Identify myopia (near-sightedness)
Scenario
Aap class mein back bench par baithe ho, but blackboard ke akshar blurry lag rahe hain. Lekin notebook bilkul clear dikh rahi hai.
Yeh common vision problem kyun hoti hai?
Mental model
Myopia mein eye lens zyada powerful ho jata hai, toh light jaldi focus ho jati hai.
Imagine karo ek projector jo screen ke thoda aage hi movie focus kar raha ho. Myopic eye mein, lens light rays ko retina par pahunchne se pehle hi mila deta hai, isliye door ki cheezein dhundhli dikhti hain.
Explanation
Myopia, jise near-sightedness bhi kehte hain, ek aisi defect of vision hai jisme ek insaan paas ki objects toh clearly dekh sakta hai, par distant objects use distinct nahi dikhti. Aise eye ka far point infinity par nahi hota, balki eye ke paas aa jata hai.
Yeh defect do main reasons se hota hai. Pehla, eye lens ki curvature ka excessive hona, jis wajah se lens ki focal length kam ho jati hai. Doosra, eyeball ka elongation ho jana, jisse retina aur lens ke beech ka distance badh jata hai. Dono hi cases mein, distant object se aane wali parallel light rays retina par focus hone ke bajaye, uske aage (in front of the retina) focus ho jati hain.
Is defect ko theek karne ke liye hum concave lens ka use karte hain. Concave lens diverging nature ka hota hai, toh yeh light rays ko thoda bikhra deta hai pehle hi, taaki eye lens unhein exactly retina par focus kar sake. Correct power ka concave lens door se aati hui image ko myopic eye ke far point par le aata hai.
Boards mein directly ray diagrams banane ko aate hain myopia ke: myopic eye, aur correction with concave lens. Inhe achhe se practice karna zaroori hai. Yaad rakhna, myopic eye ka far point infinity se less distance par hota hai.
Worked example
Problem: A student cannot see clearly beyond 2 m. Which type of lens should be used to correct this vision defect?
- Kyunki student door ki cheezein (beyond 2 m) nahi dekh sakta, iska matlab far point infinity se shift hokar 2 m par aa gaya hai. Yeh myopia ka case hai.
- Myopia ko correct karne ke liye humesha diverging lens ka use hota hai taaki light rays retina tak pahunch sakein.
Answer: Concave lens
Problem: An eye doctor prescribes a lens of power -1.5 D to a patient. Identify the defect of vision the patient is suffering from.
- Power negative hone ka matlab hai lens ki focal length negative hai.
- Focal length negative sirf concave lens ki hoti hai, aur concave lens myopia ko theek karne ke kaam aata hai.
Answer: Myopia
Key points
- Near point and Far point: For a myopic eye, near point is normal (25 cm) but far point is less than infinity.
- Correction: Requires a concave lens of suitable power to diverge light before it enters the eye.
Common mistakes
- Confusing near-sightedness meaning: Students sochte hain 'near-sightedness' ka matlab paas ka na dikhna hai. Jabki iska matlab hai paas ka 'sight' clear hai, problem door ki cheezon mein hai.
Recall questions
- Which lens is used to correct myopia?
- Where is the image of a distant object formed in a myopic eye?
- What is myopia?
Questions & answers
What is myopia? State the two causes of myopia.
Myopia is a defect of vision in which a person can see nearby objects clearly but cannot see distant objects distinctly. Causes: 1) Excessive curvature of the eye lens. 2) Elongation of the eyeball.
Approach: Define the defect clearly and list both structural reasons precisely as per NCERT.
Where is the image of a distant object formed in a myopic eye?
In front of the retina.
Approach: Explain that the eye lens is too converging or the eyeball is too long.