PHYSICS 216 Portland State University Refraction and Lenses Lab Report 3 The link is for lab instructions. https://docs.google.com/document/u/2/d/e/2PACX-1

PHYSICS 216 Portland State University Refraction and Lenses Lab Report 3 The link is for lab instructions. https://docs.google.com/document/u/2/d/e/2PACX-1vT8wrxO0YypaNN88Tk2buHRhTnJ9D_m4hmcZf12BruD5bNoQe31twfLZBoZXN7sB2ntqUoUYqjYAGns/pubThe attached file is a word document for the lab instructions in case link is not working. Lab 3 – Refraction and Lenses
Objectives
•
Understand Snell’s law of refraction
•
•
Understand what convex and concave lenses are and how they refract light by utilizing Snell’s Law
Learn how the focal length (f), object distance (do), and image distance (di) are related for a thin lens
Writeup Requirements
Overview – 4 pts, Data & Analysis – 6 pts, Questions – 8 pts, Lab 3 Survey – 2 pts
Part 1: Refraction
Preparation
Watch the lab lecture video here.
Refraction refers to light bending as it moves from one medium (material) to another. The bending of light
depends on a material property known as the index of refraction, n (note n is a unitless quantity). Materials used
for optics will typically have an index of refraction somewhere between 1.0 and 2.0. How a light ray bends when it
is refracted will also depend on the incident angle. The incident angle is measured with respect to the surface
normal.) This is described mathematically by Snell’s Law, which is given by
(3-1)
Some general results from Snell’s law that are worth noting is how light bends when it moves from one medium to
one with a lower, higher, or identical index of refraction. When moving to a lower index material, the light will
bend farther away from the normal. Conversely, light bends closer to the normal when the light moves into a
higher index. Then there will be no bending when the two materials have the same index. (See Figure 3-1.)
Figure 3-1. Refraction of light at the interface between two materials. (a) Case 1, light bending away from the
normal. (b) Case 2, light bending towards the normal. (c) Case 3, light not being bent at all.
Procedure
Watch the lab demo here.
Open the the link below to the simulated refraction experiment:
https://phet.colorado.edu/sims/html/bending-light/latest/bending-light_en.html
1. Under “Bending Light” in the PhET simulation, select “More Tools.”
2. You will see a laser pointer which is aimed at an interface between two different materials (defaults to air
and glass). Turn the laser on, and try rotating it to change the angle of incidence. Make sure you can
identify the incident, reflected, and refracted rays.
3. On the lower left corner of the screen, check the “Angles” box to show the angles of the incident and
refracted rays with respect to the surface normal (the dashed line).
1. What is the index of refraction of the upper material and the lower material (ni and nr)? What is the angle
of incidence and the angle of refraction (θi and θr)?
2. Verify that the values you measured obey Snell’s law (show how you did this, should be accurate to 2
decimal places).
4.
5.
6.
7.
8.
For the upper material, make sure “Mystery A” is selected from the “Materials” drop-down menu.
For the lower material, select “Air” from the “Materials” drop-down menu.
In the upper left-hand corner, ensure that the wavelength of your light source is set at 650 nm.
Adjust the position of the light source so that the refracted ray is visible.
Measure the angle of incidence and the angle of refraction.
3. Use your measurements and Snell’s law to determine the index of refraction of the “Mystery A” material.
You should have noticed that with this configuration, there is a range of angles where the incident light is simply
reflected from the surface, and is not refracted at all. This phenomena is called “total internal reflection,” which
can occur when light moves from a material with a higher index of refraction to a material with a lower index of
refraction (Case 1 shown in Figure 3-1).
The critical angle (θC) is the incident angle at which total internal reflection starts to occur. To find the critical
angle, set the refracted angle to 90°(this is the angle at which the ray is no longer refracted) and apply Snell’s law:
(32)
4. Use Eq. (3-2) and your answer to Q3 to predict the critical angle for interface between the material
“Mystery A” and the air.
9. Starting at an incident angle of 0°, rotate the light source until the refracted ray just disappears (this is
when the angle of refraction is 90°).
10. When this occurs, the angle of incidence is equal to the critical angle.
5. What is the critical angle you found from the simulation for this interface? How does this result compare
to your answer for the previous question?
The index of refraction is not the same for all wavelengths (colors) of light. This is why white light, which is a
combination of colors, will separate into a range of colored bands after passing through a prism. This effect is
called dispersion.
11. From the bottom of the “Bending Light” simulation, select “Prisms.”
12. Drag the triangular prism into your work area, turn on the light source, and aim the beam at the prism.
13. You can use the color slider to change the wavelength of the light source. Is the beam refracted more or
less with shorter wavelengths?
14. On the right side of the screen, switch to the white light source (see the figure to the right).
6. Which colors of the white light beam are bent the most by the prism, and which are bent the least?
7. Is the index of refraction greater for longer or shorter wavelengths?
Part 2: Refraction in Convex and Concave Lenses
Preparation
Lenses, whether they are in telescopes, eyeglasses, microscopes or magnifying glasses, work by using refraction to
redirect light in a predictable and useful way. When designed properly, lenses focus light. The shape of the lens
drastically changes its focusing properties, which is what you will explore in this lab.
