1. Accelerated Physics Lens Unit 2010
Lenses Activity Homework
5/13 Thursday Go over test Snell’s Law / Intro to Lenses
Go over homework Phet Lens WS (due MON)
5/14 Friday Exploring Lenses Activity Lens Equation WS (Front Side)
To discuss images Phet Lens WS
To learn lens equation
5/17 Monday To go over homework Ray diagrams for convex
Learn convex ray diagrams Lens Equation WS (Back side)
Start lens diagrams
5/18 Tuesday Learn concave ray diagrams Ray diagrams for concave
To start lens lab Eye article (due Thursday)
5/19 Wednesday To complete lens lab Finish lab / follow up questions
Read eye article
5/20 Thursday Quiz over reading Finish post-lab WS
Learn about vision / correcting vision problems Lens Graphing Practice
Post-lab WS
5/21 Friday Real lens activity Read and take notes over section
(Assembly (determine vision problems) 25.2-25.3 (focus on concepts NOT
Schedule) Lens applications equations)
5/24 Monday Finish lens applications Review sheet
Review for the test
5/25 Tuesday Test over lenses Study for final
5/26 Wednesday Go over test Review for the Final
Review for the final
5/27 Thursday Review for the final Review for the final
5/28 Friday First Day Of Finals Review for finals
Periods 1, 2, and 4
5/31 Monday Memorial Day – No School Review for finals
6/1 Tuesday Second Day of Finals Review for finals
Periods 3, 7, 5
6/2 Wednesday Last day of Finals Enjoy the Summer!
Periods 8, 6
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3. Lens Phet Physics Simulations
For all of these demos go to the following website http://phet.colorado.edu, click on go to the
simulations and click on “Light and Radiation”.
Geometric Optics
1. Circle the best answer for each of the following statements: (Adjust the sliders up top to verify
your answers)
(a) A lens with a higher refractive index (index of refraction) will bend the more/ less.
(b) A more/ less curved lens will bend the light more.
(c) A more/ less curved lens will have a shorter focal length.
(d) A bigger/ smaller diameter lens will allow more light in.
2. Use the sliders up top and make the refractive index 1.5, curvature radius 0.3 m, and the
diameter 1.3 m. Select “virtual image”, “ruler”, and “principle rays”. Select “change object” until
you get an arrow. Move the ruler and record the focal length.
3. Move the object so do = 20 cm, 30 cm, 40 cm, 60 cm, 80 cm. Be sure that the object is above
the principle axis.
do classify do measure classify di real image size
(do < F, di (di = 2F, vs. (bigger,
(use ruler) virtual smaller,
etc) etc)
same)
20 cm
30 cm
40 cm
60 cm
80 cm
4. Use the lens equation your measured F and the given value for do to calculate what di should
have been. How do these values compare to what you measured?
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4. Reading: What Is A Lens?
Have you ever snapped a photo or looked through a
microscope? Of your answer is yes, then you have used an optical
device.
There are many kinds of optical devices. Cameras and
microscopes are two examples. Others are projectors, binoculars,
telescopes, and even eyeglasses.
Every optical device is different. But they all have one thing
in common. Each one has at least one lens.
What is a lens? A lens is a transparent substance that bends or
refracts light in a definite way.
Most lenses are made of glass. Many lenses are made of
plastic.
Most lenses have one or two curved surfaces.
There are two main types of lenss: convex [kon-VEKS] and
concave [kon-KAVE].
• A convex lens is thicker at the center than at the edge. It
magnifies or makes things look bigger. A convex lens
converges, or brings together, light rays. The point where
the light rays meet is called the focal [FOE-kul] point. Light
that passes through a convex lens can be focused on a
screen or other surface. This forms an image of the object
that gave the light. Convex lenses are used in projectors and
cameras.
• A concave lens is thinner at the center than at the edge. It
minifies or makes things look smaller. A concave lens
spreads out light rays. They cannot form an image on a
screen. Concave lenses are often used together with convex
lenses. They help the convex lens give sharper images.
Most eyeglass lenses have a combination of concave and
convex curves
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5. UNDERSTANDING LENSES
Six lenses are shown in Figure A. Study them. Then answer the questions by writing the correct
letter.
What you need to know: Plano means “plane” or “flat.”
