Physics 1230: Light and Color
• The Eye: Vision variants and Correction
What does 20/20 vision mean?
Snellen chart
http://www.bgsm.edu/eye/cornea/normal1.htm
Examples:
20/20 vision - Normal
20/40 vision - Worse than normal.
- Can see at 20 feet what someone with normal vision can see at 40 feet
- Can only see letters that are twice as large as someone with normal vision
20/15 vision - Better than normal
20/30
Visual acuity is usually measured with a Snellen chart
Furthest distance that a person with normal vision can see object Furthest distance
Levels of Vision
* 20/20 - Normal vision. Fighter pilot minimum. Required to read the stock quotes in the newspaper, or numbers in the telephone book.
* 20/40 - Able to pass Driver's License Test in all 50 States. Most printed material is at this level.
* 20/80 - Able to read alarm clock at 10 feet. News Headlines are this size. * 20/200 - Legal blindness. Able to see STOP sign letters.
* For normal vision, the far point (or farthest point that can be seen) is usually infinity.
How does the eye form an image? (revision)
T he eye uses 2 lenses to form a sharp image on the retina
:• Cornea – Strong lens of fixed focal length – does most of the focusing • Eyelens – Weaker lens of variable focal length
- Enables us to focus on objects at different distances
eyelens
cornea
Normal Myopic (Near-sighted) Hyperopic (Far-sighted)
Common Eye problems - Near and far-sightedness
Relaxed eyes viewing a distant object
retina
Primarily determined by:
Normal Eye
THE NORMAL EYE
• For clear vision, objects focused exactly on retina
• Cornea’s curvature (or its focal length) exactly match to length of eyeball
Near-sighted eye (Myopic)
THE NEAR-SIGHTED EYE
- Cornea is too curved (focal length too short) or eyeball too long - Image of distant object forms in front of retina.
What the world looks like with myopia
• Foreground clear
Who has myopia?
Approximately one in four Americans is near-sighted, totaling 70 million people. The degree of myopia can vary from low (-1 to -3 diopters) to high (greater than 6 diopters). Myopia tends to start in the early teens (earlier for high myopes), and increases as the eye grows in length during puberty. It tends to stabilize after age 18.
Myopia is not caused by reading at an early age, prolonged reading, reading in the dark, watching TV too closely, wearing glasses too strong, wearing
glasses too weak, or other similar folklore. Some controversy exists over whether myopia can be made worse by prolonged near work as a child, but this has never been proven.
In studies, it has been correlated with higher I.Q.s., but this probably has to do with the fact that myopes tend to read more since that's where they can see the best without glasses (see below). Myopia tends to run in families, so it is probably inherited.
Concept question on myopia
To correct for myopia or near-sightedness, your glasses
need to contain:
Concept question on myopia
To correct for myopia or near-sightedness, your glasses
need to contain:
A. Positive (convex) lenses
B. Negative (concave) lenses
Correction for myopia
Relaxed myopic eye
No glasses
-This is the furthest object that can be seen clearly
With glasses with
negative lenses
Far sighted eye (Hyperopia)
THE FAR-SIGHTED EYE
- Cornea is not curved enough (focal length too long) or eyeball too short - Image of distant object forms behind retina.
What the world looks like with hyperopia
- Distant objects blurred
Concept question on hyperopia
To correct for hyperopia, do you need glasses with:
A: Convex lens
Concept question on hyperopia
To correct for hyperopia, do you need glasses with:
A: Convex lens
Correction for hyperopia
Relaxed hyperopic eye
a) Viewing nearby object -blurred
b) Viewing nearest clear object
Other facts about hyperopia
Hyperopia (i.e far-sightedness) is much less common than myopia or
emmetropia. It is typically in the +1.00 to +4.00 diopter range, rarely it can be as high as +8.00 diopters.
In contrast to myopia, hyperopia occurs when the eye is too short for the power of its optical components (i.e. lens is too weak). In hyperopia, the cornea is not steep enough and light rays hit the retina before they come
into focus. Distant objects appear blurred, and nearby objects are even more fuzzy. Most farsighted individuals need corrective eyewear to see clearly at all distances.
Correction of hyperopia requires a lens which is convex (i.e. thicker in the middle than the edges). This acts as a magnifier, and causes objects to appear bigger by 2% per diopter. For this reason hyperopes while wearing
Treatments for myopia
• Glasses
• Contact lenses
• Eye surgery → flatten the cornea
– Radial keratotomy
Radial Keratotomy
• Cut several spoke-like slits in the cornea with a diamond knife • Causes central area to flatten and focal length increase.
Photorefractive keratotomy (PRK)
• Uses a computer controlled eximer laser• Laser emits pulses lasting only billionths of a second
• Laser vaporises microscopic slivers from the cornea – theoretically making very accurate and specific modifications to its shape.
