Red light travels faster than blue light when both of them pass through a glass prism because the

Step 1: Analyze the question We're asked why red light travels faster than blue light inside a glass prism. The key idea is the link between speed of light in a medium and refractive index.

Step 2: The concept The refractive index of a medium is:

n = c / v

where c is the speed of light in vacuum and v is the speed of light in the medium.

• Rearranging, v = c / n.
• So the higher the refractive index, the lower the speed in that medium.

Step 3: Apply to red and blue light Glass shows dispersion: its refractive index depends on wavelength.

• Red light has a longer wavelength, so it is bent the least, which means a lower refractive index and a higher speed in glass.
• Blue (or violet) light has a shorter wavelength, so it is bent the most, which means a higher refractive index and a lower speed in glass.

So red travels faster because the refractive index of blue light is greater than that of red light.

Step 4: Check the options

• A. Wrong. It says red has the greater refractive index, which would make red slower.
• B. Correct. Blue has the greater refractive index, so blue travels slower.
• C and D. Wrong. Intensity (brightness) does not determine the speed of light in a medium.

Answer: B

Tip: Remember "Violet bends most, red bends least." The more a color bends, the higher its refractive index and the slower it travels in glass. This is also why a prism spreads white light into a spectrum, with red at the top (least deviated) and violet at the bottom (most deviated).