Draw ray diagrams for concave mirrors (real images)
Scenario
Projector ki light screen par padti hai aur aapko movie dikhti hai. Par agar aap object ko hilate ho, toh screen par image choti ya badi ho jati hai.
Ek concave mirror se kab choti image banti hai aur kab badi?
Mental model
Jaise-jaise object aage badhega, image peeche aur badi hoti jayegi.
Concave mirror ke front mein 4 alag zones hote hain. Object ko mirror ke paas laane par, real image mirror se door bhagne lagti hai aur size mein badi hoti chali jati hai. Saari real images ulti (inverted) hoti hain.
Explanation
Ray diagrams banane ke liye humein kisi object se nikalne wali 2 standard light rays ka rasta trace karna hota hai. Jaha yeh 2 rays reflect hokar milti hain, wahan image banti hai.
Rays ke 4 rules hain: 1) Parallel aane wali ray Focus (F) se guzarati hai. 2) Focus (F) se aane wali ray parallel ho jati hai. 3) Centre of Curvature (C) se aane wali ray wapis C par hi laut jati hai. 4) Pole (P) par hit karne wali ray same angle par reflect hoti hai.
Concave mirrors 5 alag cases mein 'Real and Inverted' image banate hain. Real image ka matlab hai ki rays sach mein milti hain aur image ko screen par pakda ja sakta hai.
Jab object C ke pichhe (Beyond C) hota hai, toh image F aur C ke beech choti (diminished) banti hai. Jab object exactly C par hota hai, toh image bhi C par banti hai aur same size ki hoti hai. Jab object C aur F ke beech hota hai, toh image C ke peeche chali jati hai aur badi (enlarged/magnified) ho jati hai. Jaise-jaise object mirror ke paas (F tak) aata hai, image door bhagti hai aur size mein badhti jati hai.
Worked example
Problem: An object is placed between C and F of a concave mirror. Where will the image form and what will be its nature?
- Object C aur F ke beech hai. Ek ray parallel le jayein jo F se guzre, aur doosri F se le jayein jo parallel ho jaye.
- Rays physically axis ke neeche intersect karti hain.
Answer: Image is beyond C; Real, inverted, and enlarged.
Problem: Where should an object be placed in front of a concave mirror to get a real image of exactly the same size?
- Same size ki real image paane ke liye, rays ko object ki jagah par hi milna chahiye.
- Yeh tabhi hota hai jab object exactly Centre of Curvature (C) par ho.
Answer: At the Centre of Curvature (C)
Key points
- Rule 1: Ray parallel to principal axis passes through F after reflection.
- Real and Inverted: Images formed by actual intersection of rays are real and always inverted (upside down).
- Object at C: Image is formed exactly at C, and is of the same size as the object.
Common mistakes
- Forgetting arrowheads: Ray diagram bina arrowheads ke meaning-less hota hai, exam mein sidhe zero marks milte hain. Always mark arrows to show the direction of light.
Recall questions
- Where is the image formed if an object is placed beyond C for a concave mirror?
- Are real images erect or inverted?
- Where should the object be placed to get a real, inverted, and same-sized image?
Questions & answers
A student wants to project the image of a candle flame on a screen 60 cm in front of a mirror by keeping the flame at a distance of 15 cm from its pole. Which type of mirror should he use?
Concave mirror
Approach: Only a concave mirror can form a real image (since it needs to be captured on a screen in front of the mirror).
If an object is placed beyond C in front of a concave mirror, where is the image formed?
Between F and C.
Approach: Draw rays from beyond C; they intersect between F and C.
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