• Procedure takes 1-2 minutes
• Procedure is pain free but 10-20% of people feel pain for weeks afterwards • 95% achieve 20/40 vision or better
LASIK
• Like PRK but performed deeper inside the cornea
• A surface flap is removed and then replaced to give access inside cornea. • Generally results in quicker and less painful recovery than PRK
Side effects of corrective eye surgery
* May still need glasses — With RK, approximately 10 percent to 30 percent of people who have the procedure require some use of corrective lenses; with PRK,
between 5 percent and 15 percent. Also, everyone's eyes degenerate with age, so by about age 40, you may find yourself needing bifocals for sharp near vision.
* Night vision may be poor: Some may see "halos" around lights, which can make night driving difficult. For some, this side effect passes. For others, it is permanent. * May see variation in visual acuity — For some, eyesight may be better in the
morning and less sharp at day's end. Some may experience light sensitivity and glare. •In very rare cases, there may be corneal scarring or rupturing — Because the cornea is weakened in RK, there is a threat of corneal rupture at the incision site if the eye is hit.
DIOPTERS AND
YOUR PRESCRIPTION
Diopters are used to measure the refractive error of the eye and the power of the lens needed to correct this error
-Negative number (myopia, nearsightedness) -Positive number (hyperopia, farsightedness).
-A -1.00 diopter myope is able to see objects at 1m clearly -A -2.00 diopter myope is able to see objects at 1/2m clearly
Your prescription contains 4 numbers:
1. Amount (in diopters) of myopia (- numbers) or hyperopia (+ numbers). 2. Amount of astigmatism.
3. Axis of the astigmatism.
Concept question - Adding lenses
Lens #1 Lens #2
F
1F
2What is the combined focal length of two convex lenses A: Shorter than either F1 or F2?
B: Between F1 and F2?
Concept question - Adding lenses
F
totalF
1 Lens #1 Lens #2F
2What is the combined focal length of two convex lenses A: Shorter than either F1 or F2?
B: Between F1 and F2?
Formula for adding lenses
F
To add lenses, add their POWERS (not their focal lengths)
Power (in diopters) = 1/F, and F is the focal length (in m)
So P
total= P
1+ P
2Example of adding lenses
F
Question: If F
1is 25 cm, and F
2is 1 m, what is F
total?
Answer: P
1= 4 D and P
2= 1 D
=> P
total= P
1+ P
2= 5 D
Another example of adding lenses
Question: If F
1is 0.5 m, and F
2is -1 m, what is F
total?
Concept Question on Adding Lenses
F
Question: If F
1is 50cm, and F
2is 1m, what is F
total?
A. -1m
Concept Question on Adding Lenses
F
Question: If F
1is 50cm, and F
2is 1m, what is F
total?
A. -1m
B. 33cm
Calculating your prescription
To calculate your prescription, calculate the
Calculating your prescription -Example
Question: A near-sighted person wants to see to infinity
but does not see clearly beyond 2 meters. What kind and
strength of eyeglasses do they need?
Answer: -0.5 D
How did I get this answer?
Convert the infinity to zero diopters, and the 2 m to
0.5 D. The difference is -0.5 D.
Why is the number negative?
Calculating your prescription – application of thin lens equation
Before correction (viewing far point)1/xi + 1/ xo = 1/Feye
xo
xi
xo’
xi
After correction (viewing new far point)
1/xi + 1/ xo’ = 1/Ftotal
= 1/Feye + 1/Fglasses
Subtract before from after:
1/xo’ – 1/xo = 1/Fglasses
Concept Question
Question: A near-sighted person wants to see to infinity
but does not see clearly beyond 1 meter. What kind
and strength of eyeglasses do they need?
Answer:
Concept Question
Question: A near-sighted person wants to see to infinity
but does not see clearly beyond 1 meter. What kind
and strength of eyeglasses do they need?
Answer:
A. -1 D
Concept Question
Question: A near-sighted person wants to see to infinity
but does not see clearly beyond 3 meters. What kind
and strength of eyeglasses do they need?
Answer:
Concept Question
Question: A near-sighted person wants to see to infinity
but does not see clearly beyond 3 meters. What kind
and strength of eyeglasses do they need?
Answer:
A. -3.0 D
B. -0.3 D
Example on Hyperopia
Question: A far-sighted person sees no closer than 0.5
meters. They want to read at the normal 25 cm distance.
What kind and strength of eyeglasses do they need?
Answer: +2D
How did I get this answer?
Convert the 0.5 m to 2 D, and the 25 cm to 4 D. The
difference is +2 D.
Why is the sign +?
Example on Hyperopia
Question:A far-sighted person sees no closer than 0.5 meters. They want to
read at the normal 25 cm distance. What kind and strength of eyeglasses do they need?
Answer: +2 D
How did I get this answer?
Convert the 0.5 m to 2 D, and the 25 cm to 4 D. The difference is +2 D.
OR
Before correction -1/xi + 1/ xo = 1/Feye So 2 + 1/L = 1/Feye After correction
