Test: Defects of Vision - UPSC MCQ

Myopia, or shortsightedness, is primarily caused by the elongation of the eyeball, leading to the inability to see distant objects clearly. This elongation affects the focusing of light onto the retina, resulting in blurry vision for faraway objects.

As Myopia is a defect of vision in which a person cannot see the distant object clearly. This can be corrected by using the concave lens of appropriate focal length

Explanation:

• Option A: (a) Myopia - Decrease in focal length of the eye lens. This is correctly matched.
  Myopia, or shortsightedness, is caused by a decrease in the focal length of the eye lens, making it more convergent. This causes distant objects to appear blurry.
• Option B: (b) Hypermetropia - Elongation of the eyeball. This is incorrect.
  Hypermetropia, or farsightedness, is typically caused by a shorter than normal eyeball or a less convergent eye lens, not by elongation of the eyeball.
• Option C: (c) Astigmatism - Genetic disorder. This is incorrect.
  Astigmatism is not necessarily a genetic disorder. It is usually caused by an irregular shape of the cornea or lens, leading to blurred vision.
• Option D: (d) Colour blindness - Concave lens for correction. This is incorrect.
  Colour blindness is a genetic disorder that affects the ability to distinguish between certain colors. It cannot be corrected by lenses like a concave lens, which is used to correct myopia. Therefore, the correctly matched pair is Option A: Myopia - Decrease in focal length of the eye lens.

Surgery to treat cataracts involves removing the clouded lens and replacing it with a synthetic new one. This procedure is safe and very effective.They're researching eye drops that may dissolve cataracts so patients don't have to go to surgery.

Longsightedness (or hypermetropia) causes the near point of the eye to be farther away from the normal near point, which is approximately 25 cm for an adult. This condition results in difficulty in focusing on nearby objects while maintaining clear vision of distant objects.

Hypermetropia can be corrected by using a convex lens. A convex lens converges the incoming light such that the image is formed on the retina.

Nearsighted individuals have image formed in front of the retina. They must correct the problem by wearing a lens which provides for some diverging of light prior to reaching the lens of the eye. This will move the image further from the lens of the eye and back towards the retina.

- Myopia (Nearsightedness): A condition where distant objects appear blurry because the image forms in front of the retina.
- Corrective Lens: A concave lens is used to correct myopia.
- Concave Lens Function: It diverges light rays before they enter the eye, moving the focal point back onto the retina, allowing for clear distant vision.

The near point of the eye is the minimum distance of the object from the eye, which can be seen distinctly without strain. For a normal human eye, this distance is 25 cm. The far point of the eye is the maximum distance to which the eye can see the objects clearly. The far point of the normal human eye is infinity.

For person affected with myopic condition, the image instead of forming on retina, it forms in front of it. In order to cure such type of defect, concave lens of power - 1.25D is used.

When object is placed at 25 cm, then a virtual image should form at 50 cm. 
So, u = -25 cm, v= -50 cm. 
So, 1/f = 1/-50 - 1/-25 
f = 50 cm = 0.5 m 
Power, P = 1/f = 1/0.5 =  +2D 
Therefore , a convex lens is needed.

This person suffers from the defect of hypermetropia.
For him u = -30cm, v = -1.5 m = -150cm
Therefore, focal length of corrective lens to be used by him is
1/f = 1/v- 1/u = 1/-150 - 1/-30 = 4/150 = 37.5cm
The positive sign shows that the lens needed is a convex lens of focal length 37.5 cm.
Hence, power of lens needed
P =1/f = 100/37.5 = 2.67D

Short-sightedness (myopia) usually occurs when the eyes grow slightly too long, which means they're unable to produce a clear image of objects in the distance.