Eye defect (Hypermetropia-Farsightedness)

Eye defect (Hypermetropia-Farsightedness)

Hypermetropia, also known as farsightedness, is an eye defect in which a person can see distant objects clearly but struggles to focus on nearby objects. This occurs because the light rays from a close object focus behind the retina instead of directly on it.

Theory:

1. Causes of Hypermetropia:
   • Shortened Eyeball: The distance between the lens and the retina is shorter than normal.
   • Decreased Curvature of the Lens: The lens is unable to become sufficiently convex to focus on nearby objects.
2. Effects on Vision:
   • The eye fails to converge light rays from close objects onto the retina, leading to blurred vision for nearby objects.
   • Distant objects remain clear because light rays from faraway sources are nearly parallel and focus properly on the retina.
3. Correction of Hypermetropia:
   • A convex lens (converging lens) is used to correct hypermetropia.
   • The convex lens converges the incoming light rays before they enter the eye, ensuring the rays focus directly on the retina.
   • The focal length of the lens is chosen based on the degree of hypermetropia.
4. Lens Formula for Correction:
   The convex lens used for correction must satisfy the lens formula:
   1/f = 1/v – 1/u
   where:
   • f: Focal length of the corrective lens
   • u: Object distance
   • v: Image distance (equal to the least distance of distinct vision, typically 25 cm)

Applications of Corrective Lenses:

1. Eyeglasses:
   Convex lenses are incorporated into eyeglasses to correct hypermetropia.
2. Contact Lenses:
   Convex contact lenses are also used for correcting farsightedness without the need for external frames.
3. Laser Surgery:
   Advanced surgical techniques like LASIK reshape the cornea to correct hypermetropia permanently.

Examples:

1. Mild Hypermetropia:
   A person with a mild degree of farsightedness can read distant signs but struggles to read a book without eyeglasses.
2. Convex Lens for Reading:
   A convex lens with a power of +2 D (diopters) allows a hypermetropic individual to focus on objects as close as 25 cm.
3. Combination Lenses:
   Eyeglasses with bifocal lenses combine convex lenses for hypermetropia and concave lenses for myopia, aiding individuals with both defects.

Real-Life Uses:

• Developing eyeglasses and contact lenses tailored to correct hypermetropia.
• Designing optical instruments for testing and diagnosing vision defects.
• Advancing surgical techniques like LASIK for permanent correction.

Observations:

• Hypermetropia becomes more common with age due to reduced flexibility of the eye lens (presbyopia).
• The severity of the defect determines the power of the convex lens required for correction.
• Proper diagnosis is essential to prescribe lenses with accurate focal lengths.
• Convex lenses provide effective correction by ensuring that light rays focus directly on the retina.