Identify hypermetropia (far-sightedness)
Scenario
Dada ji newspaper padhne ke liye use apne chehre se door karke hold karte hain.
Unhe door karke kyun padhna padta hai?
Mental model
Hypermetropia mein eye lens weak ho jata hai ya eyeball choti ho jati hai, jisse light retina ke peechhe ja kar focus hoti hai.
Agar camera ka focus point film ke peechhe nikal jaye toh image blur ho jati hai. Yahi hypermetropic eye mein hota hai, paas ki cheezon ke liye focus point retina ko cross karke peeche banta hai.
Explanation
Hypermetropia, jise far-sightedness bhi kehte hain, ek aisi vision defect hai jisme person door ki cheezein toh clearly dekh leta hai, par paas ki cheezein dhundhli lagti hain. Is defect wale insaan ka near point normal (25 cm) se aage shift ho jata hai. Unhe comfortably padhne ke liye reading material ko 25 cm se zyada door rakhna padta hai.
Is defect ke pichhe do reasons ho sakte hain. Pehla, eye lens ki focal length ka zyada lamba ho jana (lens thin ho gaya hai). Doosra, eyeball ka too small ho jana. In donon hi conditions mein, jab paas ki object se light rays aati hain, toh lens unko theek se converge nahi kar pata, aur image retina ke behind banti hai.
Ise correct karne ke liye hum convex lens (converging lens) wali spectacles use karte hain. Convex lens entering light rays ko thoda converge kar deta hai, taaki eye lens bachi hui light ko properly retina par focus kar sake. Correct power ka convex lens paas ki object (jaise 25 cm) ki virtual image wahan bana deta hai jahan hypermetropic eye ka actual near point hota hai.
Exam point of view se, aapko pata hona chahiye ki near point recede kar gaya hai, aur correction ke liye ek convex lens ki zaroorat hai. Ray diagrams practice karna mat bhoolna.
Worked example
Problem: An old man cannot read a book placed comfortably at 25 cm, but can easily see a bird sitting on a distant tree. What is this defect called and how can it be corrected?
- Kyunki aadmi door ki cheez dekh sakta hai par paas ki nahi, iska matlab uska near point recede kar gaya hai.
- Hypermetropia mein light rays retina ke peeche focus hoti hain. Isliye ek converging lens chahiye inko sahi jagah laane ke liye.
Answer: Hypermetropia; Convex lens
Problem: If a person is using spectacles with a power of +2.0 D, identify the type of lens and the vision defect.
- Positive power ka matlab focal length bhi positive hogi. Positive focal length humesha convex lens ki hoti hai.
- Convex lens ka use sirf hypermetropia yaani far-sightedness ko theek karne ke liye kiya jata hai.
Answer: Convex lens; Hypermetropia
Key points
- Shifted near point: The near point of the eye shifts farther away from the normal 25 cm.
- Correction mechanism: A convex lens is used to pre-converge the light, moving the focus back onto the retina.
Common mistakes
- Assuming focal length is too short: Hypermetropia mein focal length too long ho jati hai, isliye image retina ke peechhe focus hoti hai. Short focal length myopia ka cause hai.
Recall questions
- Which lens corrects hypermetropia?
- Where is the image formed for a nearby object in a hypermetropic eye?
- What is hypermetropia?
Questions & answers
Why is a normal eye not able to see clearly the objects placed closer than 25 cm?
Because the ciliary muscles can only contract up to a certain limit to increase the curvature of the eye lens. Beyond this limit, the focal length cannot be decreased further to focus the image exactly on the retina.
Approach: Mention the role and limitation of ciliary muscles.
Where is the image of a nearby object formed in a hypermetropic eye?
Behind the retina.
Approach: Explain that the lens converging power is too low.