What you will learn: You will determine how waves transfer energy You will describe wave reflection and discuss its practical significance
Why it’s important: Waves enable the sun’s energy to reach Earth and make possible all communication through sound. Because light waves can be reflected, you are able to see the world around you and even read these very words. Knowledge of behavior of waves is essential to the designing of bridges and many other structures.
State Standards Addressed Waves Waves have characteristic properties that do not depend on the type of wave. As a basis for understanding this concept: Students know waves carry energy from one place to another. Students know how to identify transverse and longitudinal waves in mechanical media, such as springs and ropes, and on the earth (seismic waves). Students know how to solve problems involving wavelength, frequency, and wave speed. Students know sound is a longitudinal wave whose speed depends on the properties of the medium in which it propagates. Students know radio waves, light, and X-rays are different wavelength bands in the spectrum of electromagnetic waves whose speed in a vacuum is approximately 3×10 8 m/s (186,000 miles/second). Students know how to identify the characteristic properties of waves: interference (beats), diffraction, refraction, Doppler effect, and polarization.
Chapter 14: Waves and Energy Transfer I. Wave Properties A. Classes of Waves (wave- a rhythmic disturbance that carries energy through matter or space) 1. Mechanical Waves a. Requires a medium (water, air, springs, etc.) b. Includes: sound waves, water waves, waves that move down rope or spring, etc.
2. Electromagnetic Waves a. No medium b. Consists of coupled changing magnetic and electric fields that move through space at the speed of light (3.00 x 108 m/s) 186,000 miles/s
B. Types of Waves (2) 1. Transverse wave- a wave that vibrates perpendicular to the direction of the wave motion
2. Longitudinal waves-disturbance is in the same direction as, or parallel to, the direction of the wave motion a. Includes sound waves b. Surface waves have characteristics of both transverse and longitudinal waves
C. Measuring a Wave 1. Speed a. Find speed of pulse (single bump or disturbance) b. First measure displacement of wave peak (Δd) c. Then divide by the time interval (Δt) to find speed d. In most waves (both transverse and longitudinal, speed depends only on the medium through which the waves move
2. Amplitude- the maximum displacement from its position of rest, or equilibrium a. depends on how the wave is generated, but not on its speed. b. More work has to be done to generate a wavewith a larger amplitude.
3. Wavelength- The shortest distance between points where the wave pattern repeats itself. a. Troughs- low points (troughs are spaced by one wavelength) b. Crests- high points (crests are spaced by one wavelength) c. Greek letter lambda λ, represent wavelength.
4. Period and Frequency (only apply to continuous waves) a. Period (T)- the time needed to repeat one complete cycle of motion b. Frequency (f)- number of complete oscillation each second. 1). Measured in hertz. (one hertz = one oscillation per second) 2). Equation
c. Both period and frequency of a wave depend only on its source. They do not depend on the waves speed or the medium d. More equations
II. Wave Behavior A. Waves at Boundaries 1. Speed of wave depends only on the properties of the medium, not the waves amplitude or frequency. a. Water waves- depth of the water affects speed b. Sound waves in air- temperature affects speed c. Waves in spring- depends on the spring’s rigidity and its mass per unit length.
2. Incident wave- wave that strikes a boundary 3. Reflected wave- energy of incident wave reflected backward creating returning wave
B. Superposition of Waves- two or more waves combine toform a new wave of less or greater amplitude 1. Wave Interference- can be either constructive or destructive a. Destructive interference- the superposition of waves with equal but opposite amplitudes
1). When pulse meets, displacement is zero. 2). Point N, which doesn’t move at all is called a node
b. Constructive Interference- occurs when wave displacements are in the same direction 1). Results in wave that has an amplitude larger than any of individual waves 2). Point A has the largest displacement is called the antinode.
2. Standing Waves- produced when two sets of waves of equal amplitude and wavelength pass through each other in opposite directions.
C. Refraction- the change in direction of waves at the boundary between two different media D. Diffraction- the spreading of waves around the edge of a barrier