Congratulations, you’ve made it to Lesson 6 where we’re finally going to start talking about glasses! This lesson will focus mostly on nearsightedness but you will also learn about much lesser known ‘condition’ known as emmetropia.
This lesson will continue to build on everything we’ve learned in Lessons 1-5. So if you haven’t done so already, here they are:
Refractive conditions refers to all the conditions that require us to wear glasses.
The word refraction refers to what happens to light when it passes through lenses.
Since the cornea acts like a lens, it can be said that the cornea refracts light.
I will continue to use the term ‘refract’ in the appropriate contexts, but whenever I do, you can pretty much replace that word in your head with the word ‘focus’ and you won’t miss a beat.
As we learned in Lesson 5, if the light is refracted such that it becomes perfectly focused on the retina, clear vision will be produced.
However, if the light entering the eyes is not refracted perfectly onto the retina, vision will be blurry and the eye is said to have a refractive condition. (I.e., a focusing condition)
The 3 main refractive conditions are:
- nearsightedness (myopia)
- farsightedness (hyperopia)
The lack of a refractive condition is called emmetropia.
Emmetropia Vs Ammetropia
What is Emmetropia?
Emmetropia is the ‘condition’ where light focuses perfectly inside the eyes and the vision is clear.
The word emmetropia is the name of the ‘condition’ and the word emmetropic is used to describe somebody with emmetropia. I.e., He has emmetropia, he is emmetropic.
This is not a common term and you will not see it used in the day-to-day operations of an optical store or optometry clinic. Remember, emmetropic people don’t need glasses so we usually just say that they have perfect vision instead of calling them emmetropic.
In Lesson 5, the ray diagram of the eye that we studied was in fact depicting emmetropia. Here it is again:
This diagram depicts emmetropia because the incoming parallel light is perfectly refracted by the cornea to come to a focus precisely on the retina. This is what the eye needs in order to see clearly.
What is Ammetropia?
Ammetropia is an umbrella term for anything that is not emmetropia. That means that the focal point of parallel light entering the eyes does not come to a focus on the retina. There are many refractive conditions that fall under the category of ametropia and in the rest of today’s lesson we will cover one of the most common one; myopia.
What is Myopia?
Myopia is the refractive condition that results when the focal point of parallel light entering the eye forms in front of the retina.
The word myopia is the name of the condition and the word myopic is used to describe somebody with myopia. I.e., She has myopia, she is myopia.
The more commonly used term for myopia is nearsightedness. I.e., She has nearsightedness, she is nearsighted.
Here is an example of a ray diagram of myopia:
Myopia is not just a single state. It is a continuum. It can be very low if the focal point is only slightly in front of the retina, or it can be very high if the focal point is very far in front of the retina. Here are examples of 3 different levels of myopia
|Low Myopia||Moderate Myopia||High Myopia|
What Causes Myopia?
Can you think of different things that can place an eye in a state of myopia?
There are 2 possible ways.
- The cornea is too storng (it has too much converging power)
- The eye is too long and the retina is too far back
By far and away the biggest cause is #2: The eye is too long and the retina is too far back.
The 3 diagrams above are helpful in showing different severities of myopia, but they correspond more to the cornea being the cause of myopia, which is not the case in reality.
Here is a more accurate way of thinking of myopia:
Note that in all 3 of the ray diagrams above, the focal length is the same. The focal point is exactly the same distance from the cornea in each case. The only thing that changes is the walls of the eye (and the retina) getting further and further back.
How to Treat Myopia
The goal when treating myopia is to put the focal point back onto the retina.
In myopia, the foal distance is not long enoug to reach the retina. Therefore, we have to increase the focal distace.
Since you’ve studied Lesson 3 on lens power, you know that stronger converging lesnses have shorter focal distances, and weaker converging lenses have longer focal distance.
So all we have to do to increase the focal distance of the eye is to lower the overall convergence of the cornea!
Another way of saying the exact same thing is: increase the overall divergence of the cornea.
Decreasing Convergence = Increasing Divergence
Increasing Convergence = Decreasing Divergence
The easiest way to decrease the convergence (increase the divergence) of the cornea is simply to add a diverging lense in front of it. A.k.a: Glasses!
Prescribing Lenses for Myopia
The job of an optometrist is to determine exatly what strength of diverging lens needs to be placed in front of a myopic eye in order to place the focal point back onto the retina.
Let’s do an excerise to give you an idea of how this works. Remeber this image:
Let’s pretend this is our patient. She is myopic, but what lens will she need in order to see clearly?
In this example, we will say that the focal point is exacty 0.5cm in front of the retina.
We will also assume that the cornea has a power of +50D, which gives rise to a focal point exactly 2cm behind the cornea. If you don’t understand how that works, please review Lesson 5.
The formula for lens power is:
lens power = 1 / focal distance
In this example we want the focal distance to be 2.5cm (because that’s how far the retina is from the cornea).
lens power = 1 / 0.025
lens power = +40D
This means that for the focal point to be on the retina, the cornea must have a lens power of +40D. But is has a lens power of +50D, not +40D, so what do we do?
Well, since the cornea has +10D more power than we need, we will offset that by adding a -10D lens in front of the cornea.
Total Power Needed = Power of the Cornea + Power of Glasses Needed
+40D = +50D + Power of Glasses Needed
Power of Glasses Needed = +40D – (+50D)
Power of Glasses Needed = -10D
Here is the ray diagram for the same eye with a -10D lens in front of it.
Because of the diverging power of the minus lens, it reduced the overall convergence power of the eye. This results in the focal point forming further away from the cornea. In the example above, a -10D lens is required to place the focal point exactly onto the retina.
In a nutshell, that’s what myopia is and how it is corrtected. However, during an eye exam optometrists do not do any of the calculations you saw here in this lesson. Instead, they place different lenses in front of the eyes until the vision becomes clear. When the vision is the clearest, that’s when the optometrist knows that the focal point is on the retina. The diagrams and calculations in this lesson are meant to help you better understand what is going on inside the eyes when people are nearsighted.
I have challenging question for all of you reading through this course. Test yourself by thinking about it, re-reading this lesson and previous lessons if necessary, and leave your answer in the comments section below to see if you are correct.
Why do nearsighted people need glasses to see clearly in the distance, but don’t need to glasses to see well up close?
Hint: Think of what type of lenses nearsighted people wear, what effect these lenses have on light, and how else you can create that same effect.
Leave comment below to find out the answer!
If you’re comfortable with the concepts covered in this lesson then it’s time to start learning about hyperopia! However, if you found this lesson challenging, please re-read it a few times and leave any questions you still have about it in the comments below and I will answer them for you.
==>The Optics of Vision – Lesson 7: HYPEROPIA <==