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1、Lesson 16 General description of waves Waves appear to be disturbances in some sort of medium.In the case of the clothes line the medium is the cord itself and the disturbance is a displacement of part of the cord.In a similar way,the surface of a pool of water is capable of carrying a disturbance a
2、long.Here the medium is the water and the disturbance is an upward or downward movement of the surface.Sound waves in air fit into much the same mode.The medium is the air.In general,the essential nature of a wave is a disturbance of some sort in a medium which,owing to the properties of some sort i
3、n a medium which,owing to the properties of the matter,changes either in form or position,or both,as time goes on.Let us consider the medium a little further.Although it enables a disturbance to be propagated,it itself does not move bodily along.For example the disturbance at one end of the clothes
4、line is propagated along its length,but none of the particles of which the cord is composed actually moves from one end of the line to the other.So a wave is a means of transmitting energy from one point to another without any net transfer of matter.Moreover,it is means of transmitting information f
5、rom one point to another.It may be noticed that the light waves and other forms of radiation such as X-rays,ultraviolet rays,infrared rays,and radio waves appear to require no medium whatsoever for their propagation.Indeed,they can travel through a vacuum,but we also describe them in the same way as
6、 other waves.Waves in strings may be either transverse or longitudinal.For a transverse wave,the direction or travel is at right angles to the disturbance.Let us imagine that s string is stretched horizontally and that one end is agitated with an up-and-down movement,then we would have a transverse
7、wave with disturbance in the up-and-down direction moving along the length of the string.On the other hand,a longitudinal wave can be set up by taking one end of the string and successively stretching and releasing it along its own length.In this type of wave,the disturbance is in the direction of p
8、ropagation.In fact,transverse and longitudinal waves have many properties in common with each other.Solid rods are capable of sustaining three different types of wave.The first and simplest is the longitudinal wave which is propagated because of the tensile elastic properties of the rod.It is quite
9、easy to excite such waves.If a rod of length about one meter is held firmly at its center and stroked along its length by a resined cloth,a longitudinal wave is propagated back and forth along the rod.It cannot be visually observed,but the effect of the wave can easily be audibly observed as a high-
10、pitched sound of considerable purity.Because the rod is of comparatively small length,the reflection of the waves back along the rod when they reach either end becomes very important and determines the pitch of the note heard.Transverse waves may exist in rods,but are more difficult to produce.Final
11、ly,torsional waves may exist.If a rod is twisted at one end,the the restoring torque causes a wave to be propagated.Here,the disturbance is not a linear displacement,but an angular twist.A membrane,for example the stretched skin on a drum,is the two-dimensional equivalent of stretched string.Here,as
12、 in the string when any part of the skin is pushed in a direction perpendicular to its plane and then released,transverse waves are set up.These waves,unlike those on the string,spread out from the point of the initial disturbance in way very similar to that in which surface waves on water spread ou
13、t from a point of disturbance.The simplest type of two-dimensional wave is a straight-line wave i.e.,its direction of propagation is constant over the whole surface and profile is the same along any line drawn in that direction.Point sources of waves propagating into a three-dimensional medium produ
14、ce spherical waves radiating outwards from the source at the center.This means that a detector placed at successively greater distances from the source records successively weaker responses since the total energy is being spread over the surface of a sphere of increasing surface area.This limits the
15、 range over which the detector can operate.In the case of sound waves,the point source can be represented by a loudspeaker,and the detector by a microphone or the ear.To increase the range it is possible to guide the wave through a tube so that the energy can be concentrated to a definite direction.The speaking tube is an obvious example which is technically known as a wave-guide.