PHYS 1415 Lab-10: Refraction
Name: ___________________________
Objectives
· Describe what happens to light when it shines on a medium.
· Explain light direction changes at the interface between two media and what determines the angle.
· Apply Snell’s law to a laser beam incident on the interface between media.
· Describe the effect of varying wavelength on the angle of refraction.
· Explain why a prism creates a rainbow.
How does light bend?
Light travels at different speeds in different media. As light passes at an angle from one medium to another, it changes direction at the boundary between the two media. The index of refraction of a medium, n, is the ratio of the speed of light in a vacuum, c, to its speed in the substance, v.
Speed of light in vacuum
When light enters a medium with a higher index of refraction than the medium it is leaving, it bends toward the normal. When light enters a medium with a lower index of refraction than the medium it is leaving, it bends away from the normal. This change of direction of light at the boundary of two media is called refraction.
For any light that is traveling from one medium of index of refraction n1, at angle of incidence θ1, to another medium of index of refraction n2, Snell’s law of refraction describes the angle of refraction, θ2, experienced by the light. The relationship between the indices of refraction and the angles of incident and refraction is called Snell's law:
Definitions:
· Normal line: A dashed line drawn perpendicular to the surface of the mi
or.
· Incident ray: The ray traveling from the light source to the mi
or is called the “Incident Ray”.
· Reflected Ray: The ray that bounces off the mi
or is called reflected ray.
· Incident angle: The angle formed by the incident ray and the normal line is called the incident angle.
· Reflected angle: The angle formed by the reflected ray and the normal line is called the reflected angle.
· Refracted angle: The angle formed by the refracted ray and the normal line is called the refracted angle.
Experiment
1. Open https:
phet.colorado.edu/en/simulation
ending-light . Choose “more tools”.
2. Material in the top half should be air and bottom material should be glass.
3. Move the laser so that it points vertically down. Enter your data in the table below.
Table-1:
Incident Angle
(degrees)
Reflected Angle (degrees)
Refracted angle (degrees)
% Intensity of reflected ray
% Intensity of refracted ray
0
10
20
30
40
50
60
70
80
4. Is there a relation between reflected angle and incident angle? Explain it in few sentences.
5. What do you observe with regards to intensity of the reflected ray? Do you observe a pattern with regards to change in intensity of reflected ray?
6. Set the incident angle to 45. Click and drag the “Speed” meter from the Toolbox, placing its point on the incident ray.
What is the speed of the incident ray? _______________
What is the speed of the reflected ray? _______________
7. Place the “speed” meter on the refracted ray. Note down the speeds for the following material in table 2:
Table-2
Medium
Speed (in c)
n
Ai
1.00
Wate
1.33
Glass
1.50
Mystery A
Mystery B
Index of Refraction n of a given medium is defined as the ratio of speed of light in vacuum, c to the speed of light in a medium, v.
8. Calculate the index of refraction of materials, mystery A and mystery B. Enter the calculated values in table 2 above.
9. If a student proposed the general rule, “The speed of light is a constant in all materials, regardless of n,” would you agree or disagree? Explain your answer.
10. Change the incident angle θ1 and note down the refracted angle θ2 for each trial. Record the results in Table 3.
Table 3
Trial
nglass
θ1 (degrees)
θ2 (degrees)
1
1.5
30
2
1.5
45
3
1.5
60
11. When light passes into a medium where it slows down, does it bend toward the normal or away from the normal?
12. Material in the top half should be glass and bottom material should be water.
13. Move the laser so that it points vertically down. Enter your data in the table 4 below.
Table-4:
Incident Angle
(degrees)
Reflected Angle (degrees)
Refracted angle (degrees)
% Intensity of reflected ray
% Intensity of refracted ray
0
10
20
30
40
50
When rays travel from a denser medium to a less dense medium, we can define a critical angle of incidence θc such that refracted angle θ2 = 90.
When the angle of incidence is greater than the critical angle, 100% of the light intensity is reflected. This is called total internal reflection because all the light is reflected.
14. Find the critical angle of glass and water combination.
15. What is the critical angle for the interface between Mystery A and glass? To be internally reflected, the light must start in which material?
16. What is the critical angle for the interface between Mystery B and glass? To be internally reflected, the light must start in which material?
17. Experiment to find ways to make rainbows.
a) Insert at least one setup where light passing through a prism gives a rainbow and describe why a rainbow is formed.
) Explain why only some types of light will yield rainbows.
2