# REFRACTION

Refraction of the sound is a change in the direction of propagation of the sound wave as a result of a change in the environment.

Since the change in the environment leads to a change in the speed of sound, the sound front turns when going into a new environment.

Refraction should not be confused with diffraction, even though in both cases we have a change of direction of the wave. In the first case we have refraction due to a change in the density of the medium, and in the second case – a change in direction of a wave due to a meeting with an obstacle.

Refraction can be illustrated as follows:

When moving from a denser medium to a less dense the speed of sound change to lower. When the wave falls at an angle the different parts of the wave front attack at different times the new environment.In this situation, the time in which the right side of the front will go the distance B-C, the left, which has already entered the new environment and moves with a lower speed, will go the distance

A-D. Since the path of the left side of the front is smaller than that of the right, the wave will make a turn when crossing to the new environ

ment, ie it will make a refraction.

Accordingly, if we move from less dense to more dense medium, there will be a refraction in the opposite direction. This is defined in the law of Snellius, which states:

where ? is the angle of incidence and refraction respectively, v is the wave speed in the two media, and n – the index of refraction of the medium.
From the first equation we can deduce the second:

Therefore, if we have a light beam passing from the optically less dense (a lower refractive index) in the optically denser medium (higher refractive index), the angle of refraction is smaller than the angle of incidence.

When passing the light beam from the optically denser into optically less dense medium the angle of refraction is greater than the angle of incidence.

Common example is the refraction of the light wave in water, in which the reflected light from ? object in the air at a distance from us, it will appear farther away than it actually is.t in the water, after the refraction creates optical illusion of a shallower location than the actual. Similarly, if we look underwater object in the air at a distance from us, it will appear farther away than it actually is.

This creates a lot of interesting phenomena. For example the mirages occurring under certain conditions due to the fact that the air at different temperatures have different index of refraction.In this moment the light wave will make a turn due to the transition from one medium to another.

However, the viewer perceives objects as observed by light that comes in a straight line. At this point there will be a vertical shift of the observed object in relation to it’s actual position.

In very hot air in the lower area and colder in the higher areas we can see part of the sky onto the ground and confuse it with a water surface –

a mirage of a lake in the desert mirage or a puddle on the hot asphalt.

Under conditions of temperature inversion – cold air into low areas and warm in the high, we can see objects which are on the horizon, above it – in the sky, like a land above the water, etc.

Another interesting phenomenon associated with the refraction of light, is the dispersion. In it we have dependence of the refractive index from the wavelength.

The length of the light is inversely proportional to the frequency, respectively, during the passage of a white light beam through a glass prism, the light will decompose into its component colors.

The same phenomenon can be observed in a rainbow where the water particles, when the light passes through them, refract it at different angles depending on the frequency of the wave, respectively. the color:

Another interesting phenomenon is the total internal reflection. When the angle of incidence is smaller than the angle of refraction, there can be observed a situation in which when there is more than a certain angle of attack of the new environment, the light wave will not enter into it, but will remain on the edge. If we increase the angle further, the light will break twice in the same environment, creating a reflection, rather than transition to the new environment. This is called total internal reflection.

Observing in this way the boundary between environments at a certain angle, we can not see anything beyond this border, but only the reflection of objects in the environment, in which we are:

The lenses are also objects that work on the principle of the light refraction. The varying thickness of the glass through which light rays pass leads to a different refraction and creating of focusing or dissipating effect on the observed object.

## REFRACTION OF SOUND

So far the analogy between light and sound waves often was accurate and helped to explain the specifics of the wave propagation and the interaction between the waves.

With the refraction however it is not the same case. The mechanism is the same, but the refractive index is based on various factors in light and sound waves. Since from the law of Snellius the refractive index is directly related to the speed of wave propagation in a given environment, in terms of the sound and of the light we have some difference to the speed at which sound and light propagate in different materials. Let’s see how.

In contrast to the sound the light has an end at high speeds. It is just under 300,000 km / sec and is the most- high speed across the known universe.

However, this is the speed of light in a vacuum. Its refractive index is 1. All gases, liquids and solids slow the speed of light, whereby they have a higher index:

In the situation with the sound it is not so. In less density bodies the tendency is to a lower speed of the sound, while by increasing the density usually (not always) the speed of propagation of sound increases. This results in an inverse relationship in the refraction the sound at the transition from the less dense to the denser medium and reverse, than with the light.

Here is a sample table of the propagation of sound in different environments:

Interesting is the fact that the diamond is a material that reduces dramatically the speed of light and at the same time dramatically increases the speed of sound. He practically do not increase and decrease, but holds the appropriate properties for conducting them.

Temperature changes, however, apply equally to the speed of light, as well as the speed of sound. In both cases, warmer air carries waves with higher speed. Moreover, we can observe similar temperature anomalies in the distribution both of the sound and the light.

As we see, on a hot summer day for example the sound wave will dissipate upwards due to the slow spread of sound in cold than in hot environments (the right example from the illustration). In case of temperature inversion there can be obtained a situation of very clearly hearing of an object, situated at a greater distance or height (the left example). This is because of the reverse bend of the sound as a result of its higher speed at higher altitudes, where the medium temperature is higher.

### Written byHristo

Filed under: Sound theory