Which lens or lenses …
1. Are thicker at the center than at the edge? 9. Magnify? _____
_____ 10. Minify? _____
2. Are thinner at the center than at the edge? 11. Refract light? _____
_____ 12. Converge light? _____
3. Are concave? _____ 13. Diverge light? _____
4. Are convex? _____ 14. Can form an image on a screen? _____
5. Are plano convex? _____ 15. Cannot form an image ona screen?
6. Are plano concave? _____ _____
7. Is double concave? _____ 16. Are most important for projectors and
8. Is double convex? _____ cameras? _____
Now look at figure B.
17. a. Figure B shows a [concave / convex] lens.
b. it [converges / diverges] light rays.
18. What do we call the point where light rays converge? ___________________
19. What do we call the distance between a lens and its focal point? _________________
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6. ABOUT FOCAL LENGTH
Different lenses have different focal lengths.
Focal lengths depend upon the strength of a lens.
o The stronger the lens, the shorter the focal length.
o The weaker the lens, the longer the focal length.
A strong lens has a deeper curve than a weak lens.
Two converging lenses are shown in Figure C. Study the figure. Then answer the questions by
writing the correct letter.
Which lens…
1. is more curved? _____ 6. Refracts light more? _____
2. Is less curved? _____ 7. Has the shorter focal length? _____
3. Is stronger? _____ 8. Has the longer focal length? _____
4. Is weaker? _____ 9. Magnifies more? _____
5. Refracts light less? _____ 10. Magnifies less? _____
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8. Concave Ray Diagrams
The Three Rays:
1.
2.
3.
All Ray Diagrams for Concave Lenses are the Same, Try One More:
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9. Convex Ray Diagrams
Directions:
1. Find the image AND state if it is Real or Virtual
2. Use the lens equation and compare the computed di to the drawn di.
(You will need a ruler)
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10. Concave Ray Diagrams
Directions:
1. Find the image AND state if it is Real or Virtual
2. Use the lens equation and compare the computed di to the drawn di.
(You will need a ruler)
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11. Lens Equation Worksheeet Example: A car is 2 m in front of a 1 m focal
length lens. Where will the image be located?
do = object distance from lens
di = image distance from lens do = 2m f = 1m di = ?
f = distance from lens to focal point (focal length) 1/di + ½ = 1/1
1/di = 1 – ½
1 1 1 1/di = .5
Lens Equation + = di = 2 m
do d i f
1. An object placed 30 cm in front of a converging lens forms an image 15 cm behind the lens. What
is the focal length of the lens?
2. A converging lens with a focal length of 20 cm is used to produce an image on a screen that is 2.0
m from the lens. What is the object distance?
3. For a biconvex lens, what is the minimum distance between an object and its image if the image
is
a. real?
b. virtual?
4. If a book is held 30 cm from an eyeglass lens with a focal length of
a. -45 cm, where is the image of the print formed?
b. +57 cm is used, where is the image formed?
5. The geometry of a compound
microscope, which consists of
two converging lenses is shown
below. The objective lens and
the eyepiece lens have focal
lengths of 2.8 mm and 3.3 cm,
respectively. If an object is
located 3.0 mm from the objective
lens, where is the final image
located and what type of image is
it? (Draw a picture)
6. Two converging lenses L1 and L2 have focal lengths of 30 cm and 20 cm respectively. The
lenses are placed 60 cm apart along the same axis, and an object is placed 50 cm from L1 on the
side opposite L2. Where is the image formed relative to L2, and what are its characteristics?
(Draw a picture)
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15. Six Cases of converging lenses
In the convex lens lab, you looked at the images formed by a converging lens when the object
was in different locations. Today you will draw ray diagrams for the same six object positions on
the back of this page and analyze the images in the exact same way. After you have completed
the six lens diagrams, complete the summary data table below and compare it to your results
from the convex lens lab. Then look for trends in your results.
Case 1: Object at Infinity Case 4: Object between 2F & F
Case 2: Object beyond 2F Case 5: Object at F
Case 3: Object at 2F Case 6: Object closer than F
Trends:
1. As the object gets closer to the focal point, the image location gets _____________.
2. As the objects gets closer to the focal point, the image size gets _______________.
3. The only virtual image is formed when the object is located ___________________.
The size of the virtual image is _______________. The occurs in case _______ which
is called the magnifying glass case.
