2
2
A Little Light Physics
• Light: A wave; a stream of photons, tiny particles
that each consist of one quantum of energy
The First Steps in Vision
2
A Little Light Physics (cont’d)
• Light can be
– Absorbed
2
A Little Light Physics (cont’d)
• Absorbed: Energy (e.g., light) that is taken up, and is
not transmitted at all. Photon disappears.
– Transmitted
– Diffracted
– Reflected
– Refracted
1
2
A Little Light Physics (cont’d)
2
• Transmitted: Energy that is passed on through a
surface (when it is neither reflected nor absorbed by
the surface)
A Little Light Physics (cont’d)
• Diffracted: Bent, or having waves that are spread
out, (e.g., waves of sound or light, as they pass
through a narrow aperture).
– Short wavelength sunlight is diffracted (scattered)
more as enters Earth’s atmosphere
– This is why the sky is blue
2
A Little Light Physics (cont’d)
•Reflected: Energy that is redirected when it strikes a
surface, usually back to its point of origin
–Selective reflection: some wavelengths absorbed,
others reflected (object color)
2
A Little Light Physics (cont’d)
• Refracted: Energy that is altered as it passes into
another medium, (e.g., light entering water from the
air).
– Alters direction (bends light ray)
– Lenses
2
2
Structure of Eye: Outside
2
Structure of the Eye: Inside
• Cornea
• Anterior Chamber
– Aqueous Humor
• Pupil /Iris
• Lens
– Ciliary Body
• Vitreous Chamber
– Vitreous Humor
• Retina
– fovea
• Optic Nerve
– Blind Spot
2
The Human Eye
2
Image Formation
• Focused image
– Single point in environment projects on to a single
point on retina
• Blurry image
– Single point in environment projects on to multiple
points on retina
3
2
2
Image Formation
• Refraction is necessary to focus light rays and this is
done by the lens
– The lens can change its shape, and thus alter the
refractive power: Accommodation
2
Refractive power of lens
• How strongly lens bends light
• Measured in diopters = 1/focal length in meters
• Stronger lens:
– shorter focal length
– Higher power (diopters)
2
Average eye
• cornea to retina = 1.7 cm [.017 m]
• power = 1 / .017 = 60 diopters (roughly)
Focal
Point
Parallel Light Rays
Focal Length
.017 m
4
2
Accommodation: Lens changes shape
2
• Near object: need more power ( ~ 68 diopters)
• Far object: need less power (~ 60 diopters)
2
Nearsighted: Myopia
• Focal point in front of retina
– Eye too long
– Power too strong
• Able to focus on near objects (but not far)
Normal: Emmetropia
• Image focused on retina
• Accommodation -- able to focus near and far
2
Farsighted: Hyperopia
• Focal point behind retina
– Eye too short
– Power too weak
• Able to focus on far objects (but not near)
5
2
Corrective lenses
• Myopia: concave lens
– weakens power
• Hyperopia: convex lens
– increases power
2
Near & Far Points
FP = 8
Emmetrope
NP
Myope
FP
FP = 8
2
Astigmatism
•Cornea not perfectly spherical (football shaped)
•Light bent more along one axes than other
NP
Hyperope
NP
2
Presbyopia
• Cells of lens die over time -- becomes less flexible
• Lens can’t assume stronger power
• Average near points
– 20-year-old: 10 cm
– 30-year-old: 14 cm
– 40-year-old: 22 cm
– 60-year-old: 100 cm
6
2
Presbyopic Myope
2
Corrective Surgeries
• Change shape of cornea (power)
• LASIK
With corrective lenses
FP = 8
NP
cartoon
movie
Without corrective lenses
FP
NP
7