Predict bending of light
Scenario
Ek line kheencho air se water ki boundary tak. Jab light enter karegi, toh wo left mudegi ya right?
Light hamesha normal ke paas aati hai (towards) ya us se door bhagti hai (away)? Ye kaise pata chalega?
Mental model
Ghane traffic mein chalo toh normal ke paas sikud jao, khali sadak mile toh khul ke fail jao.
Jab light rarer (air) se denser (glass) medium mein jati hai, speed slow hone ke karan wo normal line ki taraf jhuk (bend towards) jati hai. Jab denser se rarer mein jati hai, speed badhti hai aur wo normal se door (bend away) ho jati hai.
Explanation
Optics mein materials ko unki light bend karne ki capability se jancha jata hai, jise 'Optical Density' kehte hain. Jiska refractive index kam hai (jaise air), use 'Optically Rarer' medium kehte hain. Jiska refractive index zyada hai (jaise glass/water), use 'Optically Denser' medium kehte hain.
Bending ke do simple aur sunehre rules hain:
1. From Rarer to Denser (e.g., Air to Glass): Light ki speed kam ho jati hai. Aise mein refracted ray 'Normal' (perpendicular) ki taraf jhuk jati hai (Bends TOWARDS the normal). Yahan angle of incidence (i) bada hota hai aur angle of refraction (r) chota ho jata hai (i > r).
2. From Denser to Rarer (e.g., Glass to Air): Light azad hokar wapas speed pakadti hai. Isliye refracted ray Normal se door bhagti hai (Bends AWAY from the normal). Yahan angle of refraction (r) bada ho jata hai (r > i).
Yahi rule glass slab ke experiment mein dikhta hai: light jab enter karti hai toh normal ke paas aati hai, aur jab doosri side se nikalrti hai (emergent ray), toh wapis away from normal ho jati hai.
Worked example
Problem: A ray of light enters from water (n = 1.33) to diamond (n = 2.42). Will it bend towards or away from the normal?
- Refractive indices compare karke dekhein ki kaunsa medium denser hai.
- Denser medium mein light jaane ka rule lagayein.
Answer: It will bend towards the normal.
Problem: Light traveling in a glass block emerges out into the air. Compare the angle of incidence inside the glass with the angle of emergence in the air.
- Media change ko identify karein.
- Bending ka rule lagayein.
Answer: The angle of emergence (in air) is greater than the angle of incidence (in glass).
Key points
- Rarer to Denser: Speed decreases. Light bends TOWARDS the normal (angle i > angle r).
- Denser to Rarer: Speed increases. Light bends AWAY from the normal (angle i < angle r).
Common mistakes
- Confusing mass density with optical density: Kerosene oil pani ke upar tairta hai (less mass density), par optically kerosene denser hai (refractive index zyada hai). Light kerosene mein zyada mudegi pani se.
Recall questions
- When light travels from air to water, does it bend towards or away from the normal?
- In which condition is angle of refraction (r) greater than angle of incidence (i)?
- When light travels from a denser to a rarer medium, how does it bend?
Questions & answers
A ray of light traveling in air enters obliquely into water. Does the light ray bend towards the normal or away from the normal? Why?
It bends towards the normal because water is optically denser than air, causing the speed of light to decrease.
Approach: Air is rarer, water is denser. Rarer to denser = towards normal.
A ray of light travels from water to air. Will it bend towards or away from the normal?
Away from the normal, as air is optically rarer than water.
Approach: Identify water as denser and air as rarer, then apply the rule.
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