Experiment 9: Reflection, Refraction, and Total Internal Reflection
The purpose of this experiment was to study the laws of reflection, refraction and total internal reflection, to measure the focal length of mirrors with convex and concave mirrors, and to measure the index of refraction of water.
In this experiment, we learned that the angle of incidence θi is the angle that the incident ray makes in regards to the normal one. Also, the angle of reflection θr is the angle that the reflected ray produces from the normal one. Also, we discussed how the law of reflection is used to explain the behavior of the incident and reflected rays. According to Snell’s law, we ...view middle of the document...
This is known as total internal reflection. Total internal reflection occurs only when the light travels from higher-index medium toward lower-index medium.
sinθc=n2sin90°n1=n2n1 with n1>n2
First, We used a ray box as a light source and adjusted the number of white rays by maneuvering a plastic piece that allows 1, 3, or 5 rays to show. We needed to turn on the power for the light source, turn off the lights in the room and position a sheet of paper with certain types of mirrors so that the light rays could strike the mirror at different angles.
* Adjust the slit mask on the light source so one ray shines to the surface of the triangular mirror.
* Rotate the mirror
* Trace the surface of the mirror and mark points on the incident ray and the reflected ray
* Trace the incident ray and reflected ray in three different angles.
* Repeat these steps with the concave and convex mirror surfaces facing the light source.
Plane surface | Concave surface | Convex surface |
Incident θi | Reflected θr | Incident θi | Reflected θr | Incident θi | Reflected θr |
90 | 67 | 90 | 68 | 90 | 65 |
90 | 67 | 90 | 68 | 90 | 65 |
Focal length of the Mirror
* Adjust the slit mask on the light source until three parallel light rays are shining.
* Place the concave surface facing the light source.
* Adjust the mirror so that the middle reflected ray is aligned with the middle incident ray.
The place where the three reflected rays cross each other is the focal point of the mirror.
* Trace the concave mirror surface and three reflected rays.
* Measure the focal length f from the middle point on the surface to the focal point and compare it with the radius R of the curvature of the mirror.
* Use the compass to draw a circle that matches the curvature of the mirror
* Check that f = R/2
* Repeat this procedure with the convex mirror surface and note that the reflected rays are diverging and they will not cross in front of the mirror.
* Use a ruler to extent the reflected rays back behind the mirror’s surface to find the focal point.
Refraction of light passing from air through water
A refraction tank consists of a circular bath, which has a built-in scale and light source.
* Measure the angles of incident, reflection and refraction on the scale.
The water is filled up to the line of 90°-90° in the tank. The incident point of the light is at the center of the tank.
* Slowly move the light source from its top position (zero on the scale)
* Record the angles and calculate the refractive index of water nwater by Snell’s law nairsinθair= nwatersinθwater ⇒ nwater=sinθairsinθwater
Angle of incidence θair | Angle of refracted θwater | nwater | Average of nwater |
50 | 35 | 1.33 | 1.33 |
30 | 22.15 | 1.30 | |
15 | 11 | 1.36 | |
Refraction of light...