Is bacteria reproduction the slowest process?

Answer 1

The human eye has 3 receptors for color- one each for blue, green and red. These are the 3 primary colors of light. All other colors are a result of a mixture of these colors. The retina takes in light. Colors are made up of light and the How_does_the_human_eye_see_colourof an object depends on what kind of lights reflect off of the object. White or clear light is a combination of all the different colors on the light spectrum. Some object absorb all lights except say, blue. The blue light reflects of the object and is picked up by your retina, sent back through your optic nerve, and sorted in the brain. The eye detects light, and the on scale ROYGBIV (stands for the colors of the rainbow) the eye would be able to distinguish what color an object is, by light passing through only some of the colors on the scale and boucing off the eye, making only certain colors visable at one time. The human eye has three different types of cones (color receptors) in the retina. Each type is designed to respond to different wavelengths of light. Depending upon what color light is falling on the retina, the cones will be stimulated at different degrees. The brain "listens" to the comparative inputs from each type of cone as they send their response to the brain. Based upon how each cone in comparison to the others sends its signal to the brain, the brain then interprets the combined cone responses and assesses what wavelength must have fallen on the retina to produce the specific set of combined color responses to the visual cortex.Your eyes, Eye charts, Human eye model, Light box, Lens box containing: a 13-

mm aperture, spherical lenses with the powers +2.0, +7.0. +20.0 and -1.75 (all

measured in diopters), and cylindrical lenses with the powers of +1.75 and -5.50.Complete the following exploration activities with your own eyes. Everyone should do

each activity and record your results in your logbook. You can do the activities in any order,

however, be sure to clearly state what you did in your descriptions in your logbook.Look up from this How_does_the_human_eye_see_colourat an object somewhere across the room and then look back at

the paper. Were you able to clearly see both the other side of the room and the paper?

This activity illustrates the process of accommodation, or focusing, for your eyes.

Accommodation is automatically accomplished in your eye by a set of muscles that changes the

curvature of the crystalline lens. (See figure on page 3 for details.) That is, the eye actually

changes the shape of the lens.

1. Think of the glass lenses you used last week in lab. Were they able to produce clear

images for any and all object distances or were they constrained by certain parameters?

What does the ability of your eye to accommodate tell you about the focal length of your

eyes' lenses?

(b) What is Your Visual Acuity?

You can measure your own visual acuity. Stand at the taped marker in the hallway and

look at the eye chart hanging on the wall. Only one group should do this activity at a time. If a

group is already How_does_the_human_eye_see_colourwith this equipment, just move on to the following explorations and

come back to this later.

A person whose vision is rated 20/20 is seeing details at a distance of 20 feet as clearly as

a "normal" individual would. A rating of 20/15 is better than average for at 20 feet the person

can see details that would be clear for "normal" vision at 15 feet. When visual acuity falls below

20/200, the individual is considered to be legally blind.

2. Which lines can you read clearly? What is your visual acuity at twenty feet? Describe in

your own words what this rating means.

3. Do you think this test is as good as an eye test at the doctor's How_does_the_human_eye_see_colourIf not, what things do

you think should be improved to make it a better test?

Next, measure your near point. That is, measure how close you can hold this page to

your eyes and still see it clearly. The typical near point for people is 25 cm (though it can be

much closer when you are young). Record the value of your nearpoint.

4. How are the muscles changing your lens to allow you to see things this close? Why do

you think your nearpoint changes with age? Remember, the curvature of the lens is

determined by the ciliary muscles. Do you need greater or less curvature to see near

objects? That is, do you think the lens is being made thicker or thinner by the muscles?

(d) Your Blind Spot

The optic disc is the region of your eye where the optic nerve originates. There are no

light detectors on this disc. Because light striking this area goes unnoticed, it is commonly

called the blind spot. You do not actually "notice" a blank spot in your visual field because

involuntary eye movements keep the visual How_does_the_human_eye_see_colourmoving and allow the brain to "fill in" in the

missing information.

· Close your left eye and stare at the cross on Eye Chart #2 with your right eye, keeping it in

the center of your field of vision. Begin with the page a few inches away and gradually

increase the distance. Note how far the paper needs to be away from your eyes to have the

dot "disappear."

· Repeat this activity by closing your right eye and staring at the dot with your left eye.

5. How far away does the paper have to be before the dot "disappears" and then reappears

for each of your eyes?

(e) Astigmatism

Astigmatism is usually caused when the cornea or lens is out-of-round. This common

defect causes point-like objects to focus as lines and therefore blurs the image.

