Eye trouble

[Scarletblack]

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From Cracking the MCAT CBT (TPR), "Light and Optics"

A patient complains of blurred vision when viewing objects located in the distance, but reports visual clarity when viewing objects close to his eye. A physician hypothesizes that the difficulty is attributable to a condition in which the right retina is located at an abnormally large distance from the anterior surface of the right eye. Which of the following findings would best confirm her hypothesis?

A. The patient reports improvement when a convex lens is placed in front of his right eye


B. The patient reports improvement when a concave lens is placed in front of his right eye


C. The patient reports improvement when he opens his eyes more widely.

D. The patient reports that he does not experience the symptom when either eye is closed and the other remains open.

Ok so I selected C here (I was like wtf to myself?), because I thought, by opening his eyes more widely, he could increase the radius of curvature and thus the focal length.

But the true answer is B.

Can someone explain this to me? I probably don't get the idea of this question at all.

Thanks.

 

[161927]

The patient is nearsighted, so would benefit from diverging lenses. A concave lens is a diverging lens, having a negative focal distance.

 

[Scarletblack]

I know that, but I don't get the physics behind it.

 

[bluemonkey]

Opening the eyes wider will only allow more light to enter the eye. It will not have any effect on the shape of the eye or the shape of the eye's lens. Since the patient's retina is located an abnormally long distance from the surface of his eye, the lens is focusing light at a point before the retina, rather than behind the retina. To see properly, the light must be focused right at the retina. This is where supplementary lenses come in. The focal point needs to be moved back towards the retina since it is currently focusing in front of the retina. The only way to do this is to add a diverging lens in front of the eye's lens which will thus cause the new focal point to be on the retina. Given your options for lenses, a concave lens is a diverging lens, thus it would move the focal point back to the retina.

I hope this helps!

 

[unsung]

From Cracking the MCAT CBT (TPR), "Light and Optics"

A patient complains of blurred vision when viewing objects located in the distance, but reports visual clarity when viewing objects close to his eye. A physician hypothesizes that the difficulty is attributable to a condition in which the right retina is located at an abnormally large distance from the anterior surface of the right eye. Which of the following findings would best confirm her hypothesis?

A. The patient reports improvement when a convex lens is placed in front of his right eye


B. The patient reports improvement when a concave lens is placed in front of his right eye


C. The patient reports improvement when he opens his eyes more widely.

D. The patient reports that he does not experience the symptom when either eye is closed and the other remains open.

Ok so I selected C here (I was like wtf to myself?), because I thought, by opening his eyes more widely, he could increase the radius of curvature and thus the focal length.

But the true answer is B.

Can someone explain this to me? I probably don't get the idea of this question at all.

Thanks.

The easiest way to understand why a diverging lens is needed to correct near-sightedness is to draw a ray diagram.

Here's an explanation of ray diagrams:

http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/raydiag.html

Here's an even better site that lets you play with ray diagrams:

http://physics.bu.edu/~duffy/semester2/c28_lenses.html

Click show lens and ray diagram and you can play with it.

Something to keep in mind is the diverging lens (your eyeglasses) is actually creating a VIRTUAL image in front of your eyes, which your eyes can then see because it's closer than the object is. Your glasses are not creating an image that's on your retina.

If you use a converging lens, notice how the only way it can create a virtual image in front of the lens is if the object is really really close. (This is easy to see with the 2nd site if you play around with moving the object.). This is how magnifying glass works (or corrective lenses for farsightedness).

The important thing to remember is that these corrective lenses are NOT creating an image directly at your retina, they are creating virtual images which your eye can then see, which then forms the images on your retina.

Hope that helps.

 

[ecoli]

Opening the eye wider does nothing because the light is focused within the eye itself.... this is corrected for by the lens.