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Observing Refraction: Disappearing Test-Tubes and Laser


What is light? All light is a form of electromagnetic radiation which can be emitted from numerous sources like the sun. All light travels as a wave and regardless of its source follows a pattern of cyclically alternating intensity of electric and magnetic energy. Spectroscopy is the study of the interaction between electromagnetic radiation and matter. Spectroscopy is a key concept relevant for many scientific techniques and is prominent within many analytical fields of science. You will be exploring the interaction of a laser beam or light (electromagnetic radiation) with a glass block, water and oil (matter). Therefore, you will utilise spectroscopy to examine the phenomenon known as refraction. Refraction occurs when light changes direction and hence speed as it passes from one medium and into another or through a medium of varying densities. 

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Refraction is measured by observing the change in direction of light as it travels at different speeds through two materials. The change in direction of the light is dependent on the refractive index of the material the light passes through. Depending on the relative refractive index of a material light will either refract at a lesser or greater angle. If the materials share a similar refractive index, then less reflections and refraction will be observed.  Snell's law can be used to help determine the refractive index of a material.
Refraction can be readily observed by the disappearing beaker exercise because the glass beaker shares a similar refractive index as vegetable oil.  Because vegetable oil and glass share a similar refractive index less refraction and reflection occur. Therefore, when the beaker is submerged within the oil light travels straight through and does not refract off the glass surface, which causes the beaker to become invisible.

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Part 1: The Disappearing Test-Tube

1. Prepare both beakers side by side with the test tubes inside them.

2. Fill one test tube with water before filling the beaker in which it is housed in with water.

3. Repeat this process for the other beaker, instead filling the vesicles with oil.

4. Compare the visibility of the test tube in both of the beakers.

Test Tubes
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Part 2: A Laser Beam Through A Prism

1. Set up a laser beam perpendicular to the flat side of a crown glass D-shaped prism on a large piece of paper and turn the laser beam on.

2. Set up a second piece of paper perpendicular to the first piece of paper, on the opposite side of the prism to the laser beam and parallel to the prism. You will have to lean it on something like a book or box.

3. Tilt the prism so it is on a 30-degree angle with the laser.

4. Mark on the upright paper where the laser hits. Trace a straight line on the laying down paper from the laser (where the light is emitted) to the base of the prism where the light hits.

5. Remove the prism and trace a line between the marked upright paper and the end point of the line where the laser hits the base of the prism.

6. Use a protractor to measure the angle between these two lines, centring on the corner, minus this angle from 180 degrees to obtain the angle of the normal for the calculation.

7. Compare the difference between this angle and the angle that would be expected given the refractive index of air is 1.00029 and that of crown glass is 1.517.  

8. Repeat this process using a prism of quartz given the refractive index of fused quartz is around 1.475. Compare the results given using the 2 different materials.

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A link to a downloadable student question worksheet is provided below.

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