11.2 Total Internal Reflection
Introduction:
Total internal reflection of light is a method of transmitting information that is widely used in the communications industry. Information is transmitted at the speed of light through a light pipe or fibre optic cable. Light enters a medium, gets trapped inside it, and must travel the whole length of the material before it can escape. The image of the object is transmitted down the bent cable until it is clearly seen at the other end.
The angles of refraction are also increasing until they get to a point where they travel along the medium boundary between air and water. From this point on, the refracted rays cannot exit the material. They are reflected internally back into the water. The water acts as a mirror. Total internal reflection occurs only for light traveling from a more optically dense medium to a less optically dense medium.
Purpose:
To study the effect of total internal reflection and calculate the critical angle.
Equipment:
-Semicircular plastic piece
-Semicircular water container (cheese box)
-Sugar water, salt water
-Other non-toxic liquids
-Ray box with single slit aperture or laser
-Protractor, blank paper.
Procedure:
1. On a blank sheet, traced the semicircular shape of the plastic piece.
2. On the straight side on the semicircle, marked the centre and drew a normal at this point.
3. Drew in a series at 10°, 20°, and 30° from the normal.
4. Darkened the room, and shined the light along 0°. Noted where the ray emerges.
5. Moved the light source so that it followed the incident paths, and marked the angles at which the refracted rays emerged.
6. Continued to move the light source at ever-increasing angles of incidence until no more refracted rays emerged then measured and recorded this angle.
7. Noted where the light exits, and measured this angle relative to the normal.
8. Increased the angle of incidence again, and noted where the ray emerges then measured this angle.
Data & Analysis
Angle of INCIDENCE Angle of REFLECTION Sinσ1 Sinσ2 Sinσ1
Sinσ2
0 N/A
1. 10° 19° 0.17 0.33 0.52
2. 20° 31.5° 0.34 0.52 0.65
3. 30° 45° 0.5 0.71 0.70
4. 40° 61° 0.64 0.88 0.73
5. 48° 90° 0.74 1 0.74
Conclusion:
From the results, I can conclude that when light passes through a more optically dense medium to a less optically dense medium the angle of refraction is always less than the angle of incidence. When the angle of incidence was 48°, the refracted angle was 90°. This meant that 48° was our critical angle, meaning that light shown through the medium at any angle larger than that would not be able to exit the material. Instead, the ray of light would be reflected off of the water.