Most often refraction is encountered in a study of optics, with a ray of light incident upon a boundary between two media air and glass, or air and water, or glass and water. Snell's law relates the directions of the wave before and after it crosses the boundary between the two media. Notice that as the wavefronts cross the boundary the wavelength changes, but the frequency remains constant.
In the above animation a spherical wave pulse propagates in a medium where the wave speed is constant in all directions. The wave expands outwards as an ever expanding circle, with the wave traveling at the same speed in all directions. Since the wave speed is the same everywhere, there is no refraction, and the wave does not change direction as it propagates. In acoustics, however, sound waves usually don't encounter an abrupt change in medium properties.
Instead the wave speed changes gradually over a given distance. Often the change in the wave speed, and the resulting refraction, is due to a change in the local temperature of the air. For example, during the day the air is warmest right next to the ground and grows cooler above the ground. Echoes occur when a reflected sound wave reaches the ear more than 0. If the elapsed time between the arrivals of the two sound waves is more than 0. 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 of sound waves off of surfaces is also affected by the shape of the surface. As mentioned of water waves in Unit 10 , flat or plane surfaces reflect sound waves in such a way that the angle at which the wave approaches the surface equals the angle at which the wave leaves the surface.
This principle will be extended to the reflective behavior of light waves off of plane surfaces in great detail in Unit 13 of The Physics Classroom. Reflection of sound waves off of curved surfaces leads to a more interesting phenomenon.
Curved surfaces with a parabolic shape have the habit of focusing sound waves to a point. Sound waves reflecting off of parabolic surfaces concentrate all their energy to a single point in space; at that point, the sound is amplified. Perhaps you have seen a museum exhibit that utilizes a parabolic-shaped disk to collect a large amount of sound and focus it at a focal point.
If you place your ear at the focal point, you can hear even the faintest whisper of a friend standing across the room. Parabolic-shaped satellite disks use this same principle of reflection to gather large amounts of electromagnetic waves and focus it at a point where the receptor is located. Scientists have recently discovered some evidence that seems to reveal that a bull moose utilizes his antlers as a satellite disk to gather and focus sound.
Finally, scientists have long believed that owls are equipped with spherical facial disks that can be maneuvered in order to gather and reflect sound towards their ears. The reflective behavior of light waves off curved surfaces will be studies in great detail in Unit 13 of The Physics Classroom Tutorial. Diffraction involves a change in direction of waves as they pass through an opening or around a barrier in their path.
In that unit, we saw that water waves have the ability to travel around corners, around obstacles and through openings. The amount of diffraction the sharpness of the bending increases with increasing wavelength and decreases with decreasing wavelength. In fact, when the wavelength of the wave is smaller than the obstacle or opening, no noticeable diffraction occurs. Diffraction of sound waves is commonly observed; we notice sound diffracting around corners or through door openings, allowing us to hear others who are speaking to us from adjacent rooms.
Many forest-dwelling birds take advantage of the diffractive ability of long-wavelength sound waves. Owls for instance are able to communicate across long distances due to the fact that their long-wavelength hoots are able to diffract around forest trees and carry farther than the short-wavelength tweets of songbirds.
Low-pitched long wavelength sounds always carry further than high-pitched short wavelength sounds. Scientists have recently learned that elephants emit infrasonic waves of very low frequency to communicate over long distances to each other.
Elephants typically migrate in large herds that may sometimes become separated from each other by distances of several miles. Researchers who have observed elephant migrations from the air and have been both impressed and puzzled by the ability of elephants at the beginning and the end of these herds to make extremely synchronized movements.
The matriarch at the front of the herd might make a turn to the right, which is immediately followed by elephants at the end of the herd making the same turn to the right. So, it is not part of the original light stream.
The Snell's laws speak only about the part of light photons that experienced only elastic collisions in a material. But does that mean that it changes color? That depends, how you define color! As color is usually defined via wavelength i. This is not really a specific fact about electromagnetic waves. It's a fact about all waves. The basic reason for it is cause and effect.
Think of how people "do the wave" in a stadium. The way you know it's your turn to go is that the person next to you goes. When a wave travels from medium 1 to medium 2, the thing that's causing the vibration of the wave on the medium-2 side is the vibration of the wave on the medium-1 side.
It happens like that because that's what refraction is, by definition. As Rob Jeffries's answer shows, there are solutions of Maxwell's equations where a no frequency shift refraction happens across the interface, so it is possible.
When we observe such behavior, i. But we are making a tacit assumption that the interaction with the interface is elastic , i. We are also making a tacit assumption that the interaction with the interface is linear, and thus there are no multiphoton processes which would double, triple, These latter would be theoretically possible, but one can also make a handwaving argument that these latter kinds of interactions are highly unlikely given the interaction region's thin nature and if the light intensity is not too high.
At an abstract high level, it's because the boundary conditions are such that the interface between media is a timelike hypersurface. That's what breaks the symmetry between space and time. If the medium's material properties e. See my answer here to a duplicate question. Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams?
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