Physiology-Optics Flashcards
Why does your fishing rod appear to bend when you put it in the river?
The light from the air has a higher refractive index and slows down as it hits the water. This slowing down of the 1st part of the light causes the image to bend until the last part also hits the water.
What happens to the bending of light as you increase the angle at which it hits the water/
It bends more because there is more time between the first part of the light hitting the water and the last part of the light hitting the water.
How does your lens focus light?
As light passes through the convergent lens, the refractory index bends it, as it exits the lens, it gets bent further and all beams converge at a focal point. With the divergent lens, your eye sees the diverging light, and extrapolates its origin back to a focal point.
How does a cylindrical lens focus light?
Only in the horizontal plane. There is no vertical bending.
How do we measure optical power?
1/focal length in meters = diopter
How do converging lens optical power and diverging lens optical power differ?
Converging = positive. Diverging = negative.
What determines if a converging lens has a strong or weak optical power?
Strong lenses bend light right away. Weak lenses have longer focal distances.
Where does most refraction of light take place in the eye?
Air-cornea interface. The cornea has an optical power of 48 diopters. The lens has an optical power of about 20 diopters.
When is the optical power of the lens the greatest? How does this change?
At close distances. At long distances it is the weakest. The zonular fibers are attached to the lens and sclera. The ciliary muscle is attached to the zonular fibers and relieves tension on the zonular fibers that normally flatten lens. When it contracts it squeezes the lens so it is fat. This increases optic power for close distances.
How does your body accommodate for an oncoming car that you continually focus on?
As the object gets closer, the lens gets thicker so that the optic power increases and the image remains focused on the retina. Additionally, the lens moves slightly anteriorly to bring the focal point nearer to the retina. This is all mediated by the parasympathetic nervous system.
Far point
Max distance the eye can focus an object on the retina (usually infinity)
Near point. What happens to this as you age?
Minimum distance the eye can focus an object on the retina. This value is determined by your ability to squeeze the lens. As you get older the lens gets stiffer (presbyopia) and cannot get as fat. Consequently you cannot focus on close objects as well. Bifocals and reading glasses help accommodate as the lens gets stiffer.
What happens in the accommodation-convergence reflex?
As an object gets closer your ciliary muscles contract, pupils contract and lens thickens to better focus the image. As the object continues to get closer the eyes converge to reposition the image on the fovea.
Why do we squint when we see a blurred image?
The object you are focusing in on tends to be fine, but objects around it are blurred because of extra rays of light. Squinting gets rid of the extra light rays causing the problem and allows you to focus better.
Axial myopia
Parallel light is focused in front of the retina. As objects move closer, the image moves further back and becomes focused on the retina. This is why you can see things that are closer better if you are myopic.
What kind of lens do you give a patient who has myopia?
Divergent lens. It pushes the focal point further back so it no longer focuses in front of the retina, but right on the retina.
What kind of lens do you give a patient who has hyperopia?
People with hyperopia have a shorter eyeball than normal and the focal point is behind the retina. If patients are beyond accommodation and can no longer bring that image forward, they need a convergent lens to bring it forward for them.
What kind of lens do you give a patient who has astigmatism?
Cylindrical lens. Patients with astigmatism have areas of low curvature that focus images further and areas of high curvature that focus them closer. The cylindrical lens can bring those together by only shifting the horizontal or vertical image. The optometrist must determine the angle of the astigmatism to make the proper lens.
