Download Snell`s Law, Optics and Light – The Lab

Snell’s Law, Optics and Light – The Lab
Snell’s Law
Light travels at different speeds through different materials. This difference is due to a materials index of
refraction, which is determined by the structure and composition of the material. The index of refraction is
defined as the ratio of the speed of light in vacuum to the speed of light in the medium.
n
c
(vacuum speed)

v (speed in medium)
Light going into a material will be bent, with the amount of bending determined by the index of refraction and
the angle it enters at.
n1 sin 1   n2 sin  2 
n1 is the medium the light started in and n2 is the index of the material it ends in.
Place each of the six containers in the outlined section on the paper, the laser should be pointed in at a 45 degree
angle.
Orient the laser so it is pointing down the line that says normal on the angle plate. Place the container
containing the liquid such that the light is incident on the container at the center of the circle. You’ll notice that
the light going into the container does not go in the same direction it went in. Measure the two angles as shown
in the figure below, and the formula above (the index of refraction, n, for air is 1) to find the index of refraction
of each of the four materials.
Water:
Vegetable oil:
Hand Sanitizer:
Hand Soap:
Corn Syrup:
Glass:
Lenses
Things you will need:
Computer
100mm F.L. Convex Lens
200mm F.L. Convex Lens
250mm F.L. Convex Lens
Visit this site:
http://www.colorado.edu/physics/phet/simulations/lens/lens.swf
On the web simulations, set the options to Many rays and check the Virtual Image box. Change the
object until you find your favorite one.
1. What happens to the lens when you change the radius of curvature? What happens to the rays? Why?
2. What happens to the rays when you change the refractive index? What would happen if the refractive index
could be changed to 1 (same as air)?
3. What happens to the rays when you change the diameter of the lens? What happens to the image? Why?
4. Move the object around. Where is the resulting image if the object is very far away? What happens to the
rays as the object approaches the focal point? What happens when the object is closer than the focal point?
What is the orientation of the image in each case?
5. Pick up a lens and look at something far away. Move the lens closer and farther from your eye. Can you
determine the focal length this way? Measure it and compare to the writing on the lens.
6. Put the lens close to an object (closer than the focal length). What happens to the image? Where have you
seen this technique used?
7. Find a bright area (possibly outside). Point the lens at it. Get someone with a white t-shirt (or a piece of
paper) to stand close to the lens. Focus the image on their shirt. What’s the difference in the image between
the different lenses?
8. What will happen if you block half the lens? Try it. How does this compare with the computer simulation?
Finding the Blind Spot
1. Stand about 2-3 meters from the white board.
2. Have your lab partner make a 1-inch solid dot in the middle of the white board.
3. Cover your left eye with your hand and stare at the spot on the board with your right eye. (This puts the
image of this point on the fovea where vision is most acute.)
4. Have another lab partner point the laser beam at the spot on the board. (Do not point the laser beam at
anyone’s eyes, ever!)
5. While continually staring straight ahead at the dot on the board, have the other lab partner slowly move
the laser beam horizontally to the right. Do not move your eye to follow the laser beam. (This is cheating
and will not help you find your blind spot.)
6. At a certain point, the laser beam will disappear from your field of vision. When this happens, have your
lab partner mark this spot on the white board. This is because the image of laser beam falls on the blind
spot of your eye. (The blind spot is where the optic nerve reaches the retina, and there are no receptor
cells there.)
7. Have your partner continue to move the laser to the right and, eventually, the laser beam will reappear in
your field of vision. Mark this spot on the white board as well.
Alternate instructions
A
O
X
 Hold the piece of paper arms length away from your eyes.
 Close or cover your right eye and stare at the ‘X’ with your left eye. To test your right eye, cover the left





eye, and stare at the ‘A’. The rest of this procedure will use the left eye.
Slowly move this piece of paper closer to your eyes until the ‘O’ disappears. What do you observe instead
of the ‘O’? (If you continue to move the paper closer, the ‘O’ will reappear.)
Have one of your lab patterns measure the distance from the paper to your eye. We will call this distance d.
Mark ‘X’ on the dashed line below. It should be ‘d’ away from the front of the eye.
Using a straight edge, mark ‘O’, it should be two (2) inches above ‘X’.
Again using the straight edge, draw a line from ‘O’ to the back of the eye. This line should pass through the
cornea. What part of the back of the eye does it hit?
Accommodating Power of the Eye
1
.
f [m]
The human eye contains a small component, which is also called the lens. Muscles squish (un-squeeze) the lens
to make it more (less) convex and bend light farther (less).
The power of a lens is defined as P[diopters] 
Near Point
Your lens has a minimum size and a maximum size. When your eyes’ lenses are squeezed the tightest you can
focus light that is very close to your face. This is called your near point.
Measuring the distance to this point and assuming your eyes have a diameter of 2.4cm, we can determine the
focal length of your eye at its greatest.
Far Point
There is also a most relaxed state for your eyes’ lenses. When your eyes are most relaxed, you can see light
from very far away (at your far point). We can’t measure distances that long in the lab.
However, if you put another lens over your eye, then you can bring your far point much closer.
Put the lens provided on your eye. Measure the distance to the farthest clear position from you. This is your far
point.
Your TA will walk you through the algebra of determining your eye’s minimum focal length.
1
fTotal

1
1
1
1



feye flens diameter of the eye (~2.4cm) far point