The physicist's greatest tool is his wastebasket Albert Einstein

Transcription

1 Chapter 20: Waves

2 The physicist's greatest tool is his wastebasket Albert Einstein 2

3 20.1 Waves Describe transverse and longitudinal waves. Learn the properties of waves. Calculate the speed of a wave. Identify the fundamental and harmonics of a standing wave. 3

4 Waves A wave is an oscillation that travels from one place to another. Waves transport energy from one location to another. Waves can also carry information! Light and sound are waves, also radio waves! 4

5 Wave Examples Most TV remotes communicate via infrared waves. Remember waves carry energy energy that may be harnessed! X-rays penetrate soft tissue, but not hard bone. 5

6 Transverse and Longitudinal Waves Transverse Wave is a wave that vibrates perpendicular to the direction of wave motion. Waves on a string Light waves Longitudinal Wave is a wave that vibrates parallel to the direction of wave motion. Sound waves 6

7 What kind of wave is this? Transverse! 7

8 What kind of wave is this? Longitudinal! 8

9 Parts of a Wave The amplitude of a wave is the maximum amount the wave causes anything to move away from equilibrium. A crest is a high point of the wave, a trough is the low point. The wavelength is the distance from any point on a wave to the same point on the next cycle of the wave. The frequency of a wave is a measure of how often it goes up and down and is measured in Hertz. 9

10 Wave Speed The speed of a wave is the speed at which the wave s oscillations travel through a material. The material (or medium) itself never moves locations, it just oscillates up and down. The speed of a wave can also be measured as the distance travel divided by the time taken to travel that distance. v f v d t 10

11 Standing Waves A wave that is confined in a space is called a standing wave. Standing waves can be made from almost any kind of wave you probably made standing waves in your bathtub or pool as a kid. When a standing wave is produced, the medium resonates! Mediums generally have different natural frequencies. 11

12 20.2 Waves in Motion Learn how waves propagate. Describe the four wave interactions. 12

13 Wave Propagation Waves propagate, which means they spread out from where they begin. Plane waves move perpendicular to the wave fronts. Circular waves radiate outward from the center. But the direction is always perpendicular to the wave front. How could you make each type of wave to the right? 13

14 Huygen s Principle The physicist Christian Huygen saw all waves as being made up of an infinite number of tiny waves. Then, a complete wave pattern was merely the sum of these wavelets. 14

15 When a wave encounters objects Waves are affected by boundaries this is where the material or medium changes. All types of waves can experience any of the four phenomena shown on this slide. 15

16 Law of Reflection The Law of Reflection states that the angle of incidence (think incoming angle) is equal to the angle of reflection. After reflection, a wave has the same speed, frequency and wavelength, it is only the direction of the wave that has changed. What do we call the reflection of a sound wave? An Echo! 16

17 Refraction Refraction the bending of a wave occurs when a wave moves from one medium (i.e., material) into another. The frequency of the wave remains unchanged, but the speed most likely will change. This means the wavelength will also change. v f 17

18 Diffraction Diffraction occurs when a wave bends around obstacles or through openings. Diffraction is seen mostly when the obstacle or opening is roughly the same size as the wavelength of the wave. 18

19 Absorption Absorption is what happens when the amplitude of a wave gets smaller as it passes through a material. The wave energy is transferred to the absorbing material. Why does it seem to be very quiet after a snow fall? Snow is a good absorber of sound. Therefore little is reflected into the listeners ear. 19

20 20.3 Wave Interference Describe the superposition principle, and constructive and destructive interference. Review natural frequency and resonance. Learn about the relationship between wave energy and its properties. 20

21 The Superposition Principle Principle of Superposition when two or more waves overlap each other, their amplitudes combine algebraically. Constructive Interference results in larger amplitude waves Destructive Interference results in smaller amplitude waves. 21

22 The Superposition Principle These waves are in phase with each other. These waves are 180 degrees out of phase with each other. 22

23 Standing Waves & Resonance If you adjust the frequency of the vibration just right, you can set up standing waves. The wave traveling to the right will reinforce the reflected wave traveling to the left (constructive interference). Resonance in waves comes from the interference of a wave with its own reflections. N nodes (no amplitude) A antinodes (maximum amplitude) 23

24 Waves and Energy A wave is a form of moving energy. The energy of a wave is proportional to its frequency. The energy of a wave is also proportional to amplitude. 24