C HAPTER 16: W AVES AND S OUND

W AVES Much of what we see and hear is only possible because of vibrations and waves. We see the world around us because of light waves. We hear the world around us because of sound waves. If we can understand waves, then we will be able to understand the world of sight and sound.

W AVES Examples of waves: sound waves, visible light waves, radio waves, microwaves, water waves, sine waves, cosine waves, stadium waves, earthquake waves, waves on a string, and slinky waves Wavelike motion: motion of a pendulum, the motion of a mass suspended by a spring, the motion of a child on a swing

W HAT IS A W AVE ? A wave is a traveling disturbance A wave carries energy from place to place.

T YPES OF W AVES Mechanical waves propagate through a medium EX: sound waves propagate via air molecules colliding with their neighbors. electromagnetic waves, do not require a medium (can move through a vacuum) consist of periodic oscillations of electrical and magnetic fields Visible light, infrared, radio waves…

S LINKY W AVES We’ll use the slinky to make two different types of waves: transverse waves and longitudinal waves. What is the medium?

T RANSVERSE W AVES Lay the slinky out on the table. Notice the equilibrium position of the slinky. Shake the slinky from side to side along the table. What direction does the wave move? What direction do the particles of the medium move? What happens to the particles of the medium after the wave has passed?

T RANSVERSE W AVES A transverse wave is one in which the disturbance occurs perpendicular to the direction of travel of the wave

T RANSVERSE W AVES A transverse wave is one in which the disturbance occurs perpendicular to the direction of travel of the wave Examples of transverse waves: EM waves (light, radio, microwaves, etc.), strings on instruments After the disturbance has passed, the particles of the medium return to their equilibrium position

L ONGITUDINAL W AVES Lay the slinky out on the table. Notice the equilibrium position of the slinky. Push the end of the slinky forward along its length, and then pull it back to the starting point. What direction does the wave move? What direction do the particles of the medium move? What happens to the particles of the medium after the wave has passed?

L ONGITUDINAL W AVES A longitudinal wave is one in which the disturbance occurs parallel to the line of travel of the wave.

L ONGITUDINAL W AVES A longitudinal wave is one in which the disturbance occurs parallel to the line of travel of the wave. Example: sound waves After the disturbance has passed, the particles of the medium return to their equilibrium position

O THER W AVES Water waves includes both transverse and longitudinal components. The water particles at the surface move in a nearly circular path.

P ERIODIC W AVES A transverse wave may consist of more than one pulse. Transverse waves consisting of a series of alternating pulses is known as a wave train. If the oscillations are steady the resulting waveform is periodic (consist of cycles that repeat)

D ESCRIBING P ERIODIC W AVES Cycle – one complete wave pulse (shaded) amplitude, A, the maximum displacement of a particle from its equilibrium position. Can be measured between a crest and the equilibrium position Can be measured between a trough and the equilibrium position

D ESCRIBING P ERIODIC W AVES Wavelength, λ, the horizontal length of one cycle of the wave (can be in m, nm, mm,...) Measured from crest to crest, or trough to trough, or between any 2 successive points on the wave

D ESCRIBING P ERIODIC W AVES If we watched the tape on the slinky over time, we would see the graph below. Period, T, is the time required for one complete cycle (measured in seconds) Frequency, f, is a measure of the number of cycles that pass a point in 1 second (Hertz).

D ESCRIBING P ERIODIC W AVES

E XAMPLE 1: AM and FM radio waves are transverse waves consisting of electric and magnetic disturbances traveling at a speed of 3.00x10^8m/s. A station broadcasts an AM radio wave whose frequency is 1230x10^3Hz and an FM radio wave whose frequency is 91.9x10^6Hz. Find the distance between adjacent crests in each wave.

E XAMPLE 1: Solution:

E XAMPLE 2: A typical sound wave associated with human speech has a frequency of 500 Hz. The frequency of yellow light is about 5x10^14Hz. In air, sound travels at 344 m/s and light at 3x10^8m/s. A) Find the wavelength of the sound wave. B) Find the wavelength of yellow light.

E XAMPLE 2: Solution:

W AVELENGTH The fact that these characteristic wavelengths are so different explains many differences in their behaviors. THINK: why can we HEAR around the corner of a building, but cannot SEE around the corner?

W AVES CAN BEND AROUND OBSTACLES Waves can bend around obstacles whose size is comparable to their wavelength The radius of curvature of a building is ~the wavelength of sound, so sound waves bend around the corner carrying energy and allowing us to hear The radius of curvature of a building is much, much larger than the wavelength of visible light. The light waves are unable to bend around the corner. If we have a small object (like a tiny slit), light can bend around it

W AVES CAN BEND AROUND OBSTACLES What about gamma rays emitted from an atomic bomb? Gamma rays have wavelengths < m (picometers), which is less than the diameter of an atom. It would seem that they would unable to bend around obstacles, so you could hide behind anything to avoid them????

W AVES CAN BEND AROUND OBSTACLES Gamma rays are so small that they can pass through most objects, passing between the atoms that make up the object (remember most of the volume of an atom is empty space). Shielding from gamma rays must be done by materials with high atomic numbers and high density (like lead…)

A SSIGNMENT FOCUS p. 499 #3 PROBLEMS p. 500 #1-7