Optical Disk - Reflection and Refraction

Optical Disk - Reflection and Refraction

I. Brief Background

Light, as it was discovered in the 1800's, exists as electromagnetic radiation. This groundbreaking realization hit home, when the concepts of electric and magnetic fields were finally summarized in the four famous Maxwell Equations.

 In this experiment, the behavior and propagation of light was studied using a method known as geometric optics, wherein the wavelength of light is assumed to be smaller than any object or obstacle in its path.

II. What Went On

The experiment was divided into six distinct parts. Alignment and calibration of the optics was first performed, to ensure the quality of collected data and measurements. The experiment kicked off with the observation of a light ray's behavior when reflected from a plane mirror. The mirror, mounted on a rotating disk was rotated in angle increments of ten degrees, and each corresponding angle of reflection was measured. The same procedure was then done on concave and convex mirrors, with each mirror set undergoing three trials. Here are the results:

Table 1: Part 1 of the experiment

Angle of Incidence°	Angle of Reflection°
	Plane mirror	Convex Mirror	Concave Mirror
10	10	12	10
20	20	22.5	21
30	30	32.5	31

The next step practically involved the same procedure, except this time, five parallel light rays were allowed to hit the mirrors. For each of the three different mirrors, the resulting directions of the reflected lines were recorded. The mirrors were not rotated this time around. 

Fig. 1: Sketches of the reflected light paths 

The fourth part the the experiment involved the beaming of light through a semicircular prism. Using a prism allowed for the splitting of the light ray into reflected and refracted light beams. The prism was set on the rotating disk with the flat side facing the light source, and made to rotate in angled increments of 10° , starting at 10°  and ending at 50°. At each stop, the angles of the reflected and refracted light rays relative to the central axis were measured. The index of refraction was then calculated using Snell's Law. 

Table 2: Part 4 of the experiment

Angle of Incidence°	Angle of Reflection°	Angle of Refraction°	Index of Refraction
10	10	6.5	1.533
20	21	13.5	1.465
30	31.5	20	1.462
40	41.5	25.5	1.493
50	51.5	31	1.487

Next, the same thing was done for the prism, but this time with its curved side receiving the light beam. 

Table 3: Part 4.1 of the experiment

Angle of Incidence°	Angle of Reflection°	Angle of Refraction°	Index of Refraction
10	11	16	1.587
20	21	31.5	1.527
30	31	49	1.509
40	41	75	1.503
50	50

Additionally, the angle at which the refraction ray disappeared (critical angle) was also recorded. From the critical angle, the index of refraction could also be calculated for. using Snell's Law, but with the assumption that refracted light would no longer be visible (where its angle would be 90°). The speed of light inside the glass was also calculated using the gathered data and the equation v = nc where v is the speed of light in the given medium, n the refractive index, and c being the speed of light in a vacuum.

Table 4: Part 5 of the experiment

Critical Angle	43°
Index of Refraction of glass n	1.466
Speed of light inside the semi-circular glass	4.398x10^8 m/s

For the sixth and final segment of the experiment, five more prisms of varying shapes were placed one at a time on the rotating disk. Multiple parallel rays of light were then beamed at each prism. The corresponding light patterns for each prism were then recorded. 

Fig. 2: Sketches of the refracted light rays

One remarkable thing about light is that it obeys the law of reflection regardless of whether it is met by a plane or spherical mirror. This is because when a light particle (which is very, very small) hits even a curved surface, the fact that it is just so tiny makes the surface it hits practically flat; in the same sense that a curve can be seen as a connected series of infinitely small straight lines. So, to put things simply, for a travelling light particle about to reflect off a surface, that surface will always be flat regardless of its overall shape. There was, however, one aspect of the experiment that one could initially think of as disregarded. It is easy to forget that even without the glass, light around us travels through the medium of air we breathe. Being a medium, its possible effects on the passing light as well as its index of refraction should be taken into account. Well, in fact it is taken into account, and the apparent "disregard" for it is rightfully so. This is because air has a refractive index of 1, so in essence it doesn't do much at all when it comes to the bending of light passing through it. 

When passing through the planar face of the semi-circular glass, both an angle of incidence and an angle of refraction were observed. Interestingly, after recording different angles from different degrees of rotation, sufficient data was able to be gathered in order to theoretically deduce that if the contact surface of the glass were to be switched to its curved side, and the angle of incidence were to be set at the same values as the angles of refraction that were previously measured, the resulting angle of refraction would be the same as the experimentally measured angles of incidence. All it took to deduce this was to plug in the switched values into Snell's Law. But in an intuitive sense, switching the receiving surface would only rotate the glass by 180° which in effect would only change to initial direction to its opposite, but keep the initial angles as they were. Thus, resulting in the predicted angles of incidence and refraction. 

In the experiment, the obtained critical angle was 43°. Beyond this angle, what is known as total internal reflection would occur, where the supposedly refracted light is directed back into the glass and is left to continuously reflect making the refracted ray essentially disappear. 

To conclude, the long ago established theories on optics and its laws were once again verified, thus proving to stand the tests of time. Before we left, we all took pictures of the light patterns that came out of the experiment.

References:

Lab Manual Authors. Experiment 6: Optical Disk - Reflection and Refraction. N.p.: n.p., 2013. Print.