CLASS 10TH DAY 4
INTRODUCTION
Function of Lens in Human Eye
The lens of the eye is responsible for focusing images onto the back of the eye. It is normally transparent. As a normal part of aging, the lens begins to cloud and causes a gradual, painless loss in vision. Cataract removal is most often performed to better examine the back of the eye when monitoring for damage from certain diseases such as diabetes or glaucoma and to improve vision. There are two main types of cataract removal. The large majority of cataract surgeries are performed using the phacoemulsification technique.
During the phacoemulsification technique an ultrasound probe breaks the cloudy lens into tiny fragments. The fragments are vacuumed out through a tiny incision. An intraocular lens implant is then inserted to replace the natural lens that was removed. Because the incision is tiny, stitches are often not necessary and visual improvement is usually noted relatively soon after surgery. During the extracapsular technique the cataract is removed as one entire piece. This requires a larger incision and stitches.
An intraocular lens implant (artificial lens) is inserted to replace the natural lens that was removed. Recovery is usually slower, due to the larger incision. The stitches sometimes need to be removed, which is usually done in the office. After both procedures, the surgeon usually places a patch over the eye.
Which are the most common eye defects in human beings? Some of us are not able to see the objects close to us while some are unable to see the objects far away. Why is that?
Defects of vision and their corrections:
There are three common defects of vision. They are:
- Myopia (Short-sightedness or Near-sightedness)
- Hypermetropia (Long-sightedness or Far-sightedness)
- Presbyopia
Myopia: The defect of an eye in which it cannot see the distant objects clearly is called myopia. A person with myopia can see nearby objects clearly. Myopia is caused due to:
- High converging power of lens
- Eye-ball being too long
Due to high converging of the eye-lens the image is formed in front of the retina and a person cannot see clearly the distant objects. In another case, if the eye- ball is too long than the retina is at larger distance from the eye-lens. In this case also the image is formed in front of the retina even though the eye-lens has correct converging power.
Myopia or short-sightedness can be corrected by wearing spectacles containing concave lens. This is because when a concave lens of suitable power is used for the myopic eye then the concave lens first diverge the parallel rays of light coming from distant object. Therefore, first a virtual image is formed at the far point of the myopic eye.
Now since the rays of light appear to be coming from eye’s far point, they are easily focussed by the eye-lens and image is formed on retina. Concave lens is used for myopic eye so as to decrease the converging power of the eye-lens.
Hypermetropia:
Hypermetropia or long-sightedness is a defect of an eye where a person cannot see nearby objects clearly. The near-point of hypermetropic eye is more than 25 cm away. This defect of eye is caused due to:
- Low converging power of eye-lens
- Eye-ball being too short
In case of hypermetropia the image of an object is formed behind the retina and therefore, a person cannot see clearly nearby objects.
The near-point of an eye having hypermetropia is more than 25 cm. The condition of hypermetropia can be corrected by putting a convex lens in front of the eye. This is because when a convex lens of suitable power is placed in front of the hypermetropic eyes, then the convex lens first converge the diverging rays of light coming from a nearby object at the near point of the eye at which the virtual image of the nearby object is formed. Since the light rays now appear to be coming from the eye’s near point, the eye-lens can easily focus and form the image on retina. Convex lens is used for hypermetropia so as to increase the converging power of the eye-lens.
Correction of Hypermetropia: The convex lens forms a virtual image of the object (lying at normal near point N) at the near point N’ of this eye.
Presbyopia: This defect of vision usually happens in old age when ciliary muscles become weak and can no longer adjust the eye-lens. The muscles become inflexible in this condition and cannot see nearby objects clearly. The near-point of an old person having presbyopia is much more than 25 cm. Presbyopia can be corrects by wearing spectacles having convex lens.
Another point to be noted is that a person can have both myopia and hypermetropia. In such a condition, spectacles having bifocal lens are worn. The upper part of bifocal lens is concave and lower part consists of convex lens.
Cataract: A yet another defect of the eye which usually comes in old age is the cataract. The medical condition in which the lens of the eye of a person becomes progressively cloudy resulting in blurred vision.
It develops when the eye-lens of a person becomes cloudy due to the formation of a membrane over it. It decreases the vision of the eye gradually and can lead to total loss of vision of the eye. It can be restored after getting surgery .The opaque lens is removed and artificial lens is inserted in its place via operation. This defect cannot be corrected by any type of spectacle lenses.
DIRECTIONS
Imagine you are walking on the road and an elderly man asks you the direction to a nearby park. So how will you guide him to the park? Watch this video to learn more.👇 👇
Here is a small quiz to test your understanding. Try to attempt all the questions and get the highest marks possible.👇 👇
FUN ACTIVITY (REVERSI)
Each of the disks’ two sides corresponds to one player; they are referred to here as light and dark after the sides of Othello pieces, but any counters with distinctive faces are suitable. The game may for example be played with a chessboard and Scrabble pieces, with one player letters and the other backs.
The historical version of Reversi starts with an empty board, and the first two moves made by each player are in the four central squares of the board. The players place their disks alternately with their colors facing up and no captures are made. A player may choose to not play both pieces on the same diagonal, different from the standard Othello opening. It is also possible to play variants of Reversi and Othello where the second player’s second move may or must flip one of the opposite-colored disks (as variants closest to the normal games).
For the specific game of Othello (differing from the historical Reversi), the rules state that the game begins with four disks placed in a square in the middle of the grid, two facing white side up, two pieces with the dark side up, with same-colored disks on a diagonal with each other.
Convention has initial board position such that the disks with dark side up are to the north-east and south-west (from both players’ perspectives), though this is only marginally meaningful to play (where opening memorization is an issue, some players may benefit from consistency on this). If the disks with dark side up are to the north-west and south-east, the board may be rotated by 90° clockwise or counterclockwise. The dark player moves first.