How Do Glasses Work?

Video Transcript

"I am Doctor Jim Kirkconnell at Bellevue Eye Care Center in Nashville, Tennessee. How do glasses work? Well there are different classes of glasses that is number one, let us just talk very briefly about what the defect is as far as why people need glasses. One is that when light is coming from far away the the cornea and the length of the eye are mismatched that is called correlation error and so when you are near sighted it does not go all the way back, and in order to allow the light to go all the way back you have the lens which is thinner in the middle and thicker on the edge and that allows light to diverge before it hits the cornea until it comes all the way to the back in focus. When it comes to what is called hyperopia or far sightedness the problem is that the light will go behind the eye because it is too short for the focusing power of the cornea and in order to take care of that problem the lens is thicker in the middle and thinner on the edge and that allows light to converge before it reaches the cornea so it will move up to the retina where we want it to be and then astigmatism happens because the cornea is not shaped like a sphere it is actually shaped more like a spoon to where it has two focus points instead of one, you will have a light focus up and down in one spot lets say or horizontally in another spot and there is no one clear focus point. I do have examples of what these lenses look like. Glasses they they work in the sense that, we have a lens material which these days mostly it is it is optical plastic, we do not use glass much anymore but your light hits the surface and it goes through and because it is either thin in the middle to cause light to diverge or it is thicker in the middle to cause lights to converge it helps focus things for us. We have two curves on the back surface of the lens in opposite direction of the misshape of the cornea in order to correct for astigmatism and then we can have two focuses like this pair which is a bifocal. This is a traditional bifocal where there is a line so there is a different power on the bottom than the top of what is true about ninety percent of bifocal wearers today is they look like this which is you do not see a line and you do not see a line because with when you get below straight ahead seeing there is a channel in the glasses where you can see closer and closer until you have a bottom window to see for your reading area and that happens because the glasses are more steep the further down you go it it ground steeper and now we are in the fourth generation of those where it it is better than ever before but all of the no lines provide less area to see width wise than say the lined bifocal that I showed you before. So that is how glasses work."