TBR Bio Ch. 1; Passage V #33

[ajumobim]

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Astigmatism is the condition of having a non-uniformly shaped lens or cornea. This means that parallel light rays do not focus, so that a sharp image is not formed. How could this eye condition be corrected?
A) By providing a convex corrective lens
B) By providing a concave corrective lens
C) By providing a non-uniform corrective lens
D) By providing a uniform corrective lens

The book doesn't really explain why C is the correct answer and I can't find an explanation online for why non-uniform corrective lens would be used.

@NextStepTutor_3 Can you help please?

 

[NextStepTutor_3]

Yes of course, I'm so happy you asked! Lenses are both a fun and a tricky topic. To answer this question as thoroughly as possible, let's talk about the more commonly-tested conditions (myopia and hyperopia) before we get to astigmatism itself.

Let's start with myopia, or nearsightedness. I've drawn a diagram of this condition:

As you can see, the light rays (which are supposed to converge on the retina itself) converge too early (before the retina). In other words, the lens of the eye converges "too well." This means that we need a diverging, or concave, lens to correct this condition. So, choice B would be the right answer if the question had been referring to myopia.

On the other hand, hyperopia (or farsightedness) is the exact opposite - rays converge behind the retina, meaning that the lens of the eye does not have enough converging power. To fix this, we can add a converging, or convex, lens in addition to the already-convex lens of the eye. So, choice A would make perfect sense if the question had been referring to hyperopia.

Now, we can finally talk about astigmatism! Here, since the lens of the eye is irregular, there isn't a single focal point where the light rays converge (as the question states, "parallel light rays do not focus." If you look up ray diagrams for astigmatism, you'll see that there are actually multiple focal points, resulting in a blurry image. This means that you can't use either a uniform convex or a uniform concave lens to fix this condition - the light rays are converging "all over the place," so you'll need a lens that has different strength for different regions of the eye. Though you won't see the phrase "non-uniform corrective lens" in most MCAT books, this is exactly what this choice refers to - a lens that is adjusted to the irregularities of the person's lens.

 

[ajumobim]

@NextStepTutor_3 Thanks so much!! Makes so much sense now and this actually helped me understand some of the other questions in this passage as well!

 

[NextStepTutor_3]

I'm happy to help! This is one of my favorite topics haha. Let me know if you have any other questions!