BBC Bitesize - KS3 Physics - Features of waves - Revision 2
Study the different features of waves with BBC Bitesize KS3 Science. If two waves meet each other out of step, they cancel out. Two waves out of phase peak . In order to be in phase, ow out of phase, two waves must have the same frequency. What is the actual meaning of phase velocity in matter wave? This generates two circular wave patterns that collide when the wave crests meet. . the same time, they are likely the opposite of each other, and will cancel each other out. Transverse waves are like those on water, with the surface going up and down, In interference, when two waves meet, they can interfere constructively, When two such waves are of equal intensity, they will cancel each other completely.
Diffraction and constructive and destructive interference (article) | Khan Academy
If the end is free, the pulse comes back the same way it went out so no phase change. If the pulse is traveling along one rope tied to another rope, of different density, some of the energy is transmitted into the second rope and some comes back.
For a pulse going from a light rope to a heavy rope, the reflection occurs as if the end is fixed. From heavy to light, the reflection is as if the end is free. Standing waves Moving on towards musical instruments, consider a wave travelling along a string that is fixed at one end. The reflected wave will interfere with the part of the wave still moving towards the fixed end.
Typically, the interference will be neither completely constructive nor completely destructive, and nothing much useful occurs. In special cases, however, when the wavelength is matched to the length of the string, the result can be very useful indeed. Consider one of these special cases, when the length of the string is equal to half the wavelength of the wave.
The second harmonic will be twice this frequency, the third three times the frequency, etc.
The different harmonics are those that will occur, with various amplitudes, in stringed instruments. String instruments and transverse standing waves In general, the special cases the frequencies at which standing waves occur are given by: The first three harmonics are shown in the following diagram: When you pluck a guitar string, for example, waves at all sorts of frequencies will bounce back and forth along the string.
However, the waves that are NOT at the harmonic frequencies will have reflections that do NOT constructively interfere, so you won't hear those frequencies. On the other hand, waves at the harmonic frequencies will constructively interfere, and the musical tone generated by plucking the string will be a combination of the different harmonics. Figure of waves passing though single slit toward two different targets on opposite wall Because all of these pairs are the same distance apart across the slit, if we measure the path length from each pair to the same spot on the wall, each pair will have the same difference in path length.
Figure of single-slit diffraction pattern If we compare single-slit diffraction to the double-slit interference pattern, the spots are much larger and more spread out. In particular, the center bright spot is much larger than it would be for double slits with the same width.
We can view diffraction as light spreading out when it comes up to a hole or other barrier, and trying to get around that barrier. In the process of spreading out, it interferes with itself to create the pattern of light and dark spots that we call a diffraction pattern.
Double slit interference with diffraction When we talked about double slit interference, we pretended that only one light wave could go through each slit at a time. If instead we realize that there are a few light waves travelling through each of the two slits at once, then we can see that there will be a diffraction pattern for each individual slit in addition to the two-slit interference pattern.
This pattern will hold our double-slit interference pattern back, limiting how bright the bright spots can be at any given point on the wall. If we have a bright spot in the diffraction pattern, then our interference bright spots can be as bright at we want.
- Interference of Waves
- Diffraction and constructive and destructive interference
- Features of waves
But, if we have a diffraction dark spot, then the bright spots in our interference pattern cannot be any brighter than the diffraction dark spot, and will disappear altogether. Figure of single slit envelope, double slit diffraction and resulting single slit diffraction and double slit diffraction The interference pattern will come from the light from the two slits interacting, and the diffraction pattern will come from the light from each individual slit interacting with itself.
Consider the following Imagine our scenario of interference from walkie talkie signals. Say the receiver is between the person sending the walkie talkie signal and a solid rock cliff, and we know that the wavelength of the walkie talkie signal is 1 meter. The signal will reach the receiver, but then keep going past them to the cliff, and bounce off it. The signal will then come back to the receiver. That means that the distance between the receiver and the cliff will determine how the incoming and reflected waves interact with one another.
The difference in path length will be the distance past the receiver that the wave travels, plus the distance back again. Illustration of walkie talkie signals being received directly and indirectly from bouncing off rock cliff Since the wavelength of the walkie talkie signal is 1 meter, then the path difference will need to be a multiple of a whole metre for constructive interference, and a half metre for destructive interference.
Refraction The speed of a wave depends on the properties of the medium through which it travels. For example, sound travels much faster through water than through air. When a wave enters at an angle a medium through which its speed would be slower, the wave is bent toward the perpendicular. When a wave enters at an angle a medium in which its speed would be increased, the opposite effect happens. Diffraction When a wave encounters a small obstacle or a small opening that is, small compared with the wavelength of the wavethe wave can bend around the obstacle or pass through the opening and then spread out.
What happens to the energy when waves perfectly cancel each other? - Physics Stack Exchange
This bending or spreading out is called diffraction. Interference The waves from two or more centres of disturbance may reinforce each other in some directions and cancel in others.
This phenomenon is called the interference of waves. It is easy to see how this may happen. Consider two sources producing waves of the same wavelength and in phase; that is, at their origin the crests of the waves occur at the same time. If a point P is equidistant from both sources, the crests arrive at P simultaneously and reinforce each other.The Original Double Slit Experiment
Similarly, the troughs arrive simultaneously and become deeper. The same situation occurs if the distances to point P are unequal but differ by one or more full wavelengths. If, however, the distances differ by half a wavelength or by an odd number of half wavelengths, then the crests of one wave will coincide with the troughs of the other and the intensity of the resultant wave is decreased.
When two such waves are of equal intensity, they will cancel each other completely. Intermediate situations arise in those directions in which the distances traveled by the two waves differ by some other fraction of a wavelength, the waves tending either to reinforce or to cancel each other. When two waves of identical wavelength are in phase, they form a new wave with an amplitude equal to the sum of their individual amplitudes constructive interference. When two waves are of completely opposite phase, they either form a new wave of reduced amplitude partial destructive interference or cancel each other out complete destructive interference.
Much more complicated constructive and destructive interference patterns emerge when waves with different wavelengths interact. When the source of a wave moves relative to an observer, the observer notices a change in the frequency of the wave.