Dispersion occurs when white light passes through a glass prism because of the?

Analyze the question

The question asks why white light splits into its component colours (dispersion) when it passes through a glass prism.

Explain the concept

White light is a mixture of many colours (red, orange, yellow, green, blue, indigo, violet). In a vacuum or air, all colours travel at essentially the same speed. In glass, they do not.

• Each colour has a different wavelength and frequency.
• In glass, each colour travels at a different speed. Red travels fastest and violet slowest.
• The refractive index is n = c/v, so a different speed means a different refractive index for each colour.
• Violet has the highest refractive index in glass, so it bends the most. Red has the lowest, so it bends the least.

Step-by-step solution

1. White light enters the prism and refracts at the first face.
2. Because each colour has a different speed in glass, each has a different refractive index.
3. Each colour therefore bends by a different angle.
4. The colours spread out and separate, which is dispersion. They spread further after leaving the second face.

Checking the options

• A. "Hidden colours in the glass": wrong. The glass does not add colour. The colours are already in the white light.
• B. "Different speeds of the colour in the glass": correct. This is the real cause of dispersion.
• C. "Defects in the glass": wrong. A perfect prism disperses light too.
• D. "High density of the glass": wrong. Optical density affects how much light bends overall, but it is not what separates the colours.

Exam tip

Remember: "Different speed → different refractive index → different bending → dispersion." Violet bends most and red least. If a question asks about dispersion, look for an option that mentions different speeds or different refractive indices for different colours.