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Theory
Theory
Reflection and refraction are two spectacular phenomena easily observable in daily life (Figure 1 and Figure 2).
Figure 1. Reflection: trees reflected in water.
Figure 2. Refraction: "broken" hydrometer in the physics lab.
Both reflection and refraction are associated with a boundary between two media. Rays of light either bounce of the boundary (reflection) or bound because of a different optical density of the second material (refraction), or both (Figure 3).
In the figure, the source of light (a blue laser) is pointed toward a glass surface. The reflected ray is directed down and looks bluish on the table. The refracted ray exits the glass prism to the left and looks pinkish or red (the color is only as captured by the cellphone camera, it is blue in real).
Figure 3. Reflection and refraction of the blue laser light.
Reflection
Reflection
Figure 4. Reflection of the blue laser light.
Figure 4 zooms on the incident angle and the reflected angle. Notice two things:
Both angles are equal. The angles are measured between the ray and the normal line (normal line is perpendicular to the surface, here, it is the reflecting surface)
Both angles belong to the same plane. This plane is parallel to the table and lifted above the table to the level of the source of the light ray.
Refraction
Refraction
Figure 5. Refraction of the blue laser light.
In figure 5, the angles of incidence and refraction are marked on both sides of the glass surface where the blue ray enters the prism. For the second time, refraction occurs where the blue ray exits the glass prism. In that case, the refracted ray inside glass became the incident ray. The second refracted ray (the pinkish looking one) is parallel to the original ray entering the prism.
Snell's Law
Snell's Law
Diagram and physics formulas
Diagram and physics formulas
Figure 6. A diagram of reflection and refraction (source)
Reflection
The angle of reflection equals the angle of incidence.
Refraction
A ratio of the sine of angle of incidence to the sine of angle of refraction is equal to the ratio of the velocities of propagation in the two materials (v1 and v2).
Refractive index (or index of refraction) is the ratio of the speed of light in a vacuum (c) to that in another medium of greater optical density. (source)
n = c / v,
where c = 299,792,458 m/s (about 300 million meter per second)
Since as well as the inverse ratio of the indices of refraction of the two materials (n1 and n2)
Total Internal Reflection
Total Internal Reflection
When light travels from an optically more dense medium to an optically less dense medium, a total internal reflection may occur if the angle of incidence is bigger than the critical angle. The critical angle is the angle of incidence corresponding to the angle of refraction of 90°.
Total internal reflection has a very important application in fibre optics used to transmit light signals (IT, medicine).
The critical angle between two media may be calculated from Snell's Law:
n1 * sinθ = n2 * sin90°
The Simulation
The Simulation
Open the PhET simulation entitled "Intro" below.
Figure 7 below presents the screenshot of what you will see.
Figure 7. Screenshot of the Bending Light Simulation Intro
Laser
Normal line
Choose the first material. In the picture, "Air" is chosen
Choose the second material. In the picture, "Water" is chosen
Protractor
Intensity sensor
Figure 8. The source of light can be moved around to manipulate the angle of incidence. Use the red button to turn it on.
Figure 9. Protractor. You can pool the protractor and use it to measure angles. The angle of incidence (here it is 45°) and refraction (here it is 30°) are to be recorded in this experiment.
Figure 10. The intensity sensor shows that 95.03% of light was refracted, so the remaining 4.97% was reflected. You can pool it and sue to measure the intensity of rays.
Experimental Procedure
Experimental Procedure
Part A. Index of Refraction
Part A. Index of Refraction
1. Select two different materials.
2. Record the angle of incidence and the angle of refraction for at least 10 various angles of incidence. Instead of selecting random angles, plan a procedure based on some logic. Explain that logic in your lab report.
3. Create a table as shown in Figure 11 below.
4. Plot the sine of the angle of refraction vs. the sine of the angle of incidence (Figure 12). You can use Excel, Desmos, or any other graphing program.
Figure 12. Graph sineθrefr vs. sineθinc
5. Paste the graph into the lab report. Interpret the slope of the line. Refer to Snell's Law.
6. Repeat for at least two other pairs of materials.
Part B. Reflected vs refracted ray
Part B. Reflected vs refracted ray
You may have noticed that the original ray of light is partially reflected and refracted at the same time. Find an angle for which the reflected and the refracted rays have the same intensity. Use the intensity sensor (Figure 10). Use the protractor to measure the angle. Take a screenshot.
Repeat for at least two other pairs of materials. Take screenshots.
Write your observations about angles of incidence: Do the angles of incidence change for various materials? How? Why?
Extra activity: Try to explain/calculate the observed angles (hint).
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