Self study Flashcards
Explain the Law of Refraction. (Lesson 10, Chapter 3)
states that light bends toward the normal when it enters a medium more dense than the one it came from. The normal, as you’ll recall, is a line perpendicular to the surface the light ray is striking.
Outline the steps for transposing a prescription. (Lesson 11, Chapter 3
Step 1: Take the sphere number (including the sign) and the cylinder number (including the sign), and add them together to get your new sphere.
Step 2: Take the cylinder number, and change its sign to the opposite sign to come up with the new cylinder.
Step 3: Take the axis, and change it 90 degrees by either adding 90 or subtracting 90 from it. Remember that the axis cannot be 0 or greater than 180. This will tell you whether to add or subtract.
Explain how a change in vertex alters the effective power of a lens. (Lesson 8, Chapter 3)
you’ll want your client’s glasses to sit at about the same distance from the eye—12 millimeters. If the vertex distance is too close to or too far from the eyes, the power of the lenses will be too strong or too weak. We call this altered power the effective power of the lenses. If the lenses are too close to the eye, their effective power is more minus. If the lenses are too far away from the eye, their effective power is more plus.
Describe the relationship between the shape of a person’s face and the frame that will look best on it. (Lesson 7, Chapter 3
Now, here’s the key to complementing each face shape: think opposites. For instance, square frames will make a square face look even squarer. To soften a square face, go with round or oval frames instead. To review the other rules of thumb you learned today, take a look at this chart:
hyperopia
also known as Farsightedness, occurs when the eyeball is too short. As a result, light ends up focusing behind the retina. The drawing below illustrates this problem.
myopia
also known as nearsightedness. When the eyeball is too long, as shown in the drawing below, the light entering the eye ends up focusing before it gets to the retina.
astigmatism
Many medical terms originate from Latin or Greek words. Astigmatism comes from two Greek words: a, meaning without, and stigma, meaning a mark or spot. About 16 percent of the population has this focusing problem.
Astigmatism is different from nearsightedness and farsightness because it makes vision blurry at all distances. To understand why, picture a corneal surface that’s not round like a marble. Instead, the surface is curved like a grape or an olive—that is, more curved in one direction than in the direction perpendicular to it.
When light travels through an eye that’s shaped like this, the light rays focus on different parts of the retina instead of on a single spot. Thus, the eye can’t form a clear, focused picture, no matter how near or far an object is.
Here’s an example of what a person might see as a result of these multiple focus points:
Astigmatism is typically present at birth, but it may get better or worse over time. Many people have a small amount of astigmatism, and people with very mild astigmatism probably won’t even notice that they have a problem.
People with a significant degree of astigmatism will have blurry or distorted vision, and may even see “ghost images” around objects. Uncorrected astigmatism can also cause headaches and eyestrain. Luckily, we can correct this condition with glasses, contact lenses, or refractive surgery.
presbyopia
a problem that affects people’s ability to focus up close. This problem usually sets in between the ages of 35 and 45, when the lens becomes less elastic and can no longer focus near objects onto the retina. As you saw in your reading, some researchers think presbyopia may also stem from a weakening of the smooth muscles that help the lens change shape.
Presbyopia is the reason why many people need bifocals, which contain a magnifying lens that’s a different power than their distance prescription. We can also correct presbyopia with reading glasses or contact lenses.
Many people ask if presbyopia is avoidable. Unfortunately, it’s not. It’s part of the aging process, and there’s no way to stop having birthdays! About 90 million Americans wear corrective lenses to correct presbyopia, and the numbers are increasing as the Baby Boomer generation ages.
If you develop presbyopia—and odds are you will—you’ll notice that it gradually gets harder and harder to read fine print. You’ll start holding books or pill bottles at arm’s length so you can read the text, and you may develop eyestrain. You’re also likely to notice blurring when you transition between different viewing distances.
Emmetropia
This is the term we use for a person who has no refractive error. This word, too, comes from the Greek—this time from a root meaning well-proportioned.
When a person is emmetropic, the eye and cornea are perfectly spherical, and the rays of light that enter the eye focus directly on the retina without any eyeglass lenses or contact lenses in front of the eye. There are some very fortunate people who stay emmetropic their whole lives until they develop presbyopia and need reading glasses.
anisometropia
is the term we use to describe people whose eyes have different refractive powers. The first part of this word, aniso-, means unequal. As you learned in today’s reading, anisometropia creates problems with vision and can also be a challenge for the optician since the glasses lenses of a person with this problem will be of different thicknesses.
symptoms of cataracts
- the lens will start to turn yellow, brown, or cloudy in most people when they’re older.
- They may look like a yellowish or white spot on the pupil.
