Download The Eye

The Eye
Eye Dissected
 The eye is an optical lens
system.
• Two single lenses
 The goal of the eye is to focus
an image on the retina.
Corneal Lens
 The cornea is a curved
membrane with fluid behind
it.
• Two indices of refraction
• ncorn = 1.376
• naq = 1.336
 This is roughly the refraction
from air to water.
• Eye works poorly
underwater
R
 (neye  ns )
f
Crystalline Lens
 The lens is a denser material
for fine focusing.
• nlens = 1.406 at maximum
 Relaxed muscle provides little
additional focus.
• For distant objects
 Contracted muscles provide
more focus.
• Relaxes tension and creates a
bulge
• For near objects
Accommodation
 The focusing by the lens is
 The moon subtends an angle
called accommodation.
of about 0.009 rad as seen
from the earth.
 The size of the moon on the
retina comes from the image
triangle.
 The relaxed lens forms a
combination with the cornea.
• Object focus 15.6 mm in front
of cornea
• Image focus 24.3 behind
• Optical center 17.1 mm in
front of the retina
 Typical shifts in
accommodation are a few mm.
yi  si
 (17.1 mm)( 0.009 rad)  0.2 mm
Retina
 The retina is the screen for the
eye.
• Light sensitive cells
• Neurons for signal
transmission
 Ideally it should be at the focal
plane.
• Otherwise image is blurred
Pinhole Camera
 A pinhole opening can act as a
single lens.
• Forces ray convergence at the
opening point
• Iris in eye can act as pinhole
lens
 A pinhole camera is a box with
a pinhole opening to focus
light.
Single Lens Reflex
 A single lens reflex (SLR)
camera acts much like an eye.
• Shutter like iris
• Lens fixed
• Adjust distance to screen
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