Vibrations and Waves
When a wave reaches the boundary
between one medium another medium, a portion of the wave undergoes reflection.
Reflection of sound waves off of surfaces can lead to one of two phenomena - an echo or a reverberation.
A reverberation often occurs in a small room.
The reception of multiple reflections off of walls and ceilings within 0.1 seconds of
each other causes reverberations - the prolonging of a sound.
Echoes occur when a reflected sound wave reaches the ear more than 0.1 seconds after the original sound wave was heard.
If the elapsed time between the arrivals of the two sound waves is more than 0.1 seconds, then the sensation of the first sound will have died out. In this case, the arrival of the second sound wave will be perceived as a second
sound rather than the prolonging of the first sound. There will be an echo instead of a reverberation.
Reflection and transmission of light waves occur because the frequencies of the light waves do not match the
natural frequencies of vibration of the objects.
When light waves of these frequencies strike an object, the electrons in the atoms of the object begin vibrating. But instead of vibrating in resonance at a large amplitude, the electrons vibrate for brief periods of time with small amplitudes of vibration; then the energy is reemitted as a light wave.
If the object is opaque, then the vibrations of the electrons are not passed from atom to atom through the bulk of the material. Rather the electrons of atoms on the
material's surface vibrate for short periods of time and then reemit the energy as a
reflected light wave. Such frequencies of light are said to be reflected.
Wave interference is the phenomenon that occurs when two waves traveling along the same medium meet.
The interference of the waves causes the medium to take on a shape that results from the net effect of the two individual waves on the particles of the medium.
Constructive interference occurs at any location along the medium when the two interfering waves have a displacement in the same direction,
resulting in an increased amplitude.
Destructive interference occurs at any location along the medium when the two interfering waves have a displacement in opposite directions,
resulting in a decreased amplitude.
Interference in sound can occur in noise-canceling headphones. As the music is
being played, the headphones emit waves of the opposite amplitude in order to
cancel out the distracting sound waves surrounding you. This is an example of destructive interference.
If two light waves of the same color,
amplitude, and frequency are emitted, the resulting interference exhibits alternating patterns of light and dark bands,
reinforcing and neutralizing the light waves. This is a form of constructive interfernce.
● Refraction of waves involves a change in the direction of waves as they pass from one medium to another.
● Refraction, or bending of the path of the waves, is accompanied by a change in
speed and wavelength of the waves.
If the media (or its properties) are
changed, the speed of the wave is changed. Thus, waves passing from one medium to
another will undergo refraction.
For example, sound waves are known to refract when traveling over water. Even though the sound wave is not exactly
changing media, it is traveling through a medium with varying properties; thus, the wave will encounter refraction and change its direction.
Refraction makes it possible for us to have lenses, magnifying glasses, prisms and
rainbows. Even our eyes depend upon this bending of light. Without refraction, we wouldn’t be able to focus light onto our retina.
The amount of bending depends on two things:
● Change in speed – if a substance causes the light to
speed up or slow down more, it will refract (bend) more.
● Angle of the incident ray – if the light is entering the
substance at a greater angle, the amount of refraction will also be more noticeable. On the other hand, if the light is entering the new substance from straight on (at 90° to the surface), the light will still slow down, but it won’t change direction at all.
Diffraction of waves involves a change in direction of waves as they pass through an opening or around a barrier in their path.
It is a wave’s ability to bend and curve along the medium.
The greater a wave’s amplitude, the greater
its diffraction, while the smaller a wave’s amplitude, the smaller its diffraction.
Diffraction in sound works as sound waves are able to travel around corners and through
doors.
Diffraction makes it easier for us to hear
waves with lower frequencies as it encounters obstacles than those with higher frequencies, since diffraction is more pronounced with
longer wavelengths.
An example of wave diffraction in light occurs when a cloud covers the sun, making visible a thin silver lining of the sun’s light.
This occurs because the sunlight is diffracted
through the cloud; its light is able to bend around the cloud and remain visible.
If the object is transparent, then the
vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and reemitted on the opposite
side of the object. Such frequencies of light waves are said to be transmitted.
The transmission of light across a boundary between two media is accompanied by a
change in both the speed and wavelength of the wave.
The only time that a wave can be
transmitted across a boundary, change its speed, and still not refract is when the
light wave approaches the boundary in a direction that is perpendicular to it.