4. Real images are always ______________ compared to the object.
5. Real images are formed by converging lenses when the object is located ______________.
6. It is impossible to get an image when the object is located _______________.
7. The object and image are the same size and same distance from the lens when the object is
located __________________.
case Object Image Image type- Image size Image
location location real or virtual orientation-
RSU or USD
1
2
3
4
5
6
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16. Case 1: (the object is way off the page to the left <-----------)
2F F F 2F
Case 2:
Case 3
Case 4
Case 5
Case 6
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17. Seeing is Believing
Most people who wear glasses and contact lenses wish that they did not have to. Now, thanks to modern
medical science, there is an alternative to reduce your dependence on corrective lenses or possibly
eliminate them completely. Laser Vision Correction has transformed people’s lives. The ability to see
better without the hassles of contact lenses and/or glasses has inspired patients to explore new horizons in
their vision correction. They have freed themselves from the many restrictions corrective lenses create
and can enjoy this newfound freedom with improved vision.
Laser Vision Correction is not for everyone. You should make a well-informed decision when choosing
this treatment. The doctor can help you decide if Laser Vision correction is right for you.
If you decide that Laser Vision Correction is right for you, and join the thousands of people who are
already enjoying the benefits of this treatment, you will realize that Laser Vision Correction is truly a gift
of sight.
How the Eye Functions
Visual Focusing Problems (Refractive Condition) Most visual problems are caused by the way the eye
refracts (or bends) light, and then focuses the light rays. When the doctor checks your vision, he or she
considers how the parts of your eye impact your vision, including the overall shape of your eyeball, the
shape of your cornea, the power of the natural lens, and the actual length of your eye. The most common
vision problem experienced in this country is the inability to focus incoming light precisely onto the
retina. The result is blurred vision.
The Normal Eye
Your eye is like a camera, using light to form images or “pictures” in the brain. Light enters through the
clear tissue of the cornea (the outer layer of the eye), which bends (or refracts) the light rays and is
responsible for two-thirds of the focusing power of your eye. Even a slight change in cornea curvature (or
shape) has a major effect on how clearly you see.
Your pupil, located at the center of the iris (the colored portion of the eye), acts as a shutter to control the
amount of light that enters your eye. The light rays then pass through the lens of the eye, which focuses
the light onto the back of your retina (at the back of your eye). The retina sends the viewed picture to your
brain where the picture is interpreted or “seen.”
The Normal Eye
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18. The Nearsighted (Myopic) Eye
Myopia, more commonly referred to as nearsightedness, is the most common refractive condition and
affects one in four people in North America. Myopia is when people see near objects more clearly, but
distant objects are blurry. Myopia occurs when light rays entering the eye are focused in front of the
retina instead of directly on it. Myopia is usually a result of the curvature (power) of the cornea being too
strong or the length of the eyeball being too long. In the past, an eye doctor would usually recommend
glasses or contact lenses to more strongly focus the light directly onto the retina.
Myopia can be minimal, creating only slight blurring of distance vision. Patients with minimal myopia
may be able to read most of the vision chart in the doctor’s office without glasses. When myopia is
moderate, patients are barely able to see the big E on the eye chart without glasses or contact lenses. Such
eyes have myopia between 2 and 6 diopters. High myopia exceeds 6 diopters. Myopia (nearsightedness)
is often inherited; it usually starts in childhood and typically stabilizes in the late teens or early adulthood.
Nearsighted Eye
The Farsighted (Hyperopic) Eye
Hyperopic, or farsightedness, occurs when people see far away objects more clearly than those that are
near. Hyperopic is caused when light rays are not focused by the time they reach the retina. Hyperopic is
usually a result of the curvature (power) of the cornea being too weak or the length of the eyeball being
too short. Glasses or contact lenses “pull” the poorly focused image forward toward the retina. For all
individuals over 40 years of age, the focusing mechanism of the eye weakens. The focusing change
(accommodation) helps the farsighted person see well in the distance, but as one ages and this
accommodation weakens, distance vision becomes blurred. The result is presbyopia.
Farsighted Eye
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19. Presbyopia
A normal part of the aging process, presbyopia, is a gradual loss of the eye’s ability to adjust the focus.
