Physics OnlineIndex of Refraction
Snell’s law can be taken as an empirical relationship between the incident and refracted angles or it can be derived from Huygens’ principle:
n1sinθ1 = n2sinθ2
n= index of refraction = c/v
c = speed of light in vacuum
v = speed of light in medium
θ = angle between the light ray and a normal to the surface
In this lab, you will use Snell’s law to calculate the index of refraction for two different substances.
Equipment You Procure
- digital camera
- paper
- pencil
Equipment From Kits
- foam pad
- pins
- protractor
- clear acrylic block
Experimental Procedures
1)Place a clean sheet of paper over the foam pad. You may want to tape the corners to prevent the sheet from moving.
2)Place one of the clear blocks in the middle of the sheet with its largest face horizontal.
3)Draw a line around the perimeter of the block while maintaining the position of the block.
4)Stick a pin into the paper and touching the block on the long side of the block. Write “1”next to the pin.
5)Stick a second pin on the opposite side of the first pin, but not directly across from it.The pin should be touching the block. Write “2” next to the pin
6)Stick a third pin far away from the block so that it lines up with the first pin on the same side of the block and appears to line up with the second pin on the opposite side of the block when viewed through the block. Write “3” next to the pin.
7)Stick a fourth pin far away from the block so that it lines up with the second pin on the same side of the block and appears to line up with the first and third pins on the opposite side of the block when viewed through the block. Write “4” next to the pin. You might take a picture at this time.
8)Remove the block.
9)Draw solid lines connecting pins 1 to 2, 1 to 3, and 2 to 4.
10)Draw dotted lines perpendicular to the surface of the block and passing through the surface at the locations of the first and second pins. See the diagram below.
11)Remove the pins.
12)Measure the angle between the line connecting pin 1 to 3 and the dotted line coming from pin 1. This angle is θ1.
13)Measure the angle between the line connecting pin 1 to 2 and the dotted line coming from pin 1. This is θ2.
14)Use Snell’s law to calculate the index of refraction for the block. Assume that the index of refraction for air, n1, is 1. Compare your result to the accepted value of 1.5 ± 0.1.
15)Measure the angle between the line connecting pins 2 and 4 and the dotted line coming from pin 2. This angle is θ1.
16)Measure the angle between the line connecting pins 1 and 2 and the dotted line coming from pin 2. This is θ2.
17)Use Snell’s law to again calculate the index of refraction for the block. Assume that the index of refraction for air, n1, is 1. Compare your result to the accepted value of 1.5 ± 0.1.
18)Repeat steps 1 through 17 two more times with different locations for the first two pins.
Note: you will have a total of 12 measurements of angles and 6 calculations of an index of refraction. There are 3 different starting locations for the pins, each with 4 measurements of angles. If you do not provide 12 measurements of angles and 6 calculations of an index of refraction, you will not receive full credit on your report.