Explain blue color of sky
Scenario
Aasmaan ki taraf dekhne par wo humein saaf neela (blue) dikhta hai, halanki suraj ki dhoop white hoti hai.
Yeh blue color kahan se aa gaya?
Mental model
Atmosphere ke fine particles chhoti (blue) light waves ko har disha mein bikhra dete hain (scatter), jabki badi (red) waves seedhe nikal jati hain.
Jab sunlight hamare atmosphere se guzarti hai, wahan present nitrogen/oxygen molecules chhoti chalon (shorter wavelengths like blue) se takra kar unhe charo taraf faila dete hain. Hamari aankhon tak yehi faili hui blue light aati hai, isliye aasmaan blue lagta hai.
Explanation
Blue color of the sky scattering of light ki ek perfect illustration hai. Jab sunlight Earth ke atmosphere se pass hoti hai, toh wo wahan maujood gas molecules (mostly nitrogen and oxygen), dust, aur water vapour se interact karti hai. Rayleigh scattering ka law kehta hai ki wo particles jinka size light ki wavelength se chhota hota hai, wo shorter wavelengths ko much more strongly scatter karte hain.
White light saare rangon ka mixture hai. Usme blue light ki wavelength choti hoti hai, jabki red light ki wavelength blue se kareeb 1.8 times lambi hoti hai. Isliye atmosphere ke fine particles blue light ko red light ke comparison mein much more strongly scatter karte hain.
Yeh bikhri hui (scattered) blue light har direction mein radiate hoti hai. Jab hum upar dekhte hain, toh yehi scattered blue light humari aankhon mein enter karti hai, jisse humein lagta hai ki pura aasmaan neela hai.
Agar earth par koi atmosphere nahi hota, jaise space mein, toh koi scattering bhi nahi hoti. Aise mein aasmaan bilkul kala (dark) dikhayi deta, jaisa ki space station mein astronauts ko dikhta hai. Sunrise aur sunset ke time pe light ko atmosphere mein lamba distance travel karna padta hai, tab saari blue light raste mein hi scatter ho jati hai aur sirf bachi hui red light hum tak pahunchti hai, isliye suraj laal dikhta hai.
Worked example
Problem: Why does the sky appear dark to an astronaut flying at a very high altitude?
- Bahut high altitude par ya space mein hawa/atmosphere almost negligible hota hai.
- Bina atmosphere ke light ko scatter karne wala koi particle nahi hota, isliye koi color hamari aankhon tak har direction se nahi aata.
Answer: At high altitudes, there is no atmosphere to scatter the light, so the sky appears dark.
Problem: During sunrise, the sun looks reddish but the rest of the sky is still somewhat blue. Based on scattering, which colour has travelled a longer distance through the atmosphere to reach our eyes from the sun?
- Sunrise par suraj horizon par hota hai, isliye uski light ko atmosphere ki thick layers se lamba rasta cross karna padta hai.
- Itne lambe raste mein shorter waves (blue) pehle hi scatter out ho jati hain aur bachi hui red light seedhi hum tak aati hai.
Answer: Red colour reaches our eyes as most of the blue light is scattered away.
Key points
- Scattering efficiency: Shorter wavelengths (blue) are scattered much more effectively by fine atmospheric particles than longer wavelengths (red).
- Absence of atmosphere: Without an atmosphere, no scattering occurs and the sky appears totally dark.
Common mistakes
- Confusing scattering with reflection: Kabhi kabhi students likhte hain ki sky ocean ke blue color ka reflection hai. Yeh bilkul galat hai, sky ka neelapan sunlight ki atmospheric particles dwara scattering se aata hai.
Recall questions
- Which wavelength of visible light is scattered the most by atmospheric gas molecules?
- What phenomenon makes the clear sky appear blue?
- Which phenomenon causes the sky to appear blue?
Questions & answers
Explain giving reason why the sky appears blue to an observer from the surface of the Earth. What should the appearance of the sky be during the day for an astronaut staying in the international space station orbiting the Earth?
The sky appears blue because fine particles in the atmosphere scatter the shorter wavelengths (blue light) more strongly than the longer ones (red light). For an astronaut in the ISS, there is no atmosphere to scatter light, so the sky appears dark.
Approach: Mention the relation between particle size, wavelength, and scattering intensity.
Why does the sky appear dark to an astronaut?
Because there is no atmosphere in space to scatter sunlight.
Approach: Link the presence of an atmosphere to the scattering effect.
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