The photo shows a magnifying lens that is concentrating sunlight onto a small spot. This spot is an image of the Sun. In this small spot the sunlight is converted into thermal energy. It becomes very hot - hot enough to start burning.
There are also lenses in your eyes and in cameras. These lenses focus light at one point that is called the focal point of the lens. By focusing light, these lenses make images.
In this lesson you will study how lenses create images and how images form in your eyes. You will also compare the make-up of the human eye to that of the camera.
Play the Science 8 Multimedia CD, provided to you on the inside cover of Module 1, on a computer. Once the first screen appears, select the title "Real Images." This will give you an introduction to how a lens can form an image on a screen. Note: Don't try to remember all the details.
Types of Lenses
Light rays refract as they enter and come out of glass. That's because glass is a medium that is denser than air. A piece of glass with rounded surfaces can make light rays converge or diverge. This specially shaped piece of glass is a lens.
Turn to page 208 of the textbook and read the top half of the page.
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The lens faces are like parts of a sphere, so all incoming parallel light rays diverge or converge to a point. And the more the surfaces are curved, the closer the focal point is to the lens.
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2. Refer to Figure 3.29 on page 208 of the textbook. Explain why a light ray passing through the centre of either lens does not bend.
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Hold a concave lens over the words on this page. Move the lens slowly up and down at a variety of distances until you see a clear image. Now try the same procedure using a convex lens.
Find Out Activity
See for Yourself!
Refer to the activity on page 208 of the textbook.
Follow the steps of "Procedure." Record your observations in your notebook.
3. Why does a single drop of water work better as a convex lens than does a larger amount of several drops?
4. Answer the questions from "What Did You Find Out?"
The Eye and the Camera
Read pages 210 and 211 of the textbook to find out what lenses do in the eye and a camera. You will also discover how lenses are used to improve vision.
5. Muscles in the eye change the shape of the lens to view both distant and nearby objects. The contraction of these muscles lengthens and thins the lens. Predict whether the lens would thicken or thin to view a distant object. Explain your prediction or use a ray diagram to support your prediction.
6. a. Define far-sighted.
b. How is far-sighted vision corrected?
7. a. Define near-sighted.
b. How is near-sighted vision corrected?
c. Eyeglasses can be made of either glass or plastic. This fact led a student to ask the following question: Is plastic the best optical material to use in eyeglasses? Rephrase the student's question so that it can be tested.
8. Create a table that compares the functions and components of a camera to those of the human eye. Include the terms aperture, eyelid, and pupil in your table. Try to infer the function. Read ahead on pages 215 to 219 if you are unsure of the functions.
Note: Some eyeglasses are made of "high-index" material that allows the lens curvature to be small and the lens to be thin but still provide strong correction.
9. Read "Cool Tools" on page 214 of the textbook. How does a digital camera differ from the camera you just analysed?
In the next investigation you will learn how a basic camera works. This investigation should be done under the supervision of an adult because it involves fire.
I n v e s t i g a t i o n 3E
The Camera
Refer to the "Inquiry Investigation" on pages 212 to 214 of the textbook.Read the beginning of the investigation. "Prediction" is about the distance you need between the lens and a screen to have a sharp image of an object. Think about this: Does this distance have to change when the object comes closer to the lens?
Avoid any risk of accidental fire. Ask your teacher or home instructor to help you whenever you are using the candle.
Follow steps 1, 2, and 3(a) of "Procedure" from "Part 1."
10. Answer step 3(b) of "Procedure."
Follow step 3(c).
11. Continue with steps 1 and 2 of "Procedure" from "Part 2." While you do the steps, place your data in the table made in question 10.
12. Complete question 1 of "Analyze."
Turn to Assignment Booklet 3B. Complete question 1 from Section 2.
Going Further
Take "Investigation 3E" a little further! Try "Math Connect" and "Computer Connect" on page 214 of the textbook.
Putting It in Focus
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Typically for fixed-focus cameras, you have to make sure that what you are taking a picture of is more than two metres away. If it's closer than that the photo will be fuzzy. In other words, it will be out of focus. To take close-up shots you need a camera with an adjustable focus. Such a camera is generally a bit more expensive than a fixed-focus camera.
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13. Define the following terms: accommodation, near point, and far point.
14. What is both the near point and the far point for an adult with normal vision?
Letting in the Light
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You may have seen a photograph in which everything is too light or too dark. A camera needs to admit the proper amount of light in order to work; if not, photos look overexposed or underexposed.
You may have found everything hard to see after coming indoors on a bright, sunny day. This illustrates that your eye must also admit the proper amount of light. Otherwise, your vision suffers.
Read page 216 and the top of page 217. You will learn how the amount of light entering a camera or an eye is controlled.
15. Define the following terms about vision: diaphragm, aperture, shutter, iris, pupil, and iris reflex.
Seeing the Image
Read the rest of page 217, all of page 218, and "Career Connect" on page 220 of the textbook.
16. How is the area at the "blind spot" different from the rest of the retina?
17. What purposes do the aqueous and vitreous humours have in the eye?
18. What does an optometrist do?
Laser Light
Did you know that laser light is involved when you play music on a CD or watch a DVD movie? A laser beam is used to read the information from the spinning disk.
In your textbook, laser light is described as a coherent light. To understand the idea of coherence you need to understand the wave model of light. This goes beyond what is expected of Science 8 students.
Just accept that because laser light is coherent light, it is stronger than normal light and can be brought to a sharper focus. Because of these special properties, laser light is used in eye surgery.
