Name: ___________________________
Group Members: ___________________________
___________________________
___________________________
Reflection & Refraction
Purpose and Objectives
Much of optics can be understood without referencing the wave nature of light. Instead, we can treat
light as traveling in straight lines until it hits something, at which point the light can reflect or refract,
often some of each.
In today’s lab, you will do several explorations, starting with simple reflection from a flat mirror and
progressing to refraction through a piece of glass or plastic. You will actually be tracing out the paths
that light takes. Your learning objectives for this lab are as follows:



To explore reflection and refraction
To use a simple method to trace light rays.
To see how Snell’s Law determines the angle of refraction
Materials
Plane Mirror
Cardboard
Push Pins
thick pane of glass
Graph Paper
Procedure
Part I – Reflection from a Plane Mirror
When you look at the reflection of an object in a flat (plane) mirror, you see an image formed
behind the mirror. The image is formed by your brain tracing light that reaches your eye
backwards in straight lines. We can find the location of an image by finding out where two light
rays came from or appear to have come from. In order to trace out the light rays, we will use a
method that is pretty straightforward but is a little difficult to understand from just reading about
it. The idea is that you can use pins to define a line along which you are looking. If you are
looking at an image along a certain direction and you place two pins such that they appear to line
up with each other and the image, they will define the line that the light ray takes from that image.
If you look at the image along another direction and define the line with the pins again, you will
get two lines that intersect at the image, as in the figure below.
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Figure 1
1. Place a piece of paper on the cardboard. Place a plane mirror on top of the paper, standing
upright. Draw a line on the paper along the reflecting surface of the mirror (backside of the
mirror).
2. Stick a pin in the cardboard a few centimeters in front of the mirror. This pin will serve as your
object.
3. Observe the image in the mirror, looking from the left side of the object in the mirror. Place two
pins in line with this image (left side of figure). Pay no attention to the location of the object.
4. Select an angle on the right side of the object and repeat the last step with two additional pins in
the board along a new line to the image (right side of figure).
5. Remove the mirror and draw lines through the two sets of alignment pins to locate the image
(O’).
6. Measure the angles of incidence and reflection.
7. Measure the distance from the object to the mirror and the distance from the image to the
mirror.
Part II – Refraction
A light ray that impinges on some material that is not opaque will get at least partially transmitted.
But as it enters the material, the direction of propagation changes, which is “refraction.” If the
material is shaped like a block, the light will be refracted at the surface, travel through the
material, and refracted again at the second surface, exiting the material at the same angle that it
was incident. The result is that the light ray will be shifted in position, but not in angle (see figure
2.) If you are looking at a ray coming through the block, it will appear to come from a different
place that it actually did. You can use a similar method as above to trace out the complete path of
the light.
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Figure 2
1. Place a glass or plastic block on a piece of paper and trace its outline.
2. Place two pins on one side of the block to form a line. This line should be at an angle of 40 – 50O
from the normal to the surface of the block.
3. Look through the block at the pins in such a way that the two pins align. Set two more pins on
the near side of the block that appears to be in line with the first two on the far side.
4. Remove the block and trace the path of a light ray through each set of pins. Extend the lines
such that they intersect with the block outline. Connect the two intersecting points, which will
show you the direction that the light ray took within the block. Your drawing should resemble
figure 2.
5. Measure the angles θ 1 and θ2. Use Snell’s law to calculate the index of refraction of the block at
each interface.
Data
Part I
Distance between object and mirror
(cm)
Distance between image and mirror
(cm)
Left Ray
θi
θr
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Right Ray
Part II
1st Interface
2nd Interface
θ1
θ2
nglass
Analysis
1.
What type of relation exists between the incidence angle and the reflected angle? Support your
answer by referring to the results.
2.
What type of relation exists between object distance from the plane mirror and the image
distance from the plane mirror? Support your answer by referring to the results.
3.
According to the law of reflection, the angle of incidence should equal the angle of reflection.
Find the percent difference (error) between your measured values for the angle of reflection and
the angle of incidence. How well do you feel your experimental results compare to the law of
reflection? Considering your drawing ability and sighting ability, is your result reasonable?
4.
What type of relation exists between index of refraction for each interface? Support your
answer by referring to the results.
5.
Describe how the ray behaved as is passed through the first interface. Describe how the ray
behaved as it passed through the second interface. Why did it behave the way it did at each
interface.
6.
What is the percent difference between your two values of the index of refraction? What is the
average value for the index of refraction?
7.
Calculate the speed of light in your glass plate using the average value of nglass.
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