- To understand how cataracts affect vision, imagine looking through cloudy glasses lenses. This is similar to how people with cataracts see all the time.
treatment for cataracts
- Rather than removing cataracts right away, doctors often wait until a patient’s vision becomes so blurry that it significantly interferes with eyesight. In the meantime, you’ll want to help patients select glasses that will make the most of their imperfect vision.
- Night vision is a particular problem for people with cataracts, especially when they’re driving. This is because extra glare occurs when light bounces off the opaque lens of the affected eye. To help people cope with this glare, you’ll want to recommend antireflective coatings on their lenses.
- In addition, you’ll want to do everything possible to help people slow the progression of their cataracts. For instance, you should recommend that they wear a UV coating on their lenses or use UV-coated sunglasses as much as possible. We’ll talk in detail about these lenses later, but for now, just be aware that they serve an important medical purpose.
- There are certain tinted lenses (for instance, amber-colored lenses) that also help enhance the daytime or nighttime vision of people with cataracts. Polarized sunglasses will reduce glare in addition to protecting the eyes.
symptoms of glaucoma
As glaucoma progresses, a person’s peripheral (side) vision becomes more and more reduced. Eventually, the person develops advanced tunnel vision. However, the central portion of the vision remains intact.
treatment for glaucoma,
Currently, treatments for glaucoma include eye surgery and eye drops that help lower the pressure in the anterior part of the eye. The unfortunate thing about glaucoma is that it’s only rarely curable with surgery. And as you learned in your reading, doctors can’t reverse any vision loss that’s already occurred.
Since glaucoma primarily affects peripheral vision, you’ll want to recommend glasses that enhance people’s ability to see to the sides. Your patients or customers should avoid thick-rimmed frames, and the frame parts that hook over the ears (called the temples) should be as thin as possible.
You’ll also want to recommend an antireflective lens coating to improve overall vision, especially at night. And be sure to suggest sunglasses to help people with glaucoma preserve the vision they have left. They certainly won’t want to develop a cataract in addition to having glaucoma! The eye medications that doctors use to treat glaucoma will often cause patients’ eyes to become red, so you might want to recommend a very light lens tint for cosmetic reasons.
treatment for macular degeneration.
When you’re assisting people with macular degeneration, you’ll notice that most of them have prescriptions with very high magnification. This is so we can enhance their peripheral vision as much as possible, helping to make up for their poor central vision.
As a result, these people’s lenses tend to be very thick and heavy. So instead of recommending a very large frame, you’ll want to recommend a thinner (but sturdy) frame and thinner lens materials. This will draw attention away from the thickness of their lenses.
Certain antireflective lens coatings can also help people with macular degeneration see a bit better. And, as always, you’ll want to persuade people with this condition to wear sunglasses in order to protect their remaining vision.
symptoms of macular degeneration
one test used to measure the progression of macular degeneration. It’s called an Amsler grid, and it’s a test patients with this disorder can use to keep track of their own vision changes.
To use the grid, patients cover one eye and look at the dot in the center. Macular degeneration that’s progressed will cause some lines in this grid to look wavy or gray or disappear altogether, and these vision impairments will become more dramatic as the condition worsens. Try it yourself below (wearing glasses if you use them)—but remember that this very informal test can’t diagnose macular degeneration. Only a thorough eye exam by a doctor can identify this condition.
Amsler grid
Amsler Grid
Of course, an accurate diagnosis of macular degeneration requires extensive testing. But the Amsler grid is a useful way for patients to let us know if their vision is staying the same or getting worse.
Even people with severe macular degeneration don’t become totally blind, because they still have useful peripheral vision. But because we rely heavily on our central vision for driving, reading, watching TV, and even socializing with other people, living with macular degeneration is a huge challenge.
concave lenses (minus lenses)
After going through a concave lens, light rays diverge and focus farther out than they normally would. We call the image created by a minus lens a virtual image.
convex lenses (plus lenses)
After going through a convex lens, light converges before it reaches the retina of a normal-length eyeball. We call the image created by a plus lens a real image.
how concave lenses alters the way in which light converges in the eye.
A minus lens bends the light less, so the light converges farther back in the eye, reaching the retina of a nearsighted person whose myopic eyeball is longer than normal.
how convex lenses alters the way in which light converges in the eye.
The light going through a plus lens will correct the vision of a hyperopic (farsighted) person because the light will converge sooner and focus onto the retina of a short hyperopic eyeball as shown below:
advantages of traditional bifocal
Well, some people prefer lenses with wider segments, because these provide more reading area. Also, wider segments raise the optical center of the lens so there’s less image jump when the eye moves from the distance portion to the near portion.
disadvantages of traditional bifocal
One drawback of bifocals is that when the eye looks down and crosses over the segment line, there’s a sudden “jump” or shift in the position of the image the person is viewing. This is known as image jump. Most people get used to this over time, so it doesn’t bother them after a while. As an optician, you’ll want to let people know about this when they’re adjusting to their first pair of bifocals.
