Describe dispersion of white light
Scenario
Baarish ke baad aasmaan mein beautiful rainbow banta hua dekhte ho.
Safed dhoop (white sunlight) achanak se saat (7) alag alag colors mein kaise toot jati hai?
Mental model
Prism ek running race track jaisa hai jahan white light ke saare colors ek saath entry lete hain, par unki bending speed alag hone se wo separate ho jate hain.
White light mix of different colors (wavelengths) se bani hoti hai. Jab ye kisi glass prism jaisi cheez se guzarati hai, toh har color alag angle par bend (refract) hota hai, aur isi bending difference se band of colors banta hai.
Explanation
Jab safed light (white light) kisi transparent medium jaise glass prism se paas hoti hai, toh wo apne constituent colors mein split ho jati hai. Is phenomenon ko dispersion of light kehte hain. White light asal mein saat (7) alag colors ka mixture hoti hai: Violet, Indigo, Blue, Green, Yellow, Orange, aur Red (VIBGYOR).
Dispersion ka main reason yeh hai ki light ke alag-alag colors ki speed kisi bhi transparent medium (jaise glass) mein alag-alag hoti hai. Vacuum ya air mein sab colors ki speed lagbhag same hoti hai, isliye wo saath travel karte hain aur white dikhte hain. Par glass mein aate hi red light sabse tez chalti hai aur violet sabse slowly.
Kyunki unki speeds alag ho jati hain, wo alag-alag angles par bend (refract) karte hain. Red light sabse kam bend karti hai, aur violet light sabse zyada bend karti hai. Isi wajah se prism se bahar nikalte waqt yeh saare colors fan out ho jate hain aur ek distinct spectrum banate hain.
Sir Isaac Newton ne sabse pehle glass prism ka use karke sunlight ka spectrum obtain kiya tha. Unhone ek doosra inverted prism pehle waale prism ke aage rakha aur dekha ki saare rang wapas milkar phir se white light ban gaye (recombination of spectrum). Rainbow banna bhi dispersion of sunlight ka hi ek natural example hai, jisme pani ki boondein (water droplets) chote prisms ki tarah kaam karti hain.
Worked example
Problem: Which colour of white light suffers maximum bending when passing through a glass prism?
- Hamein pata hai ki dispersion mein alag alag rang alag angles par bend karte hain. Jis rang ki wavelength sabse kam hoti hai, wo sabse zyada bend hota hai.
- Spectrum (VIBGYOR) mein sabse neeche violet hota hai, iska matlab wo sabse zyada refract hua hai.
Answer: Violet colour
Problem: What happens when you place a second identical prism in an inverted position with respect to the first prism, allowing the spectrum to pass through it?
- Pehla prism white light ko 7 rangon mein tod deta hai (dispersion).
- Inverted doosra prism unhi 7 rangon ko wapas merge karke recombine kar deta hai kyunki bending ka direction reverse ho jata hai.
Answer: The spectrum recombines to form white light again.
Key points
- Spectrum: The band of the coloured components of a light beam is called its spectrum (VIBGYOR).
- Cause of Dispersion: Different colors of light bend through different angles with respect to the incident ray, as they pass through a prism.
Common mistakes
- Mixing up the order of VIBGYOR: Kuch students base of prism ke hisaab se Red ko neeche bana dete hain. Hamesha yaad rakhna, Violet sabse zyada bend karta hai, isliye wo prism ke base ki taraf (sabse neeche) hota hai.
Recall questions
- Which colour of light bends the least upon passing through a glass prism?
- What is the band of seven colours obtained from the dispersion of white light called?
- Which color of light bends the most during dispersion through a prism?
Questions & answers
Why does a glass prism leave a spectrum of colors while a glass slab does not?
In a glass slab, the refracting surfaces are parallel. The opposite bending at the two surfaces cancels out the dispersion, recombining the colors. In a prism, the refracting surfaces are at an angle, causing the separated colors to diverge further.
Approach: Compare the parallel faces of a slab to the angled faces of a prism.
Why does a glass prism split white light into a spectrum?
Different colors of light travel at different speeds in glass, causing them to bend at different angles.
Approach: Explain based on the dependence of refractive index on wavelength.