Turn to pages 246 and 247 of the textbook and read "Laser Light."
19. List two ways in which laser light is used in eye surgery.
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Are all eyes created equal? Different environments and lifestyles demand specially adapted optical equipment. The more you "look" at eyes and their associated structures, the weirder the structures become.
To start your investigation of what may sometimes seem like science fiction, read "Did You Know?" on pages 231 and 254 of the textbook and "Off the Wall" on page 234.
20. See if you can answer these trick questions.
a. What colour is the grass that deer eat? Explain your answer.
b. Will that fish truly appreciate your colourful new swimsuit? Explain your answer.
c. Should bumblebees wear sunglasses with UV protection? Explain your answer.
Have fun thinking about these trick questions.
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Find out more interesting and weird things as you do the next investigation!
I n v e s t i g a t i o n 3F
What an Eyeful!
Refer to the "Think and Link Investigation" on page 219 of the textbook.
21. Use the investigation, except step 3 of "What to Do," as a guide to research and to compare the eyes of a different type of animal. If you are not in a science class, then share your findings with your family, your friends, and your instructor.
Going Further
To learn about light-based technology careers, try "Internet Connect" on page 220 of the textbook.
22. To test your understanding of the concepts in this lesson, answer questions 1 to 4 of "Topic 4 Review" on page 220 of your textbook.Looking Back
In this lesson you investigated how concave and convex lenses affect light, and you also studied how these lenses correct eye defects. You then looked at the structure and function of both the parts of the eye and a camera. You also investigated the intriguing eyes of a variety of animals.
Turn to Assignment Booklet 3B. Complete questions 2 to 5 from Section 2.
1. A lens is a curved piece of glass, plastic, or other transparent material used to make light rays converge or diverge.
A concave lens is a lens that is thinner in the middle than it is at the edges. It causes refracted rays to spread out or diverge.
A convex lens is a lens that is thicker in the middle than it is at the edges. It causes refracted rays to bend toward each other or converge.
2. The light ray going through the centre of either lens is perpendicular to the surface of the lens material, which is made of glass. For this light ray the angle of incidence is 0°. For this angle there is no change in the direction of the light ray due to refraction.
3. A single drop of water has a curved surface. A larger number of drops has a flat surface.
4. Textbook questions 1 and 2 from "What Did You Find Out?" on page 208:
1. A convex lens is used to make a magnifying glass.
2. Answers will vary. Samples follow.
(a) A concave lens causes radiant energy to diverge. This lens could be used in a car headlight or in a lantern.
(b) A convex lens causes radiant energy to converge. This lens could be used in a spotlight or in a narrow beam flashlight.
5. A thinner convex lens causes less refraction. To see distant objects, the eye muscles contract. This thins the lens - less refraction is necessary to focus the image. For nearby objects the muscles relax, causing the lens to thicken. More refraction is necessary to focus the image.
6. a. Being far-sighted means you're unable to focus the image of a nearby object on your retina because your eyeball is too short.
b. Far-sighted vision is corrected with a convex lens.
7. a. If you're near-sighted you're unable to focus the image of a distant object on your retina because your eyeball is too long.
b. Near-sighted vision is corrected with a concave lens.
c. Answers may vary. The question should be similar to this: What effect does the type of plastic (MV) have on the angle of refraction (RV)?
8. The following is an example of how your table could look.
| Comparing Structures | ||
| Function | Camera Component | Eye Component |
| allows light in | aperture | pupil |
| controls amount of light entering | diaphragm | iris |
| opens and closes to let light in or prevent it from entering |
shutter | eyelid |
| refracts light to focus image | rigid lens | flexible lens |
| light-sensitive screen for image formation | film | retina |
| adjusts lens for near and far objects | ffocusing ring | ciliary muscle |
9. Digital cameras do not use film. Images are stored on a silicon chip as a series of electrical charges. The brighter a point of the image, the higher the stored electric charge. A computer measures the resulting design of tiny electrical charges and converts them into binary numbers - these are the codes computers use to recreate the images.
13. An accommodation is an automatic adjustment of the lens in the eye to bring images of objects from various distances into focus on the retina.
The near point is the shortest distance at which an object is in focus.
The far point is the greatest distance at which an object is in focus.
14. The near point is 25 cm, and the far point is infinity.
15. The diaphragm is the camera part that changes the size of the aperture and, therefore, the amount of light entering the camera.
The aperture is the opening in a camera that lets light in.
The camera part that opens briefly to let light through the aperture is the shutter.
An iris is the band of muscle in the eye that controls the pupil's size.
The opening in the eye where light enters is called the pupil.
The iris reflex is the automatic change in the pupil's size in response to varying light conditions.
16. The retina has no cells that sense light at the blind spot. This is the point where the optic nerve enters the retina.
17. The fluids or humours keep the eye from collapsing and help to refract light entering the eye.
18. An optometrist prescribes lenses to correct vision problems.
19. Laser light is used in eye surgery to
20. a. Deer see only black and white colours, so the grass they eat must be gray.
b. Fish have a highly developed sense of colour, so any truly fashion-conscious fish will have an opinion.
c. If bumblebees wore sunglasses, they would not be able to detect the ultraviolet radiation patterns that help them identify flowers.
focal point: the point at which parallel rays reflected by a mirror or refracted by a lens meet or appear to meet
focus: to bring light to a point; to bring rays of light together to produce a clear image