Two essential characteristics of lenses are focal point and focal length. The focal point is the point where light rays
parallel to the principal axis of the lens cross (or point away from in the case of a diverging or negative lens) after
passing through the lens. The focal length is the distance between the focal point and the center of the lens.
Here is some other lens-related terminology:
•
Principle axis: A line that passes through the center of a lens, and is perpendicular to it.
•
•
Real image: An image formed by a lens is said to be real when light rays actually pass through the image.
Virtual image: An image formed by a lens is said to be virtual when light rays do not pass through the
image.
Procedure
Open the ‘Lenses’ simulation found at the following
link: https://www.geogebra.org/m/dGdEMuUW#material/cPqeYrJQ
The simulation allows users to modify the focal length of a lens, and the placement of an object relative to the
lens. When the simulation loads, a blue dot labeled ‘f’ appears to the right of the lens. By clicking and dragging
this blue dot, the user can adjust the focal length. Dragging the dot away from the center of the lense
increases the focal length with points on the right corresponding to a converging lens, and points on the left
corresponding to a diverging lens.
The simulation provides 3 different light rays, each emanating from a point labeled ‘E’ positioned at the tip of an
arrow labeled ‘Object”. The user can adjust the position of this point with respect to the optical axis and the center
of the lense by simply clicking and dragging it around. The parallel light ray is shown as the blue arrow and as its
name would suggest, it propagates from the object to the lens parallel to the optical axis, then is refracted when
passing through the lens. The axial light ray shown in red passes directly through the center of the lense. Finally the
focal ray in yellow propagates from the object, passes through the focal point, and eventually is refracted by the
lens. A light ray that appears as dashed represents extensions of the solid ray.
1. Place the object on the left side of the lens, and the focal length selection dot to the right side of the lens
(creating a positive lens). Make sure the focal distance is less than the object distance.
8. Observe the position of the image relative to the object and lens. Does this indicate if the image formed is
real or virtual?
2. With the object still on the left side of the lens, and move the focal length selection dot to the left side of
the lens to create a negative lens.
9. How is the image formed by the negative lens different from the one formed by the positive lens?
Lenses that have a spherical surface will focus parallel light rays differently depending on the distance that the
rays are from the optical axis. This characteristic is referred to as “spherical aberration”. To observe this, open
the ‘lens refraction and spherical aberration’ simulation found at the following link:
https://www.geogebra.org/m/a2rNFfHA Try adjusting different parameters in the simulation such as the lens
index of refraction until you can clearly see that the 7 refracted rays converge at different points along the
optical axis.
10. Of the 7 rays, which converge closest to the lens? (The rays near the optical axis, or near the edge of
the lens?)
Part 3: Focal Length of a Thin Lens
Preparation
Watch the lab demo video here
For many applications, like microscopy, lens systems will not be focusing parallel rays of light. Thus, we need to be
able to predict how light will focus for a nearby object. For a lens that has a thickness much less than the radii of
curvature, we can use the thin lens equation which relates the focal length f, to the distance of the object do, and
the distance of the image di
3-3)
(
Another important quantity of this system is magnification. This is simply the ratio of the height of a resulting
image hi, and the height of the original object ho. This is geometrically the same as the ratio between the
distances di, and do.
(3-4)
Procedure
To explore the properties of this this lens equation, open the following simulation: https://www.walterfendt.de/html5/phen/imageconverginglens_en.htm
1. Make a table similar to Table 3-1 with columns object distance (d0), image distance (di), object size
2.
3.
4.
5.
6.
(ho), image size (hi), magnification (M), inverse of the object distance (1/do) and inverse of the image
distance (1/di).
Select a focal length for the simulation by typing a value into the white box to the right of ‘focal
length’ (values between 10 and 15 cm work well).
Click and drag the object represented by the blue arrow all the way to the left until it’s placed at
location zero cm on the optical axis. Its height can be similarly changed by clicking and dragging with
its exact height listed in the green data field to the right.
A screen represented by a vertical line can be placed at the 100 cm mark by clicking and dragging.
With the object and screen location set, click and drag the lens until you see the green image is
superimposed at the same location as the screen. If the image height is so large that it goes out of
frame, simply adjust the object height until both appear.
Record the image distance and object distance supplied in the green data field. Additionally, record
the object and image height (note: if the image is inverted, record a negative height).
11. Is the image formed real or virtual? Explain.
7. Move the lens until you find another position where you see a focused image (do not move the light
source or the screen), again you might need to adjust the object height so that both object and image
fit within the simulation window.
8. Repeat the previous steps after moving the light source or screen closer together. Do these steps 4
additional times to fill out your table. As you continue to move the screen toward the light box and
you should notice that at a certain distance you can no longer create a clear image on the screen.
Table 3-1. Data used to determine the focal length of the thin lens.
Data Point
di (cm)
Measured Values
do (cm)
hi (cm)
ho (cm)
M
Calculated Values
1/di (cm-1) 1/do (cm1
)
1
2
…
N
Analysis
Solve Eq. (3-3) for 1/di. This equation will give a linear relationship for finding focal length (y = mx + b).
12. For the equation you just wrote down, what are the corresponding m, b, x and y variables?
Make a plot of 1/di vs. 1/do with your data. Find the x- and y-intercepts of this line.
13. How do the x- and y-intercepts of your plot compare to the focal length you chose?

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