Lab 6:Refraction and Reflection - Geometric OpticsINSTRUCTIONS

Introduction

examine how rays of light behave when they go through one medium to the other. Doing so, we will be testing the laws of reflection and refraction and determining the critical angle for total internal reflection.

You will be using an online simulation on the PhET website called Bending Light:https://phet.colorado.edu/sims/html/bending-light/latest/bending-light_en.html

Theory

The law of reflection predicts that the angle made by an incident ray should be equal to the angle made by the reflected ray:

_i=_r (6.1)

Refraction is governed by Snell's Law which states that when light goes from one medium with index of refraction to another medium with index of refraction , the angle of incidence and the angle of refraction are related by:

n_1 sin(_1 )=n_2 sin(_2) (6.2)

Here, these angles are measured with respect to the perpendicular to the interface between the two media; we call this line the normal line.

Figure 6.1: Default view of the simulation panel

One implication of Snell's Law is that there exist angles at which refraction is not possible. This can be algebraically achieved only when which, at first look, shows that sin can be greater than 1. However, this is not plausible mathematically for any real angle . As such, no light will propagate through the second medium when such conditions are met; therefore, all of the light is reflected at the interface between the media. This is called "total internal reflection". The smallest angle of incidence that satisfies this total internal reflection is called the critical angle.

n_1 sin_c =n_2 sin90_c=sin^(-1) (n_2/n_1 ) (6.3)

Procedure

Go to the link provided above to launch the simulation. Then, double-click on More Tools.

Snell's Law - Two Known Media

• Take some time to familiarize yourself with the tools of the simulation.
• The default media are water with index of refraction and glass with index of refraction .
• Make sure you have a check mark on "Normal" on the lower left menu. You can toggle it on and off in order to be sure what the normal line is. Once you are done playing with it, leave it with the check mark on.
• Now, take a protractor from the left menu and align the 90's with the interface line between the media and align the 0's with the normal line.
• Turn on the laser by pressing on the red button.
• Set the incident angle to 25^o and predict what the reflected angle should be, using equation 6.1, and what the refracted angle should be, using equation 6.2 and record your predictions on the Data Table 6.1.
• Take a screenshot of your experimental observations and paste it in the report.
• Now, measure the values of the reflected angle (experimental ) and that of the refracted angle (experimental ) using the protractor and record these values on the Data Table 6.1.

Snell's Law - One Unknown Medium and Known Medium

• Leave the laser ON.
• Then change the top medium into "Mystery A" and leave the bottom as glass.
• Then, set the incident angle to 35^o.
• Measure the refracted angle and calculate the index of refraction of the "Mystery A" using equation 6.2.
• Take a screenshot of your experimental observations and paste it in your report.

Critical Angle

• Now, change the top medium to glass and the bottom medium to water.
• For these two media, predict what the critical angle should be, using equation 6.3.
• Now, move the laser until the very moment the refracted angle disappears.
• Then, measure the incidence angle that satisfies this condition.
• Now, compare your predicted critical angle with the experimental critical angle.
• Take a screenshot of your experimental observations and paste it in your report.

Data Table 6.1

Two Known Media Incident _1=25^o	Predicted r:	Experimental r:	% difference:
Predicted 2 :	Experimental 2:	% difference:
Unknown Medium Incident _1=35^o	n2= 1.500	2:	n1:
Critical angle	n1:	n2:
Predicted c:	Experimental c:	% difference:

Data Analysis

Show your data analysis work on your lab , as per the instructions from the procedure.