Main content
Course: Class 10 Physics (India) > Unit 2
Lesson 3: Defects of vision and correctionPresbyopia
Let's explore what are the symptoms and causes of Presbyopia. We will also see how bifocals correct them. Created by Mahesh Shenoy.
Want to join the conversation?
Sir, can you tell me some mnemonics to memorise it ?(3 votes)
as you said if a person is suffering from presbyopia and some other defect related to distant vision then flat and convex(converging) lens both are used but instead why cant we only use converging lens?(2 votes)- Sir, at1:40you said that the ciliary muscles apply force on the lens to curve it but what I have studied is that muscles can only push not pull so how can they cause the lens to bend?(2 votes)
It depends on the object as if it is far so our eye lens need to become thin but if the object is closer so our eye lens need to become thick, this is performed by ciliary muscles. Thus, cillary muscles don't only push but also pull(1 vote)
- As you said when a person suffering from presbyopia use biofocal lenses but why can't we use only converging (convex) lens(1 vote)
- We know that in presbyopia we suffer from both things. If we are watching a distance object, we need to look closer; we need a diverging lens.(1 vote)
Two children went to the park with their grandfather.On reaching the park, the children joined others to play,
while their grandfather after taking 3 rounds of the park, sat on the chair, took out newspaper from his bag
and began to read with the help of his spectacle. After sometime, he realized that was too long to see the children,
He looked around, but though he has worn spectacle, he couldn't see anything. He realized that he had forgotten his other spectacle,
which he used to see the faraway places, were left at home. He began calling them by their names , bit due to the large distance
, his voice was not reaching the children. Another man sitting beside him realized the problem and helped him to reach the children.
1. Name the eye defect with which the grandfather was suffering from?
2. Which type of spectacles should he wear, so as to avoid calling the other?
3. Explain the cause of disease.
4.Explain with the diagram.(1 vote)- A1. The grandfather was suffering from Presbyopia.
A2. He should wear bi-focal glasses to avoid calling the other.
A3. This leads to age, it doesn't depend on genetics, and anyone can lead to this problem.(1 vote)
1:40Video transcript
have you seen these glasses in which it seems like there are two kinds of lenses they're called as bifocals two lenses but why would people use them well these are used by people who are suffering from an eye condition called the presbyopia due to this they are unable to see things clearly which are close by now this sounds very familiar to another eye condition that we might have learned hyper Metropia farsightedness even in that condition people can see things far away but they are unable to see things clearly which are close by right well the effect is the same but the cause of that is different and that's why they are named differently so the cause for press myopia is the loss of accomodation power so let's explore this in detail now first let's recall what accommodation is accommodation is where our eyes adjust their optical power depending upon the Vivian distance for example if you're looking at things which are far away the rays of light from that object would be pretty much parallel to each other and now to focus them onto the retina notice the bending required is pretty small so the optical power needed is pretty small and as a result our eyes will be relaxed but if you're looking at things which are close by now the incoming light is no longer paddle they are diverging away and as a result to again focus them onto the retina notice now the bending required is more and so more optical power is needed this is now achieved by the ciliary muscles over here which are not shown in the diagram these ciliary muscles push on that lens and then the lens bulges out increasing its curvature and thereby increasing the optical power this is called as accomodation now imagine if we lost the power of accomodation imagine if our lens lost ours lost its elasticity what would happen then well if you're looking at things far away our eyes will be relaxed no problems here but when things come close by now we have problems ball this lens because the lens has lost his elasticity it has become hard now the lens will not bulge it'll it's hard to push it and as a result it will the optical power will not increase and as a result the rays of light will not Bend enough and so he cannot focus on the retina and that's why due to this we will not be able to see things which are close by so the cost for presbyopia is the loss of accomodation power on the other hand if you're dealing with hyper Metropia or even myopia for that for that matter that is not due to accomodation their accomodation a deck that has nothing to do with the accomodation those those conditions are due to abnormal shape of the eyeball itself of the eyeball itself or or the abnormal shape of the eye lens and we've spoken about this in great detail in a previous video so if you need more clarity on that then it'd be a great idea to go back and watch that video and come back over here but in short presbyopia is due to some is related to accomodation whereas myopia and hyperopia are related to the abnormal shapes of the eyeball and lens another difference between presbyopia and the other two is how you even get these defects myopia and hyperopia are found to be pretty genetic it also depends upon the environment you are in so you may or may never get myopia or even hyper Metropia in your life that's totally possible but presbyopia is almost unavoidable for most people you see presbyopia has nothing to do with your genetics or anything even though your eyes are perfectly normal in the young age as one enters into their 40s or mid-40s we slowly start losing the elasticity of our islands and eventually even the ciliary muscles that you know carry out accomodation that tends to wear out and so how do we correct them well to correct it we need to provide extra optical power extra converging power when you are looking at things close by and that's what these bifocals do this lens over here which is at the bottom of the glass this this is actually inverted this is the bottom when you wear it so this lens which is near the bottom is a converging lens so this is converging converging lens providing the extra optical power which is needed over here and things are close by and the reason it's it's it's it's provided at the bottom is because whenever we are looking at things close by like when we're reading something our eyes usually tend to be towards the bottom right when you're reading something think about it and if you're looking at things far away then we will be using this part of the glasses right when you're looking straight straight ahead so this part of the glass is used to correct any defects we might have for distant vision so for example if we have say myopia as well then this part of the glass will correct myopia but if you don't have any other defect that we don't require any optical support here then this glass will not do anything it'll just be flat and so that's how bifocals are useful in correcting both presbyopia and any other defects that our eyes might have relate to distant vision and by the way these bifocal glasses are pretty outdated today they are replaced by what is called progressive glasses so in progressive glasses the difference is we don't have two distinct lenses as you can see over here you have one single lens whose curvature keeps changing progressively so at the bottom the curvature meets the demands of presbyopia and as we go towards the top the curvature meets the demands for distant vision maybe myopia or any other problems related distant vision