Presbyopia is due to the natural stiffening of the lens in the eye on near objects. This prevents the lens
from changing focus so that one can clearly see both distance and near objects. Presbyopia usually begins
to occur around the age of forty and it is commonly corrected by the use of reading glasses or bifocals.
Patients with myopia should realize that even correcting myopia would not eliminate the potential need for
reading glasses when reaching middle age.
Astigmatism
Astigmatism is the result of having a corneal surface that is not regular in shape. The eye is unable to
focus clearly at any distance because of this irregular focusing surface. Individuals with no astigmatism
have corneas that are shaped like basketballs while individuals with astigmatism have corneas that are
shaped more like footballs. There are many possible types of astigmatic corneas, which is why the doctor
must examine your eyes. People with astigmatism also are often myopic or hyperopic.
The eye reading analysis questions
You should be able to answer all the questions using the article. I have included websites to help you with
some of the question (if necessary).
http://www6.district125.k12.il.us/science/reg_physics/lensPCP/Vision.mov
1. Fill in the blanks. http://www.freezeray.com/flashFiles/eye.htm
2. Which part(s)
of the eye bend the
light?
3. Which part of
the eye do you
want the image to
form?
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20. 4. How does your eye focus on a near or far object? (What actually happens?)
http://micro.magnet.fsu.edu/primer/java/humanvision/accommodation/index.html
5. If you are nearsighted where does the image form when looking at an object that is far away? What
type of lens is needed to correct this? http://www.freezeray.com/flashFiles/eyeDefects.htm
6. If you are farsighted where does the image form when looking at an object that is near? What type of
lens is needed to correct this? http://www.freezeray.com/flashFiles/eyeDefects.htm
Normal Vision 7. Many older people become farsighted. Why?
8. If you were nearsighted with an astigmatism,
which correct surgeries would work?
Nearsighted Vision Corrected Nearsighted Vision
Farsighted Vision Corrected Farsighted Vision
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21. Lens Applications
The Refracting Telescope:
Hold a convex lens in each hand and look through
both of them. Try moving your hands back and forth
until you see a clear image through the lenses. (Note:
You may have to switch lenses so that the thicker one
is closer to your eye, or vice versa.)
1. Which lens worked better to have closer to
your eye? (As the eyepiece?)
2. How does your eyepiece focal length compare to the other (objective) lens’ focal length?
3. Is your image right side up or up side down?
4. Your image should be bigger, is it?
5. Telescopes are typically used to observe things like stars. We can assume then that the object
distance will be infinite. Where will the image location be for the objective lens?
6. What will the image size from the objective lens be?
7. Pay attention here… the image from the objective lens will now become the object for the
eyepiece lens (observe the diagram above). What you see through the eyepiece is really an
image of an image. You want the final image to be magnified, so the image formed by the
objective should be inside what? (hint: look at chart)
8. Complete the following: The image formed by the objective lens should be inside the
______________ of the _______________ lens.
Binoculars:
1. With binoculars you want the final image to be
right side up, but still larger. You can
accomplish this two ways. One way is to use a
convex lens and a concave lens. Hold up a
convex and concave lens and look through
them like you did before. Does it work better to
have the concave or convex lens closer to your
eye (eyepiece)?
2. So the concave lens is the _________ lens and
the convex lens is the ___________ lens.
3. The use of a third convex lens placed in-
between the objective and eyepiece lenses can
also flip the final image so that it is right side
up. The problem with this is you need a much
longer space – this is how a spyglass works.
(Harr mates! A spyglass is what pirates typically
used.) To get around this binoculars use
reflecting prisms to lengthen the distance that
the light travels. Examine the diagram to verify this.
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22. Compound Microscopes
1. A compound microscope uses two convex lenses both with short focal lengths. In contrast to a
telescope where the object is very far from the objective lens, a microscope has the object much
______ to the objective lens.
2. The image from the
objective lens is real and
larger, so where does the
object need to be
located? (hint: look at
your chart)
3. The final image you see
in a microscope (like the
telescope) is really an
image of an image. Also
in case you don’t remember the final image you see in a microscope is up side down. So the
image formed by the objective lens should be inside the _________ of the __________ lens.
