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
Answer
Why
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.
Why the others are wrong
- (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.
Concept
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.
Key facts
- 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.
Study next
Common traps
- 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.
Related PYQs
A glass prism splits white light into different colours. This phenomenon is called dispersion of light by prism. Which one of the following statements is correct?
- (a) Red light will deviate the most and it is because of the reflection of light.
- (b) Violet light will deviate the most and it is because of the refraction of light.
- (c) Red light will deviate the most and it is because of the refraction of light.
- (d) Violet light will deviate the most and it is because of the reflection of light.
Answer(b) Violet light will deviate the most and it is because of the refraction of light.
Same prism-dispersion concept (violet deviates most) in a 2021 NDA GAT item.
Practice
- 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.