INTRODUCTION TO WAVES Dr. Watchara Liewrian
What are Waves? Rhythmic disturbances that carry energy without carrying matter
Types of Waves Mechanical Waves need matter (or medium) to transfer energy A medium is the substance through which a wave can travel. Ex. Air,water,strings, solids, liquids; gases Electromagnetic Waves DO NOT NEED matter to transfer energy They do not need a medium, but they can go through matter (medium), such as air, water, and glass
Disturbance moves (propagates) with velocity v (wave speed) The wave speed is not the same as the speed with which the particles in the medium move TRANSVERSE WAVE: particle motion perpendicular to direction of wave propagation LONGITUDINAL WAVE: particle motion parallel/antiparallel to direction of propagation No net motion of particles of medium from one region to another: WAVES TRANSPORT ENERGY NOT MATTER
transverse and longitudinal wave motion
Transverse Wave
longitudinal wave
WATER WAVE (both transverse and longitudinal wave motion)
Waves - Terminology Frequency The number of vibrations or oscillations per unit time. Unit Hertz (Hz) derived. Period Time required for one complete cycle or to move the linear distance of one wavelength. Unit second (s) Fundamental. Amplitude The maximum displacement of a particle of the medium from the rest position. Unit meter (m) Fundamental. Wavelength The distance traveled by one wave in one period. The distance between two consecutive points in phase. Unit Meter (m) Fundamental.
Parts of a Transverse Wave The crest is the highest point on a wave.
Parts of a Transverse Wave The trough is the valley between two waves, is the lowest point.
Parts of a Transverse Wave The wavelength is the horizontal distance, either between the crests or troughs of two consecutive waves.
Parts of a Transverse Wave The amplitude is the peak (greatest) value (either positive or negative) of a wave. The distance from the undisturbed level to the trough or crest.
An ocean wave is an example of a mechanical transverse wave
Parts of a Compressional Wave (Longitudinal) The compression is the part of the compressional wave where the particles are crowded together.
Parts of a Compressional Wave (Longitudinal) The rarefaction is the part of the compressional wave where the particles are spread apart.
Parts of a Compressional Wave (Longitudinal) The wavelength is the distance from compression to compression or rarefaction to rarefaction in a compressional wave.
Traveling Wave Characteristics Frequency is the reciprocal of Period f = 1/T or T = 1/f Determine the frequency of a wave with a period of 0.01667 sec. f =1/T, f = 1/ (0.01667 sec) = 60 Hz What happens to the period of a wave as its frequency increases?
Wave Speed wave equation Wave equation can be derived from the kinematics equation v = d/t If d=λ(wavelength) and t=t(period), then Since f = 1/T, then v= λ/t v = f λ
Wave Speed A sound wave in a steel rail has a frequency of 620 Hz and a wavelength of 10.5 m. what is the speed of sound in steel? v=f λ v= 620hz (10.5 m) v= 6510 m/s
Waves on a Pond A pebble thrown into a pond will produce concentric circular ripples which move outward from the point of impact. If a fishing float is in the water, the float will bob up and down as the wave moves by. This is a characteristic of transverse waves. Such waves obey the wave relationship. http://ef.engr.utk.edu/hyperphysics/hbase/sound/wavplt.html
SUMMARY
Wave Phenomena Huygens Principle: - Wavelets! This principle uses the wave concepts to explain periodic wave phenomena. Reflection Refraction Diffraction Wave-fronts Sun Ripple
Reflection Law of Reflection The angle of incidence equals the angle of reflection. Θ i = θ r The incident and reflected rays lie in the same plane with the normal.
Refraction Sudden change in direction of a wave as it changes speed. It must enter obliquely to change direction!
Refraction In both cases the speed of the wave has decreased. This is indicated by the decrease in wavelength!
Refraction of Sound When a wave slows down it bends closer to the normal. When a wave speed up it bends away from the normal.
Diffraction The bending or spreading out around the edges of a barrier or obstruction. Does the speed change? No! You can see the wavelength is constant.
Diffraction The extent of the diffraction depends on the ratio of the wavelength to the opening of the hole. Diffraction ~ λ/d Tsunami Waves
Interference Constructive Interference Destructive Interference Waves DO NOT bounce! Energy passes through.
Superposition The Algebraic sum of the amplitudes of two or more waves which form interference. Waves which arrive in phase form constructive interference. Waves which meet out of phase form destructive interference.
Standing or Stationary Waves Conditions need: Same Amplitude Same frequency Opposite Directions Caused by both Constructive and Destructive interference. Nodes Destructive Antinodes - Constructive