Test your own eyes for astigmatism using the figure in Eye Chart #2. Look with one eye

at the center of the pattern. Sharply focused lines appear dark and those that are not in focus

appear dimmer or gray.

Record your observations for your own eyes.

Now that you have explored some of the remarkable properties of your own eyes, you

will attempt to model the physical properties of the human eye with the provided human eye

model.

Here is a brief description of the parts of the human eye along with their counterparts in

the eye model.

Image of a Human Eye Image of the eye model you will use in lab

Part of a

Human Eye

General Description Part of the

Eye Model

Cornea The first and most powerful lens

of the eye's optical system

Meniscus lens C

(Fixed in the eye model)

Iris Controls the amount of light

intensity that enters the eye's

optical system

Aperture insert

(Placed at position G1)

Pupil The variable opening in the iris Aperture insert

(Placed at position G1)

Crystalline Lens Second lens of the eye's optical

system

Lens insert

(Placed at position L)

Ciliary Muscle Muscles controlling the curvature

of the crystalline lens

Vitreous Humor Clear colorless jelly that fills the

eyeball

The eye model is filled

with water.

Retina Light sensitive membrane

distributed over the back of the

eyeball

Curved screen

(Placed at position R)

Fovea The most sensitive region of the

retina

Dashed markings on

the curved screen.

Optic Nerve Conducts visual stimuli to the

How_does_the_human_eye_see_colour

Shown as the spot on

the curved screen.

The power of a lens is often measured in the unit of diopters (for instance, eyeglass

prescriptions are given in units of diopters). The power of a lens is computed by taking the

reciprocal of its focal length when the focal length is measured in meters.

Lens power (diopters) =

1

f (m)

6. Compute the power of the converging lens(es) that you used in the last lab. If a lens has a

higher power, then does it have a longer or shorter focal length?

Remove any lenses that may have been left in the model from the last class (positions L ,

G and S). Check that the curved screen, which simulates the eye's retina, is placed in the

"normal" position (R). That is, place the screen in the middle of the three possible positions.

Take your eye model into the hallway and fill it with water at the sink before doing any

of the following activities. Fill it so the model's cornea is completely covered, but don't fill it so

full that water spills over the top.

! · Please be careful not to spill water in the hallway.

· If any water is spilled, please notify your instructor right away so it can be

cleaned up before anyone slips.

7. Why do you think you use water in your eye model? How does this water relate to the

human eye? What physical properties might it simulate?

Set up the model so that it is "looking" toward a partly-open window or other bright object

4 or 5 meters away. Use an object with features that you can recognize in the image (like

your lab partner standing in front of the window). Don't use a bare light bulb, which only

looks like a bright spot and does not have any distinguishing features.

• Describe the object and describe precisely how the image looks on the retina. Comment on

important features like the size of the image, if it is right side up or upside down, etc.

• Find a spherical lens to insert into the groove L that gives a clear, sharp image of the far

away object on the retina.

· Record both the power of this lens in diopters and its focal length in meters. Describe how

the new image looks on the retina. Note the characteristics of the image including: whether

it is erect or inverted, the image size compared to object size, ...

8. What part of human vision are you currently modeling with this set up?

• Without changing anything in the eye model, turn the model so it is looking at a near object

(namely, the light box). Position the light box with the radially-slotted pattern 35 cm in

front of the model's cornea.

· Sketch the image of the light box on the retina and describe how it looks.

9. How does the quality of the image compare to the image that was formed when the

model was looking far away?

• Replace the crystalline lens with one that makes the image of this near object clear.

• Record your observations and lens choice. Carefully describe this image along with any

notable characteristics.

10. How does the crystalline lens needed for the model to have clear far vision compare to

the lens needed to clearly view near objects?

11. Does the image that is formed on your retina differ at all from what your brain tells you

that you are seeing? Explain.

12. How does the process of accommodation for your eyes compare to and differ from the

process of accommodation of the eye model?

!

· During the remainder of this lab, you will be modeling the eye's function when

it is looking at near objects. Therefore, you must leave the crystalline lens for

near vision in place (position L) for the remainder of the lab.

· Feel free to verify that you have the correct lens by comparing with another

group or asking your lab instructor.

13. Using what you know about image formation with lenses and with this eye model, do you

think the optic nerve in either of your eyes is located at the center of retina, between the

center and your nose, or between the center and your ear? Explain the reasoning for your

choice, using sketches when helpful.

Hint! Think about the results of your blind spot test!

14. Based on your reasoning, is your eye model a human right eye or left eye? Explain and

draw a sketch of your evidence.

Two of the most-common defects that occur with human vision are farsightedness and

nearsightedness. These two conditions are briefly defined here.

