Question

Explain the causes and correction methods (with diagrams) for Hypermetropia (long-sightedness) and Myopia.

Hint:

Myopia → Cannot see far → Concave lens Hypermetropia → Cannot see near → Convex lens

Answer 1

Concept: Defects of vision occur when the eye lens does not focus light properly on the retina. Two common defects are Myopia (short-sightedness) and Hypermetropia (long-sightedness).

1. Myopia (Short-sightedness)
Definition: A person can see nearby objects clearly but distant objects appear blurred.
Causes:

• Eyeball becomes elongated
• Eye lens has too much converging power (short focal length)

Image Formation: Image of distant object forms in front of the retina.
Correction: Corrected using a concave (diverging) lens which diverges incoming rays so they focus on the retina.
Diagram (Myopia and its correction):
\[ \begin{tikzpicture}[scale=0.9] % Retina \draw (4,-1) -- (4,1) node[right]{Retina}; % Eye lens \draw[thick] (2,0) circle (0.5); % Rays converging early \draw[->] (-1,0.5) -- (1.5,0.2); \draw[->] (-1,-0.5) -- (1.5,-0.2); \draw (3,0) node{Image}; \node at (2,-1.5) {Myopia: Image before retina}; \end{tikzpicture} \]

2. Hypermetropia (Long-sightedness)
Definition: A person can see distant objects clearly but nearby objects appear blurred.
Causes:
 

• Eyeball is too short
• Eye lens has less converging power (long focal length)

Image Formation: Image of nearby object forms behind the retina.
Correction: Corrected using a convex (converging) lens which converges rays before they enter the eye so the image forms on the retina.
Diagram (Hypermetropia and its correction):
\[ \begin{tikzpicture}[scale=0.9] % Retina \draw (4,-1) -- (4,1) node[right]{Retina}; % Eye lens \draw[thick] (2,0) circle (0.5); % Rays not converged yet \draw[->] (-1,0.5) -- (2.5,0.2); \draw[->] (-1,-0.5) -- (2.5,-0.2); \draw (4.5,0) node{Image}; \node at (2,-1.5) {Hypermetropia: Image behind retina}; \end{tikzpicture} \]

Summary Table:
\[ \begin{array}{|c|c|c|} \hline \textbf{Defect} & \textbf{Image Position} & \textbf{Correcting Lens} \\ \hline Myopia & In front of retina & Concave lens \\ \hline Hypermetropia & Behind retina & Convex lens \\ \hline \end{array} \]