Reflection and 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.

Describe the effect of varying wavelength on the angle of refraction.

Explain why a prism creates a rainbow.

Apply Snell’s law to a laser beam incident on the interface between media.

Open https://phet.colorado.edu/en/simulation/bending-light . Choose “more tools”.

Material in the top half should be air and bottom material should be glass.

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 90 Is there a relation between reflected angle and incident angle? Explain it in few sentences.

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?

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? _______________

Place the “speed” meter on the refracted ray. Note down the speeds for the following material in table 2:

Table-2

Medium Speed (m/s) n

Air 1.00

Water 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. n=cvCalculate the index of refraction of materials, mystery A and mystery B. Enter the calculated values in table 2 above.

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.

Calculate sin θ1 and sin θ2 for each trial. Record the results in Table 3.

Calculate n2 for each trial.

Table 3

Trial nglassθ1 (degrees) θ2 (degrees) Sin θ1 Sin θ2 n2 = Sin θ1 / Sin θ2

1

1.5 30 2

1.5 45 3

1.5 60 The relationship between the indices of refraction and the angles of incident and refraction is called Snell’s law: n1sinθ1= n2sinθ2Compare your calculated n2 with the given index of refraction, nglass. Do they agree? Explain why it does or doesn’t.

When light passes into a medium where it slows down, does it bend toward the normal or away from the normal?

Material in the top half should be glass and bottom material should be water.

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. Applying Snell’s law:

Critical angle θc=sin-1n2n1When 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.

Calculate the critical angle of glass and water combination. Show your calculation.

What is the critical angle for the interface between Mystery A and glass? To be internally reflected, the light must start in which material? Show your calculation.

What is the critical angle for the interface between Mystery B and glass? To be internally reflected, the light must start in which material? Show your calculation.

Click on the “Prisms” tab. Set up the experiment as shown in the figure below. Measure the angle of incidence (q1) and the angle of refraction (q2). Calculate the index of refraction.

Simulation tip: Remember that the Normal dashed line and protractor will help take measurements.

Does the index of refraction vary as you change the wavelength of light.What is the angle of the ray that leaves the glass square (emergent ray) relative to the ray that enters it?

What can you say about the path of emergent ray relative to that of the incident ray?

19. Experiment to find ways to make rainbows.

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.