Understanding the Eye Defect: Seeing Near, Blurry Far

The question describes a common eye condition where a person can see objects that are close by clearly, but objects that are far away appear blurry. This specific type of vision problem is known as myopia, or nearsightedness.

In a healthy eye, light from distant objects is focused precisely on the retina, which is at the back of the eye, allowing a clear image to be formed. In a myopic eye, the light from distant objects is focused in front of the retina instead of directly on it. This misalignment causes the distant image to be blurry.

Let's analyze the possible reasons given for this defect:

• 1. Excessive curvature of the eye lens: The eye lens focuses light onto the retina. If the lens has excessive curvature, it bends light rays more strongly. This increased bending (or power) causes the light from distant objects to converge and focus too early, specifically in front of the retina. This condition leads to myopia.
• 2. The eye-ball has become too small: If the eyeball is shorter than normal, the retina is closer to the lens. Light from distant objects would tend to focus behind the retina in this case. This condition, where distant objects are clear but near objects are blurry, is known as hyperopia (farsightedness), which is the opposite of the defect described in the question.
• 3. The focal length of the eye lens is too large: The focal length of a lens is inversely related to its power. A larger focal length means the lens has less power (bends light less strongly). If the focal length is too large, light from distant objects will not be bent enough and will focus behind the retina. Like a small eyeball, this also leads to hyperopia, not the myopia described.
• 4. Elongation of the eye-ball: If the eyeball is longer than normal, the retina is further away from the lens than it should be. The light from distant objects focuses at the correct distance from the lens, but because the retina is further back, the focal point falls in front of the retina. This condition also causes myopia.

Based on this analysis, the reasons that can cause myopia (seeing near objects clearly but distant objects blurrily) are excessive curvature of the eye lens (making the lens too powerful) and elongation of the eye-ball (making the eye too long). Both of these conditions cause the light to focus in front of the retina.

Let's summarize the effects:

Therefore, options 1 (excessive curvature of the eye lens) and 4 (elongation of the eye-ball) are the possible reasons for the described eye defect.

Revision Table: Eye Defects

Additional Information: The Eye and Vision Correction

The human eye works like a camera, with the cornea and lens focusing light onto the retina. The retina converts light into electrical signals sent to the brain. The power of the eye's lens can change slightly (accommodation) to focus on objects at different distances, but this mechanism can only compensate for small defects.

Myopia is typically corrected using a concave lens. A concave lens diverges the incoming light rays slightly before they enter the eye. This divergence compensates for the eye's excessive focusing power or length, ensuring that the light is then focused correctly onto the retina.

Hyperopia is corrected using a convex lens. A convex lens converges the incoming light rays slightly before they enter the eye. This convergence compensates for the eye's insufficient focusing power or short length, ensuring the light focuses on the retina.

The focal length ($f$) and power ($P$) of a lens are related by the formula $P = \frac{1}{f}$, where $f$ is in meters and $P$ is in diopters (D). A lens with a shorter focal length has higher power and bends light more strongly. In myopia, the eye's overall power is too high relative to its length, or its length is too great for its power. Corrective lenses adjust the total optical power of the eye system.