Identify cause of refraction
Scenario
Pani ke glass mein adhi doobi hui pencil mudi hui (bent) ya tooti hui lagti hai.
Pani ke andar jate hi pencil kyu toot jati hai? (Ya sirf roshni hume dhokha de rahi hai?)
Mental model
Traffic jam aate hi gaadi ki speed kam hona.
Jab light hawa (khali rasta) se paani ya glass (traffic) mein jati hai, toh uski speed kam ho jati hai. Speed badalne ke is jhatke ki wajah se light apna raasta thoda sa mod (bend) leti hai. Is mudne ko hi refraction kehte hain.
Explanation
Light normally hamesha straight lines mein travel karti hai. Par ye rule tabhi tak apply hota hai jab tak light ek hi tarah ke medium (jaise sirf hawa, ya sirf pani) mein travel kar rahi ho. Jab light ek transparent medium se doosre transparent medium mein cross karti hai (jaise hawa se glass mein), toh wo apni direction change kar leti hai. Bending of light ki is ghatna ko Refraction kehte hain.
Refraction ka main reason hai 'Speed of Light' mein change aana. Light vacuum ya air mein sabse tezi se travel karti hai (`3 \times 10^8 m/s`). Lekin jab wo pani ya glass jaise dense (ghane) materials mein enter karti hai, to particles ke interference ki wajah se uski speed dheemi (slow down) ho jati hai.
Socho ek car ek tar road (high speed) se chalte hue achanak ret (sand) par aa jaye. Car ki raftaar kam ho jayegi aur wo thoda sa skid karke mud jayegi. Light ke sath bhi bilkul waisa hi hota hai. Medium change hone par light ki boundary par bending hoti hai.
Isi wajah se swimming pool ki bottom upar uthi hui lagti hai, aur neebu (lemon) pani ke glass mein rakha ho to bada dikhta hai. Ye sab light ke mudne ke optical illusions hain.
Worked example
Problem: Why does a coin kept at the bottom of a glass bowl full of water appear raised?
- Coin se aati light water se air mein ja rahi hai (medium change).
- Is wajah se light bend hoti hai aur coin ki virtual image upar banti hai.
Answer: Because of the refraction of light when it travels from water to air.
Problem: Does refraction occur if light enters a glass slab completely perpendicularly (at `90 degrees` to the surface)?
- Bending tabhi hoti hai jab light surface par ek angle par (obliquely) takrati hai.
- Agar wo seedhi takraye (`90 degrees`), toh speed badalti hai par path seedha hi rehta hai.
Answer: No, it passes undeviated (straight through), though its speed decreases.
Key points
- Refraction: The bending of light when it passes obliquely from one transparent medium to another.
- Cause of Refraction: The change in the speed of light as it enters a different medium.
Common mistakes
- Thinking light bends inside the medium constantly: Light sirf aur sirf boundary par (surface/interface par) mudti hai. Pani ke andar ghusne ke baad woh wapas straight line me hi travel karti hai (alag speed par).
Recall questions
- What is the primary cause of the refraction of light?
- What phenomenon makes a pencil look bent in a glass of water?
- What causes the refraction of light?
Questions & answers
A ray of light traveling in air enters obliquely into water. Will the speed of light increase or decrease?
Decrease
Approach: Water is optically denser than air, so light travels slower in it.
Does the frequency of light change during refraction?
No, only its speed and wavelength change.
Approach: Frequency is a property of the source, so it remains constant.
Continue learning
Next: Apply laws of refraction (Snell's Law)