Lens unit 2010 5

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Accelerated Physics Lens Unit 2010 Lenses Activity Homework 5/13 Thursday Go over test

Go over homework

Snell’s Law / Intro to Lenses Phet Lens WS (due MON)

5/14 Friday Exploring Lenses Activity To discuss images To learn lens equation

Lens Equation WS (Front Side) Phet Lens WS

5/17 Monday To go over homework Learn convex ray diagrams Start lens diagrams

Ray diagrams for convex Lens Equation WS (Back side)

5/18 Tuesday Learn concave ray diagrams To start lens lab

Ray diagrams for concave 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 Learn about vision / correcting vision problems Post-lab WS

Finish post-lab WS Lens Graphing Practice

5/21 Friday (Assembly Schedule)

Real lens activity (determine vision problems) Lens applications

Read and take notes over section 25.2-25.3 (focus on concepts NOT equations)

5/24 Monday Finish lens applications Review for the test

Review sheet

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 Periods 1, 2, and 4

Review for finals

5/31 Monday Memorial Day – No School Review for finals 6/1 Tuesday

Second Day of Finals Periods 3, 7, 5

Review for finals

6/2 Wednesday

Last day of Finals Periods 8, 6

Enjoy the Summer!

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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 (do < F,

etc)

measure di

(use ruler)

classify di (di = 2F,

etc)

real vs.

virtual

image size (bigger, smaller, 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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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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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?

_____

2. Are thinner at the center than at the edge?

_____

3. Are concave? _____

4. Are convex? _____

5. Are plano convex? _____

6. Are plano concave? _____

7. Is double concave? _____

8. Is double convex? _____

9. Magnify? _____

10. Minify? _____

11. Refract light? _____

12. Converge light? _____

13. Diverge light? _____

14. Can form an image on a screen? _____

15. Cannot form an image ona screen?

_____

16. Are most important for projectors and

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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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? _____

2. Is less curved? _____

3. Is stronger? _____

4. Is weaker? _____

5. Refracts light less? _____

6. Refracts light more? _____

7. Has the shorter focal length? _____

8. Has the longer focal length? _____

9. Magnifies more? _____

10. Magnifies less? _____

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Convex Ray Diagrams

The Three Rays: 1. 2. 3. One Special Case for Convex Ray Diagrams

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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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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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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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Lens Equation Worksheeet do = object distance from lens di = image distance from lens f = distance from lens to focal point (focal length)

Lens Equation

�

1do

+1di

=1f

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)

Example: A car is 2 m in front of a 1 m focal length lens. Where will the image be located?

do = 2m f = 1m di = ? 1/di + ½ = 1/1 1/di = 1 – ½ 1/di = .5 di = 2 m

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More Ray Diagrams Directions:

1. Draw Ray diagram 2. Compare to the lens equation

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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

location Image location

Image type- real or virtual

Image size Image orientation- RSU or USD

1

2

3

4

5

6

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Case 1: (the object is way off the page to the left <-----------)

Case 2:

Case 3

Case 4

Case 5

Case 6

2F 2F F F

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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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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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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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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

7. Many older people become farsighted. Why? 8. If you were nearsighted with an astigmatism, which correct surgeries would work?

Normal Vision

Nearsighted Vision Corrected Nearsighted Vision

Farsighted Vision Corrected Farsighted Vision

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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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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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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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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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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 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.

2F 2F F F

a) b) c)

2F 2F F F

a) b) c)

2F 2F F F

a) b) c)

2Fo 2Fo Fo Fo Feye Feye

objective lens eyepiece lens

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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.