In the dispersion of white light by a common glass prism, which one among the following is correct ?
- A.Red light deviates the most because red light has highest speed in prism
- B.Blue light deviates the most because blue light has highest speed in prism
- C.Red light deviates the most because red light has lowest speed in prism
- D.Blue light deviates the most because blue light has lowest speed in prism
Correct answer
Explanation
The correct answer is D, Blue light deviates the most because blue light has lowest speed in prism. In glass, light of shorter wavelength travels more slowly, so the glass has a higher refractive index for it and bends it more.
Refractive index n = c/v, where v is the speed of light in the medium. In a vacuum all colours travel at the same speed, but in glass the speed depends on wavelength. Violet and blue, with short wavelengths, are slowed the most and deviate the most; red, with the longest wavelength, travels fastest in glass and deviates the least. This difference is what spreads white light into the spectrum VIBGYOR, with red at the top of the band and violet at the bottom.
A is wrong: red deviates the least, not the most. B is wrong: blue is the slowest colour in glass, not the fastest. C is wrong: red deviates the least, and it is the fastest colour in glass, not the slowest.
Exam tip: in a prism, shorter wavelength means slower speed, higher refractive index and more deviation; violet bends most and red bends least.
Practice Questions
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