Refraction can be described as the bending of waves as they move from one medium to another one, with a different density. As light travels through a given medium, it does so in a straight line. However, when light passes from one medium into a second medium, the light path bends and the process of refraction occurs. The refraction occurs only at the boundary. Once the light has crossed the boundary between the two media, it continues to travel in a straight line. Only now, the direction of that line is different than it was in the former medium. Light rays are refracted when they go from air to water since water has a higher density than air.
The image above shows the waves bending as they change media. Since the waves in the second medium are closer to one another and have gone closer to the normal, one can assume that this is the medium with a higher density. The same will happen when light goes from air to water.
The amount by which light bends when it enders a medium is known as its refractive index. The refractive index of air, for example, is approximately 1.00 while that of pure water is about 1.33. This means that when light goes into water from air, it will bend towards the normal. It is a fact that the refractive index usually increases with the density of the medium. Have you ever thought of why we need goggles to see under water? The answer to that is simply because water has the same refractive index as the vitreous humour in your eyes, and thus, refraction does not happen to focus the image on the back of your retina and the image cannot be formed.
How Our Eyes Work:
The process of vision begins when light rays that reflect off objects and travel through the eye's optical system are refracted and focused onto a point of sharp focus.
The process of vision begins when light rays that reflect off objects and travel through the eye's optical system are refracted and focused onto a point of sharp focus.
Just as a camera's aperture (called the diaphragm) is used to adjust the amount of light needed to expose film in just the right way, the eye's pupil widens or constricts to control the amount of light that reaches the retina.
In dark conditions, the pupil widens. In bright conditions, the pupil constricts.So that we can see, light enters our eyes and is refracted to focus on the back of the retina. In the retina, there is a high density of photosensitive cells which will respond to the exposition to light. In order to focus the light, a lot of refraction needs to happen in the lens as well as the vitreous humour. If water has the same refractive index as one's eyes, that refraction would not happen.
Density and Pollution
As previously mentioned, the higher the density of an object, the more refraction occurs and therefore, the larger the refractive index. In pure water, which has no impurities, the refractive index is approx. 1.33. However, if impurities were found in the water, obviously, the density of the medium would increase and therefore, increase its refractive index. Thus, polluted, impure water, due to its increased density, will refract light more than pure water.
Where Our Project Comes in
Since the aim of our project is to test for water pollution and compare two samples of water, a way of doing so is by applying what was said in the previous paragraph. Our group will therefore test for refraction in the two samples of water, from Ipanema beach and the Lagoa Rodrigo de Freitas and thus, the one with the higher level of refraction is the denser and therefore, most polluted one.
As the topic of refraction is indeed interesting, the previously mentioned section of the eye and how one cannot see under water was only put as an additional research to our investigation, as the members of the group indeed found this topic really interesting and decided to add to the project by including it.
Method
In order to test for the refreactive index, there is a variety of methods to be used. The first one is to half fill a beaker with water. We would use two beakers, one for pure water and another for sea water. then place a straw with a diagonal orientation in the water so that some of it is not submerged. refraction should be clear but now, we would have to compare the distortion of the straw in both cases. Since we are testing for impurities, we have also to use the process of centrifuging in the samples to be tested. this would remove at least some of the salt in the waters. These would make the test unfair since the sea water would obviously have a larger salt content.
Density and Pollution
As previously mentioned, the higher the density of an object, the more refraction occurs and therefore, the larger the refractive index. In pure water, which has no impurities, the refractive index is approx. 1.33. However, if impurities were found in the water, obviously, the density of the medium would increase and therefore, increase its refractive index. Thus, polluted, impure water, due to its increased density, will refract light more than pure water.
Where Our Project Comes in
Since the aim of our project is to test for water pollution and compare two samples of water, a way of doing so is by applying what was said in the previous paragraph. Our group will therefore test for refraction in the two samples of water, from Ipanema beach and the Lagoa Rodrigo de Freitas and thus, the one with the higher level of refraction is the denser and therefore, most polluted one.
As the topic of refraction is indeed interesting, the previously mentioned section of the eye and how one cannot see under water was only put as an additional research to our investigation, as the members of the group indeed found this topic really interesting and decided to add to the project by including it.
Method
In order to test for the refreactive index, there is a variety of methods to be used. The first one is to half fill a beaker with water. We would use two beakers, one for pure water and another for sea water. then place a straw with a diagonal orientation in the water so that some of it is not submerged. refraction should be clear but now, we would have to compare the distortion of the straw in both cases. Since we are testing for impurities, we have also to use the process of centrifuging in the samples to be tested. this would remove at least some of the salt in the waters. These would make the test unfair since the sea water would obviously have a larger salt content.
- Add the same amount of centrifuged water to three different beakers, remembering to lable them.
- Put a straight object into the water to show refraction. A good idea is to use a straw with the red marks on it since this would create a more specific point of comparison between the three samples
- Repeat the same process for the other sample of water.
- The results will be given through the amount of refraction that happens in each sample, that is, the amount that the red line is distorted.
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| Refraction is obvious but enhanced by the red orientation |
Results
Below are the results obtained, the pictures can be compared in order to concude that the water from teh Lagoon has a larger refractive index than that of the one in Ipanema well as that of distilled water. If you are still unsure, we can prove this even further!! The refactive index is also directly related to the density of the susbtance, the more dense it is, the higher is the refractive index. To discover density, we use the formula density=mass/ volume. The same volume was tested in each case (75cm³) and we found that from these three, the water from the Lagoa Rodrigo de Freitas has the higher density.
Lagoon | Ipanema | Pure Water | |
Mass (g) | 79.97 | 77.77 | 74.89 |
Volume tested (cm³) | 75.00 | 75.00 | 75.00 |
Density (g/ cm³) | 1.07 | 1.04 | 1.00 |
Conclusion
From the results obtained from testing the density of water, we could conlude that the water has a higher density in the Lagoon. It is interesting to observe that this was a common conclusion in the other experiments. This means that in terms of refraction in our eyes, it would be harder to focus in the water of the Lagoon since it has a higher density than teh vitreous humour in our eyes. That means that even though more refraction would happen in comaprison to in pure water, light would me moving into a less dense subtance causing it to refract wrongly. Rather than focusing, thsi would couse light to spread further.
Sources:
http://www.physicsclassroom.com/class/refrn/u14l1b.cfm
http://www.all-science-fair-projects.com/
http://www.whyzz.com/how-do-our-eyes-work
http://www.allaboutvision.com/eye-exam/refraction.htm
Centrifuge the samples of water collected in order to remove the excessive salts but maintain the impurities.
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