4. Borrow a thick lens from a different group. Draw an arrow on your page and stand up and try to
construct a compound microscope using the two lenses. From #2, you should know about how
far to hold the objective (the bottom) lens from the arrow. Adjust the “eyepiece” until you get a
clear, up side down image. Your eye also needs to be close to the eyepiece.
The Projector:
1. The arrangement of lenses for a slide or
movie projector is shown in the diagram.
What type of lenses are the condenser
and projection lenses?
2. A projector needs to focus a sharp image
on a screen. Would this image be real or
virtual?
The Camera:
1. With cameras do you want to project an object on
a screen? (Hint: Think about the film.)
2. Is the image real or virtual?
3. Would you use a convex or concave lens?
4. If we know that a glass lens only has one specific
focal length, how is a camera able to focus on a
near or far object? (Hint: think about older
manual focus cameras. When you turn the screw
mount, what are you really moving?)
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23. GRAPHING LENSES IS PHUN!
Using the following data, graph 1/di vs 1/do:
do 5 cm 10 cm 15 cm 20 cm 25 cm 30 cm 40 cm 50 cm
di -3.33 cm -5 cm -6 cm -6.67 cm -7.14 cm -7.5 cm -8.0 cm -8.33 cm
1 / do
1 / di
1. DRAW A BEST FIT LINE!
2. Is this data for a convex or a concave lens? How do you know?
3. a. Using the lens equation and the above data, calculate the focal length of the lens. Show work
b. Describe how you can figure out the focal length by looking at the graph.
4. Label the following sections on the graph. (Hint: You may not be able to do all of the following)
a. The image is right-side-up?
b. The image is upside-down and bigger?
c. The image is the same size as the object.
d. The image is the smaller than the object.
5. A concave lens will always give an image that is ____________ and _____________.
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24. Using the following data, graph 1/di vs 1/do.
do 5 cm 10 cm 20 cm 25 cm 30 cm 50 cm 80 cm 100 cm
di -7.5 cm -30 cm 60 cm 37.5 cm 30 cm 21.43 cm 18.46 cm 17.65 cm
1 / do
1 / di
1. DRAW A BEST FIT LINE
2. a. Describe how you can figure out the focal length by looking at the graph.
b Using the lens equation and the above data, calculate the focal length of the lens Show work
3. Why is there no data for do = 15 cm?
4. Label the following sections on the graph.
a.The image is right-side-up?
b.The image is upside-down and bigger?
c.The image is the same size as the object. (one point)
d. The image is the smaller than the object.
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25. Lens Review
1. An object is placed 50 cm in front of a converging lens with a focal length of 20 cm.
a. Where would the image be?
b. (circle the correct answer) The image would be real / virtual and bigger / smaller.
2. Draw the ray diagrams below and describe the images as:
(a) real / virtual, (b) smaller / larger, and (c) RSU or USD
a)
b)
2F F F 2F c)
a)
b)
2F F F 2F c)
a)
b)
2F F F 2F c)
3. A microscope makes use of _____ con_____ lenses.
4. (a) Draw the ray diagram finding the image for the objective lens. (b) Use that image to be the object
for the eye piece and draw the ray diagram finding the final image for the eye piece lens.
objective lens eyepiece lens
2Fo Fo Fo 2Fo Feye Feye
25
26. 5. An object is placed 50 cm in front of a diverging lens with a virtual focal length of 20 cm.
a. Where would the image be?
b. (circle the correct answer) The image would be real / virtual and bigger / smaller.
6. What does a positive do mean? What does a negative do mean?
7. What does a positive F mean? What does a negative F mean?
8. An object 3 cm tall is placed 20 cm from a converging lens. A real image is found 10 cm from the lens.
(a) What is the image size? (b) What is the focal length of the lens?
9. An object is placed 10 cm from a lens with a focal length of -2 cm. (a) What type of lens is it? (b)
Where is the image located? (c) Is the image real or virtual? (d) Is the image bigger or smaller?
10. An object is placed 25 cm from a lens with a focal length of 5 cm. (a) What type of lens is it? (b)
Where is the image located? (c) Is the image real or virtual? (d) Is the image bigger or smaller?
11. A __________sighted person focuses light before the retina. To correct this you would use a
__________ lens.
12. A __________ sighted person focuses light after the retina. To correct this you would use a
__________ lens.
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