Question

(a) What is hypermetropia?
(b) Write any one cause of hypermetropia.
(c) With the help of a suitable ray diagram, explain how hypermetropia is corrected?

Hint:

Hypermetropia = far-sightedness (near objects blurry). Cause: short eyeball or weak lens. Correction: convex lens (converges rays to focus on retina).

Answer 1

Part (a): What is hypermetropia?
Step 1: Definition.
Hypermetropia, also known as far-sightedness or long-sightedness, is a common refractive error of the eye in which a person can see distant objects clearly but nearby objects appear blurred.
Step 2: Reason for blurred near vision.
In hypermetropia, the image of a nearby object is formed behind the retina instead of directly on it. This happens because the eye lens cannot focus the light rays properly.
Step 3: Final answer for part (a).
Hypermetropia is a vision defect where nearby objects appear blurred while distant objects are seen clearly. Part (b): Any one cause of hypermetropia.
Step 1: List possible causes.
Hypermetropia can be caused by:

• Too short eyeball (axial hypermetropia) - the distance between lens and retina is too small
• Too flat cornea (refractive hypermetropia)
• Weak ciliary muscles
• Decreased curvature of the eye lens
• Aging (presbyopia is different but similar)

Step 2: Select one cause.
The most common cause is: \[ \boxed{\text{The eyeball is too short (too small in diameter from front to back).}} \]
Step 3: Explanation.
When the eyeball is too short, the distance between the lens and the retina is reduced. Therefore, the image of nearby objects is formed behind the retina because the lens cannot converge the rays sufficiently to focus on the retina. Part (c): Correction of hypermetropia with ray diagram
Step 1: Principle of correction.
Hypermetropia is corrected by using a convex lens (converging lens) of appropriate power. The convex lens converges the incoming light rays before they enter the eye, so that after refraction by the eye lens, they focus exactly on the retina.
Step 2: Ray diagram - Hypermetropic eye (without correction).
and
Step 3: Ray diagram - Correction using convex lens.

Step 4: Explanation of correction.

• A convex lens is placed in front of the hypermetropic eye.
• The convex lens converges the incoming light rays from a nearby object before they enter the eye.
• These slightly converged rays are then further converged by the eye lens to focus exactly on the retina.
• Thus, the person can see nearby objects clearly.

Step 5: Power of lens.
The power of the convex lens required depends on the degree of hypermetropia. It is usually measured in diopters.
Step 6: Final answer for part (c).
Hypermetropia is corrected by using a convex lens of appropriate power, which converges the light rays before they enter the eye so that the image forms on the retina.