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Refraction (pt 1) Refraction Refraction of Water Waves

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1 Refraction (pt 1) Refraction Refraction of Water Waves
Light Rays in Glass Atmospheric Refraction Apparent Depth Refraction is responsible for why objects look bent in water and why the setting sun looks flattened

2 Refraction of Water Waves
Reflection involves a change in direction of waves when they bounce off a barrier; 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. Refraction Analogy A way to visualize refraction of waves is to follow the motion of a marching band line as it moves at an angle from one type of marching surface to another. Let’s say that the line moves from cement onto a muddy football field. The first musicians to enter the field will be slowed down by the mud but the musicians on the end of the line carry on at full speed. The result will be that the line turns towards a line drawn perpendicular to the boundary between the two surfaces (normal line). Also note that the marching lines will get closer together but the number of lines passing a point per unit time will stay the same. Refraction of Water Waves We have seen that the speed of a wave is dependent upon the properties of the medium through which the waves pass. So if the medium (and its properties) are changed, the speed of the waves are changed. The most significant property of water which would effect the speed of waves traveling through it is the depth of the water. Water waves travel fastest when the medium is the deepest. Thus, if water waves are passing from deep water into shallow water, they will slow down. And as mentioned, this decrease in speed will also be accompanied by a decrease in wavelength. So as water waves are transmitted from deep water into shallow water, the speed decreases, the wavelength decreases, and the direction changes. The frequency of the waves will however stay the same. If the waves’ travel from shallow water to deep water, the waves bend in the opposite direction as they speed up and the wavelength increases. normal line

3 Light Rays in Glass Light can be perceived as a series of wave fronts just light like the band analogy is composed of a series of marching lines. Often it is more useful to draw the direction of the moving wave fronts as a line drawn perpendicular to the wave fronts called a ray. When light moves from a less optically dense medium (lower n) to a more optically dense medium (higher n), the light rays bend towards the normal line. The reverse is true when moving from a more optically dense medium to a less optically dense medium. See opposite. Example Light is incident on a glass prism as shown opposite. Draw the path of the light ray a) in the prism and b) after it leaves. First draw the normal line where the incident ray hits the glass prism. Glass is more optically dense than air so the light will slow down and bend towards the normal line and then continue in a straight line in the material as there is no change in the medium. When the ray hits the boundary between glass and air it will speed up again as it moves into air and therefore bend away from the normal.

4 Atmospheric Refraction
This involves changes in wave properties including direction as a wave moves through the atmosphere. Sound in Air Sound waves are refracted when parts of the wave fronts travel at different speeds. This happens in uneven winds or when sound is traveling through air in uneven temperature. On a warm day the air near the ground may be appreciably warmer than the air above. Since the sound travels faster in warmer air, the speed of sound near the ground is increased. The refraction effect thus bends the sound gradually away from the ground, making it appear that sound does not carry well. On a cold day (or night) the reverse is true and the higher speed of the wave fronts above cause a bending of the sound toward earth. When this happens, sound can be heard over longer distances. Think of the times that you have heard the whistle of the train at night as it rolls into Dow but haven’t heard it other nights. Light The setting sun often looks oval shaped. Why is this? The sun actually has sunk below the horizon and yet is still visible. This is because light is refracted by the Earth’s atmosphere. Since the density of the atmosphere changes gradually, the refracted rays bend gradually to produce a curved path. The same thing occurs at sunrise, so our daytimes are about 5 minutes longer because of atmospheric refraction of light. When the sun (or moon) is the near the horizon, the rays from the lower edge are bent more than the rays from the upper edge. (see diagram). The eye traces these rays back in a straight line so it looks like the sun (or moon) is squashed in a vertical direction.

5 Mirage Another application of atmospheric refraction of light is a mirage. This is very much like the atmospheric refraction of sound in that hot air near the ground will speed up part of the light wave front nearest to it and thus produce a gradual bending of the light ray upwards. An example is shown opposite. Light from the top of the tree is refracted away from the ground towards the observer’s eye. The eye and brain together play a trick on you as they perceive the light to have come in a straight line which it did not. The eye thus forms an inverted image of the tree and the brain interprets this as a tree standing on the edge of water. A motorist experiences a similar situation when driving along a hot road. It appears as if there are shimmering pools of water on the road but light from the sky is really being refracted through a layer of hot air. Convection currents add to this effect by giving the light a shimmering appearance.

6 Apparent Depth Have you ever looked down into a fountain or swimming pool and have seen an object sitting on the bottom but when you went to fish it out found out that it was much deeper than it seemed. This is because the light from the object is refracted as it crosses the boundary from water into air. According to our refraction rules the light waves will speed up and thus bend away from the normal. The lens in the eye will focus these rays and the brain will interpret the rays as have come in a straight line and thus have traveled from an object that was a lot closer than in reality. Just like a mirage, this is an illusion, but is very much real in that the light rays exist, and so can be photographed. Have you ever felt that a thick frosted glass mug seems to contain less liquid refreshment than it looks? Well, you are right! Your brain perceives the light to have come from the edges of the glass but careful inspection will show that it didn’t. Great marketing skills do the rest! Example Where must the spear fisherman aim in order to hit the fish? He must aim his spear below where he sees the image of the fish as the light reflected from the fish bends away from the normal as it moves into air. If the fisherman throws along his line of sight the spear will shoot above the fish’s head.

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