Waves.notebook. April 15, 2019

Transcription

1 Waves You will need a protractor! What is a wave? A wave is a vibratory disturbance that propagates through a medium(body of matter) or field. Every wave has, as its source, a particle vibrating or oscillating. Examples include sound waves made by tuning forks or guitar strings and a radio wave can be made by vibrating an electron through a transmitter. 1

2 Waves and Energy Transfer Waves transfers energy from one place to another by repeated small vibrations of particles of a medium or through small changes in a field. The source provides the vibrations but no actual mass is transferred from the source. Only energy is transferred from the source. Two types of waves will be discussed: 1. Mechanical Waves requires a material medium to flow through. (i.e. Water waves and sound waves) 2. Electromagnetic Waves Do not require a medium and can move through a vacuum(empty space) 2

3 Pulses and Periodic Waves A wave can be classified as either a pulse or a periodic wave. A pulse is a single short disturbance. When a pulse moves through a medium and strikes the boundary between two media, some of the wave gets absorbed and part of it is reflected. A periodic wave is several repeated pulses that are evenly timed. 3

4 Types of Wave Motion A wave is a vibratory disturbance that propagates through a medium. If the propagation is parallel to the disturbance, this is called a longitudinal wave. Sound is the most important example here. If propagation is perpendicular to the disturbance, this is a transverse wave. All electromagnetic waves are transverse. Water waves are transverse. 4

5 Characteristics of Periodic Waves Frequency the complete series of changes at one point as a wave passes is called a cycle. The number of cycles, or complete vibrations per second is called the frequency, f. The unit for frequency is known as the Hertz(Hz) or (s 1 ) Frequency of a sound wave determines pitch while in light determines color. Period is the time it take for the wave to do one complete cycle. Period is denoted by symbol T and has the unit of seconds(s). It is the inverse of frequency. Amplitude(height of the wave) is a measure from the equilibrium position(0 position) to its maximum height. In a transverse wave this is from midpoint to crest, or midpoint to trough. Amplitude in sound determines the loudness, while in light determines the brightness. Wavelength the distance between two points in a wave that are considered in phase(two crests, two troughs) 5

6 Phase: Two points on successive wave cycles that are displaced by the same position. 6

7 Speed of a wave: The speed of a wave is equal to frequency times the wavelength This formula is valid for waves in all media. The speed of a wave depends on the media it is traveling through. Sound travels at different speeds in water and in the air. Speed of sound in air: 346 m/s at 25 degrees Celsius Speed of sound in water: 1498 m/s Speed of light in air/vacuum: 3 * 10 8 m/s 7

8 Periodic Wave Phenomena Wave fronts when water drips into a faucet it forms concentric rings that radiate outwards, these concentric rings create wave fronts, point on a wave that are considered in phase Doppler Effect The perceived change in frequency as the source of the wave moves towards or away from an observer. 8

9 Interference Waves can interfere with each other. When they will combine under the principles of super position. Their amplitudes will either construct or destruct depending on the shape of the previous waves. Constructive interference: The amplitudes of the waves will work together to create a greater amplitude. Points of maximum constructive interference create an antinode. Destructive interference: The amplitudes will work against each other and create less amplitude. If two waves have equal and opposite amplitudes they will destruct completely and form a node, a section of 0 amplitude. 9

10 Wave interference = crest = trough Yungs Double Slit Experiment showed light has wave properties. Light shown through two slits created a diffraction pattern, bright dark bright dark. Standing Waves Standing Waves when two waves with the same amplitude and frequency travel in opposite directions a standing wave is formed. It is called standing wave because it looks stationary. The standing wave will have nodes and antinodes. If both ends are fixed there will be one less antinode than node. 10

11 Resonance All elastic bodies have a natural frequency, a frequency that they vibrate when disturbed. When a periodic wave is applied to an elastic body the vibration increases. This will cause an increase in the objects amplitude(more amplitude more energy). This is called resonance. The frequency of the wave must be the same as the objects natural frequency. Example: A singer breaking glass with their voices. The collapse of the tacoma narrows bridge Diffraction The spreading of a wave into the region behind a barrier in the wave's path is called diffraction. As parallel wave fronts move toward a barrier it will bend around a barrier and create concentric semicircular fronts. 11

12 Recap Sound longitudinal frequency = pitch amplitude = loudness faster in water than in air Light transverse frequency = color amplitude = brightness 12

13 Electromagnetic Radiation Classically, electromagnetic radiation consists of electromagnetic waves, which are synchronized oscillations of electric and magnetic fields that propagate at the speed of light, which, in a vacuum, is commonly denoted c. In homogeneous, isotropic media, the oscillations of the two fields are perpendicular to each other and perpendicular to the direction of energy and wave propagation, forming a transverse wave. The wavefront of electromagnetic waves emitted from a point source (such as a light bulb) is a sphere. The position of an electromagnetic wave within the electromagnetic spectrum can be characterized by either its frequency of oscillation or its wavelength. Electromagnetic waves of different frequency are called by different names since they have different sources and effects on matter. 13

14 EM Spectrum 14

15 Light The human eye can only perceive a small fraction of the entire electromagnetic spectrum. The EM spectrum ranges from gamma rays to radiowaves with gamma rays having the highest frequency. Ray Diagrams To explore how light behaves when traveling through different mediums we will use ray diagrams. We will imagine the wave fronts of light as a single ray. Unless otherwise told we will assume the EM wave is traveling through a vacuum. We will find that waves traveling through different media will have different speeds. V = (lambda)(frequency) 15

16 Because speed of a wave depends on the media it is traveling through we will use the frequency of light to determine its color. (not the wavelength!) ex: A beam of light traveling through space has a wavelength of 650nm. Determine the color of the light. 16

17 What happens when waves hit a boundary between two media? Our analysis thus far has been of waves traveling through a specific uniform medium. But what happens when waves travel between media? Reflection When a wave reaches the boundary(interface) between two media part of the wave gets reflected. (In sound waves this is an echo.) Reflection angle depends on the angle the original wave made with the boundary's normal. Law of reflection: Angle of incidence is equal to the angle of reflection Incident ray: the original ray, interacting with a boundary Angle of incidence: angle the incident ray made with the normal to the boundary. 17

18 Diffuse Reflection Diffuse reflection is the reflection of light or other waves or particles from a surface such that a ray incident on the surface is scattered at many angles rather than at just one angle 18

19 Using a protractor to find angle of reflection

20 Refraction When a ray strikes a boundary part of the ray will be reflected and part of ray will penetrate through the boundary. When light travels through a medium that isn't a vacuum, the speed of the light will change, therefore there will be a change in the wavelength, THE FREQUENCY REMAINS UNCHANGED. (frequency is determined by the source) The absolute index of refraction The absolute index, n, is the ratio of the speed of light in a vacuum,c, to the speed of light in a material medium,v. The index of refraction is unitless. The greater the value n, the more optically dense the material is and therefore the slower the light will travel through that medium. The indices given on the reference table are for a specific frequency of light, we will learn later that colors of light will experience different indices. 20

21 Use the reference table to determine the speed at which light will travel in water. 21

22 Refraction The bending straw/ pencil. Why does light bend? Light will try to travel the path of least time. We can measure how much the light will bend using snell's law. 22

23 Example: A ray of monochromatic yellow light(f = 5.09*10 14 Hz) travels at a speed of 1.24*10 8 m/ s while in a medium, determine the medium. 23

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