Farsightedness

Someone with farsighted vision is only

able to clearly see objects far away.

Farsightedness (hypermetropia) occurs if a

person's eyeball is "short." This results in

parallel light being focused behind the retina.

Nearsightedness

Someone with nearsighted vision is only

able to clearly see near objects.

Nearsightedness (myopia) occurs if a person's

eyeball is "long." This results in parallel light

being focused in front of the retina.

Becoming an optometrist for an eye model...

· Set aside your model of a "normal" eye from Activity #3.

· Your lab instructor has two patients, Martha and George, who are in need of eyeglasses.

· Request a patient from the instructor so you can complete the following activity.

· If you find that a patient is already busy with another doctor, then continue on to Activity

#5 until the patient is available.

· Be sure to return the patient to your instructor once you have finished.

15. Complete a study of your patient as he/she is looking at the light box from a distance of

35 cm. As part of this study, be sure to answer all of the following How_does_the_human_eye_see_colour. Explain

the evidence that led you to your conclusions.

· Record your patient's name.

· Is this patient nearsighted or farsighted?

· What impact does their visual defect have on their ability to form a clear image?

Give a careful description and/or sketch.

· What shape of lens is needed to correct this defect?

· Using the lenses provided in your box, find an appropriate lens to correct this

patient's vision. Keep in mind that you are only licensed to determine a prescription

for this patient (position S1), you are not licensed to do surgery! (That is, do not

remove the lens L!)

· Make a note of your prescription and the resulting image formation.

16. Repeat this activity for the second patient, How_does_the_human_eye_see_colourthe same questions listed in #15.

In the human eye, astigmatism is generally caused by a slight cylindrical curvature of

the cornea. Thus, a change in the model's cornea would perhaps be the logical way of

producing this effect. However, this is impractical since the cornea of the model is a fixed lens.

However, the same effect can be accomplished by inserting an additional lens.

• Return to using your "normal" eye model from Activity #3.

• Put the object box at 35 cm. Insert a cylindrical concave lens (-5.5 diopters) immediately

behind the cornea, producing astigmatism.

• Remember - the crystalline lens you found for near vision in the first activity should still be

in place.

• Turn the cylindrical lens a little to make only one line of the image sharp.

· Make a sketch of the blurred image and record the lenses that you are using.

Becoming an optometrist for an eye model...

Your eye model no longer represents normal vision, but vision with astigmatism.

Assume you are an optometrist and need to prescribe a corrective lens (i.e., glasses) to correct

this patient's vision.

· Place in front of the cornea the correcting convex cylindrical lens (1.75 diopters) and turn it

until the image is again sharp.

• Change the angle of the rear lens and repeat.

17. Explain how you think one lens is able to correct this vision defect. Note, the axis of a

cylindrical lens is defined as the line along the thinnest part of the lens.

Activity #6: Modeling Compound Defects with the Eye Model

Astigmatism is often accompanied by farsightedness or nearsightedness You will now

model these compound defects as well as attempt to correct for them.

• Be sure to finish Activity #4 before starting Activity #6!

• In order to study this phenomenon, place a concave cylindrical lens (-5.50 diopters) at G1

immediately behind the cornea with its cylindrical axis vertical.

• In addition, place the retina in the position to give myopia. Make a note of how you model

this eye defect.

Becoming an optometrist for an eye model...

Assume you are an optometrist and need to prescribe corrective lenses (i.e., glasses) to

correct this eye's vision.

· Correct the eye's vision by choosing the proper combination of eyeglass lenses (S1 and S2).

· Record the kinds of lenses used and the results of the lens combination.

In actual practice the two correcting lenses are combined into a single eyeglass lens

In the eye disease known as a cataract, the lens becomes opaque. When this condition

exists, the crystalline lens is often removed.

• Return the eye model so that it represents a "normal" eye.

• Remove the lens L from your model.

18. Is vision still possible for someone who has a lens removed? Explain.

19. What do you need for the eye to see clearly? With this eyeglass lens, at what distance(s)

is the image still distinct? Would another lens allow vision at another distance? Test

your hypothesis and describe the results.

Activity #8: Wrap Up

20. Describe in your own words at least two ways that the eye model is a good model for

the human eye and at least two ways that the eye model is not a good model for the

human eye.

Please do the following before you leave...

• At the conclusion of this experiment, be sure there are no lenses left in the model.

• Empty and rinse the eye model at the sink in the hallway and dry it with paper towel.

• Be sure to also dry each lens and put them all back into their small metal box.

• You should only have one of each type of lens and aperture at your lab station.

• Be sure all "patients" have been returned to your instructor.