ex. Sound waves travel longitudinally. 3

4 Transverse and Longitudinal Waves Crest Crest Crest Trough Trough Compression Rarefaction 4

5 Parts of Waves Crests The highest points on a transverse wave. Troughs The lowest points on a transverse wave. Compression The region of a longitudinal wave in which the distance separating the vibrating particles is less than their equilibrium distance. Rarefaction The region of a longitudinal wave in which the vibrating particles are farther apart then their equilibrium distance. 5

6 Mechanical Waves All waves that travel through matter are known as mechanical waves (also sometimes called matter waves). Sound waves and water waves are examples of mechanical waves. The speed of a matter wave in a particular medium is a characteristic property of that medium. While the disturbance, and the energy it carries, moves through the medium, the matter does not experience net movement. Instead, each particle in the medium vibrates about some mean (or rest) position as the wave passes. 6

7 Electromagnetic Waves Electromagnetic waves transmit energy just as mechanical waves do. However, they do not require a medium and can travel through a vacuum. All electromagnetic waves travel at the same speed in a vacuum, 3.00x10 8 m/s. Electromagnetic waves have an electric component and a magnetic component, each being perpendicular to the other and both perpendicular to the direction of propagation. 7

8 Electromagnetic Waves 8

9 Describing Waves Amplitude (A) The maximum displacement from the equilibrium position. (measured in m) Amplitude (A) Amplitude (A) Amplitude (A) 9

10 Describing Waves Wavelength (λ) the distance between any two successive points in phase. (measured in m) Wavelength (λ) Wavelength (λ) Wavelength (λ) 10

11 Describing Waves Frequency (f) the number of cycles per unit time. (measured in Hz or s 1 ) Period (T) the amount of time for one complete cycle. (measured in s) Speed (v) the distance per unit time a wave travels in its direction of propagation. (measured in m/s) 11

12 The Wave Equation 12

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15 Problems: Textbook Page 341, #1 to 4 Textbook Pages 349/350, # 5 to 9 15

16 The Five Properties of All Waves 16

17 The Ripple Tank To study waves, we will use a ripple tank. A ripple tank works by having a wave generating probe produce a continuous wave in the water. The ripple tank has a glass bottom that allows light to be projected down through the water and the glass onto a screen below the table 17

18 Virtual Ripple Tank etank/ripple.htm 18

19 Wave Fronts and Rays The portions of water surface whose particles are all in the same phase of motion are called wavefronts. The direction of travel is indicated by a directed line segment called a ray. 19

20 Property 1: Rectilinear Propagation Waves travelling in a uniform medium propagate in straight lines perpendicular to the advancing wave fronts. 20

21 Property 2: Reflection The bouncing back of a wave from the boundary of a medium. 21

22 Property 3: Refraction The bending of a wave as it passes from one medium to another in which it has a different wave speed. 22

23 Property 4: Diffraction The spreading of a wave beyond the edge of a barrier or aperture. Note that diffraction effects are most noticable when the wavelength of the wave is comparable to the size of the barrier or aperture. 23

24 Property 5: Interference The superposing (overlapping) of one wave onto another when they are in the same place at the same time. 24

25 Constructive Interference When two waves are added together and the resulting displacement for a given particle is greater than it had been in either of the waves being added, the waves are said to interfere constructively. 25

26 Destructive Interference When two waves are added together and the resulting displacement for a given particle is less than it had been in either of the waves being added, the waves are said to interfere destructively. 26

27 Interference Patterns 27

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30 The Five Properties of All Waves 1. Rectilinear Propagation Waves travelling in a uniform medium propagate in straight lines perpendicular to the advancing wave fronts. 2. Reflection The bouncing back of a wave from the boundary of a medium. 3. Refraction The bending of a wave as it passes from one medium to another in which it has a different wave speed. 4. Diffraction The bending of a wave as it moves around a barrier or through an aperture. 5. Interference The superposing (overlapping) of one wave onto another when they are in the same place at the same time. 30

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