Transcript Unit 3: Light and Optical Instruments

Unit 3: Light and Optical
Instruments
Lesson 5: The
Human Eye
J. Pulickeel
January 2007
Eye Spy
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The lens in the human eye a convex lens
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This focuses light rays from an object by refraction, into a point
•
The light refracts through the lens onto a light sensitive area
called the retina
Eye vs Camera
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The human eye and the camera have a lot in common
•
In order for us to see, light is reflected into our eye, refracted through the lens
and focused on the retina.
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In a camera, the lens refracts the light and the film senses the light.
Bringing in the Light
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The iris and pupil of the eye diaphragm and aperture in a camera do
the same thing. Both control the amount of light that enters the eye
•
On a cloudy day, not enough light will be present to take a picture with a
camera. By opening the aperture, you let more light in.
•
Similarly, if it is dark, the iris increases the size of the eyes opening (the
pupil) to let more light in.
Shut up and Focus!
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In a camera, if an object moves closer to the film, the lens must move
farther to keep the image focused. This is what you do when you turn
the focusing dial.
•
In the eye, the ciliary muscles change the shape of the lens. If the
object comes closer, the muscles make the lens bulge in the middle.
Shut up and Focus!
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As people become older, the lens stiffens and loses it ability to change
shape. Thus old people have to hold books farther away, or wear
convex glasses
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Most adults focus on things 25 cm away. Most babies focus on things 7
cm. away.
I.C.U
• At the back of the camera is the film, and the image is focused
onto it.
The light energy cause the chemicals on the film to change and
record the image
• In the human eye the retina senses the light. The retina cells
create small electrical impulses that trave to the brain through
optic nerves
Cool Facts!
The point where the optic nerve enters the
retina does not have any light sensing cells.
This is known as your blind spot!
Corrective Lenses
• In order to see an image clearly, the lens
must focus the image to a point on the
retina
• Near sighted people have trouble seeing distant
objects. The image point is formed before the
retina.
• For other people, the eye has a shorter shape
and the image has not formed by the time the
light reaches the retina.
Summary