Snell’s law

The law of refraction relates the angle of incidence (angle between the incident ray and the normal) to the angle of refraction (angle between the refracted ray and the normal). This law, credited to Willebrord Snell, states that the ratio of the sine of the angle of incidence, i, to the sine of the angle of refraction, r, is equal to the ratio of the speed of light in the original medium, vi , to the speed of light in the refracting medium, vr , or sin i /sin r = vi / vr . Snell's law is often stated in terms of the indexes of refraction of the two media rather than the speeds of light in the media.

Refracted index = sine (i) – incidence ray

sine (r) – refraction ray

Total internal reflection:

Waves going from a slower to a faster medium speed up and bend at the boundary, e.g. light going from glass to air. Beyond a certain angle (the critical angle) all the waves bounce back into the glass - they are totally internally reflected.

All waves that hit the surface beyond this critical angle are effectively trapped. The critical angle for most glass is about 42° and 49° for water.

Total internal reflection takes place only when-

i) the rays are traveling in a dense medium towards a less dense medium.

ii) the angle of incidence is greater than the critical angle.

Refractive index = sine 90°

sine C (critical angle)

THEREFORE

Refractive index = 1

sine C (critical angle)

Plan:

I am going to measure the angle of refracted light that is shone through a glass block. I am going to vary the angle of incidence, for example 10°, 20°, 30° etc. I will keep the same shape of glass block (rectangular) and the same material (glass). I am also going to keep the same light source (ray box) and the same distance from the ray box to the glass block. Using Snell’s law I am going to find the refractive index.

Prediction:

I predict that when the angle of incidence increases the refracted angle will also increase. I predict this due to the fact that because glass has a higher density than air the ray of light will be refracted away from the normal and both angles will increase. As the angle of incidence increases, so will the angle of refraction.

Method:

I set up a ray box and a glass box in top of a white plain piece of paper. I then shone the ray box so that a line of light shone at an angle of 10° into the glass block. I marked the incident ray of 10° then measured the emergent ray.

I repeated this with the angles of the incident ray ranging from 20° to 80°.

We then saw that the ray was refracted where it entered the glass and where it left the glass.

Result Table:

Graph:

Evaluation:

The graph loosely supports my prediction, which was as you increase the angle of incidence you increase the angle of refraction. It did show this but it didn't show that as you double the angle of incidence you double the angle of refraction. It showed that when angle I is 40 degrees angle r is 19 degrees and when angle I is 80 degrees angle r is 30 degrees this means that my prediction was wrong but not totally because if you use Snell's law it works. Snell found that if you take the sine of angle I and divide it by the sine of angle r you get the refractive index which should be the same for every angle for that material, in my case, glass.