Which colour is deviated least when white light passes through a triangular prism?

Analyze the question: When white light passes through a prism, it splits into its component colours (dispersion). We need the colour that bends (deviates) the least.

Concept:

• The refractive index of glass depends on wavelength. Shorter wavelengths have a higher refractive index, so they slow down more and bend more.
• Cauchy's relation shows this: n = A + B/λ², so a smaller λ gives a larger n.
• The greater the refractive index, the greater the deviation.

Order of colours (VIBGYOR):

| Colour | Wavelength | Deviation | |--------|-----------|-----------| | Violet | shortest (~400 nm) | most | | Indigo | ↓ | ↓ | | Blue | ↓ | ↓ | | Green | ↓ | ↓ | | Yellow | ↓ | ↓ | | Orange | ↓ | ↓ | | Red | longest (~700 nm) | least |

Solution:

• Red has the longest wavelength, so it has the lowest refractive index in glass.
• Therefore red is bent the least, and violet is bent the most.

Among the options (green, indigo, red, violet, yellow), red is the answer.

Tip: Remember "Red is Relaxed", which means it bends the least. In the VIBGYOR order, violet is bent most and red is bent least. You can also remember that the colour at the bottom of the spectrum from a prism is violet and the one at the top is red, with red closest to the original direction of the light.