Presentation on theme: "Lenses The Eye. Iris: the colored part that surrounds the pupil and regulates the amount of light entering the eye. Pupil: the opening of the eye. Cornea:"— Presentation transcript:
Iris: the colored part that surrounds the pupil and regulates the amount of light entering the eye. Pupil: the opening of the eye. Cornea: the transparent covering, focuses the eye. Retina: tissue in the back of the eye that is sensitive to light.
The Retina Not uniform Fovea: the spot on the retina where we have the most distinct vision Blind Spot: the place where light produces no vision, where the nerves from the retina leave the eye.
Finding Your Blind Spot Page 474 x oo Close your left eye and focus on the “o” As you slowly move the book closer to your eye, you will see…
Normal Vision If you have 20/20 vision, it means that at a distance of 20 feet, you can read a certain line (labeled 20) on the chart and that your vision is normal. If you can only see the top line clearly (the one labeled 200), then you have 20/200 vision. This means that you must be 20 feet from the chart to see what most people can see at 200 feet. By the way, if someone's vision is 20/200 or worse, then they are legally blind.
Normal Vision Your eye can accommodate to clearly see objects from infinity (the far point) down to 25 cm (the near point).
Farsighted The eyes form an image behind the retina. The eyeball is too short. Have to hold things more than 25 cm away to be able to focus them.
Farsighted Can correct this defect by using a converging lens.
Nearsighted Can see nearby objects. The eye forms an image before the retina, not on it. The eyeball is too long. The far point is not infinity.
Nearsighted Can be corrected by using a diverging lens.
Astigmatism The cornea is curved more in one direction than the other. Like a football. The eye does not form sharp images. Need cylindrical corrective lenses that have more curvature in one direction.
LASIK After applying anesthetizing drops, the surgeon marks the eye to indicate where the flap will be cut and then replaced.
LASIK A suction ring holds the eye still and pressurizes it so it is firm enough to cut.
LASIK The microkeratome slices a tiny flap in the cornea.
LASIK The flap is moved out of the way, but it is still attached.
LASIK The laser removes tissue to reshape the cornea. The flap is replaced on the eye.
Finding Your Prescription Measure how close / far away you have to hold a piece a paper away from your eye to see it clearly.