Top 10, 10th Sci. Lenses..3🎷

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Defects of vision and their corrections.

 What to do about vision defects?

Are there any eye exercises?

Due to the loss of accommodation power, some people cannot see things.

Sometimes the eyeball is elongated or the eyeball is flattened, resulting in visual defects.
If we write a to-the-point (मुद्देसूद) answer while writing a big question like five marks, there is no need for recitation पाठांतर.

In the next question we are going to write the answer to the point so even if the answer is long, there will be no need for recitation.

1. Nearsightedness/ myopia (निकटदृष्टीता.)

1. Definition:- a person can see the nearby object clearly but can't see a distinct object properly this defect is called myopia.
2. Reasons A. Due to the elongation of the eyeball, the distance between the lens and the retina increases.
B. The curvature of the cornea and the eye lens increases so that the converging power of the lens remains large.
3. Image:- due to elongation of eyeball the image of a distant object forms in front of the retina.
4. Correction/ Solution/remedies:-
 Myopia defects can be corrected by using spectacles with a suitable concave lens.

🎷 Farsightedness or Hypermetropia  (दूरदृष्टीता)

1. Definition:- persons can see distant objects clearly but cannot see nearby objects distinctly this defect is called hypermetropia means Farsightedness.

2. Reasons:-
 A. Eyeball flattened due to which distance between lens and retina decreases.
B. Converging power of the lens becomes less due to the curvature of the cornea and the eye lens decreases.

3. Image:- the image of nearby objects gets formed behind the retina.

4. Correction/ solution/ remedies:-
A hypermetropia defect can be corrected by using a suitable focal length of a convex lens.

🎸 Presbyopia:- the ciliary muscles near the lens lose their ability to change the focal length of the lens. Old people cannot see nearby objects clearly due to the shifting of the near point of the lens further from the eye.
Old people suffer from nearsightedness as well as Farsightedness. Bifocal lenses are required to correct such defects. In a bifocal lens, the upper lens is concave which corrects nearsightedness and the lower lens is convex which corrects farsightedness.



🎺. The apparent size of an object.

When two objects of the same height are placed at different distances from the eye, their angles with the eye are different. A nearby object of the same height appears larger to the eye, and a distant object of the same height appears smaller to the eye.
Ex. A plane with more than 200 people in it looks like a firebox at high altitude. The moon and the sun of enormous ( प्रचंड) size appear (भासणे) small to our eyes.

🎷 Simple microscope

A convex lens with a small focal length produces a virtual, erect, and bigger image of an object in place between focal lengths. 
Use:- a. for watch repair,
b. Testing precious gems (मौल्यवान रत्नांची पारख)
c.Palmistry to see the lines on the hand.

🎺.  Compound microscope

construction:- The compound microscope is made up of 2 convex lenses, an objective, and an eyepiece. In a compound microscope, an objective lens has a smaller focal length than an eyepiece. Due to the combined effect of the two lenses, we get higher magnification. The two lenses are fitted along the same line between a metallic tube.
Uses:- to see minute objects like bacteria, animal cells, plant cells, hydra, volvox, spirogyra, and blood cells.
 

🎷   Telescope

Telescopes are of two types. a. refracting telescope in which lenses are used.
b. Reflecting telescope:- in which mirrors and lenses are used.
Opposite to that of the compound microscope now objective lens has a larger diameter and larger focal length to get the maximum amount of light coming from distant objects. The image formed by the objective lens acts as an object for the eyepiece. The size of the eyepiece is smaller and its focal length is also less. The principal axis of the objective lens and eyepiece are along the same straight line.
Uses:- A. To see the moon, planets, comets, stars, etc.

🎻   Persistence of vision.
When Images are created on the retina we can see an object. When an object is moved away from the eye, the image of the object remains on the screen for 1/16 seconds. The sensation on the retina persists for a while, this is called the persistence of vision.
Ex. 1. If we draw a cage from one side of the cardboard and a bird from the other side and rotate it with a thread, it will look like the bird is in the cage.
2. If we rotate the burning incense stick rapidly in a circular motion, we will see a red circle.

* How do we perceive/ identify different colours?
In our eyes many light-sensitive rod and cone cells are present. Rod cells respond to the intensity of light and give information about the brightness or dimness of the object. 

Cone cells give information about the colour of the object. Cone cells are inactive in the faint light, so we cannot perceive colours in dim light. Some individuals do not have specific cone cells, so they do not perceive colors and are called colorblind.

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