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 — D, blue light deviates the most because it has the lowest speed in the prism. In glass, violet/blue light travels slowest, so it has the highest refractive index and bends (deviates) the most; red travels fastest, has the lowest index, and deviates the least. This spread produces the spectrum.
- (a)Red deviates most (highest speed) — Wrong twice: red actually deviates the LEAST, and it has the highest speed in glass, which goes with the least bending.
- (b)Blue deviates most (highest speed) — The deviation part is right, but the reason is wrong — blue has the LOWEST speed in glass (highest refractive index), not the highest.
- (c)Red deviates most (lowest speed) — Wrong both ways: red deviates the least, and red has the highest (not lowest) speed in the prism.
Dispersion occurs because a medium's refractive index depends on wavelength. In glass, n is larger for shorter wavelengths (violet/blue), so those colours travel slower and refract more; longer wavelengths (red) travel faster and refract less. A prism therefore spreads white light into a spectrum with red deviated least and violet most.
Two facts fixed together: in glass, higher refractive index → slower speed → more deviation, and this applies to blue/violet. So 'blue deviates most because it is slowest in the prism' is the fully-correct statement.
- Refractive index of glass is higher for shorter wavelengths (violet/blue).
- Higher index means slower speed in the medium and greater deviation.
- Red deviates least; violet/blue deviates most.
- Dispersion by a prism produces the visible spectrum.
- Thinking red deviates most — it deviates the least.
- Linking greater speed to greater bending — it is the opposite.
Asked as which colour deviates most through a prism and why, testing index–speed–deviation links.
No directly related past PYQ was found.
- practice — not a real PYQ
Through a glass prism, which colour of white light bends the least?
- (a)Violet
- (b)Blue
- (c)Green
- (d)Red
Answer(d) Red (highest speed, lowest refractive index in glass).
- practice — not a real PYQ
In glass, light of shorter wavelength (violet) travels
- (a)faster than red
- (b)slower than red
- (c)at the same speed as red
- (d)at the speed of light in vacuum
Answer(b) slower than red.