Question

Explain the dependence of the deviation of light produced by a glass prism on the wavelength or colour of light.

Answer 1

The angle of deviation produced by a glass prism is inversely proportional to the wavelength of light. When different colours of light travel through air or a vacuum, they all travel at the same speed. However, when they enter a denser medium like a glass prism, their speed decreases, which causes the light to refract or bend.

This dependence can be understood through the relationship between speed, frequency, and wavelength:

Speed (v) = Frequency (f) × Wavelength (λ)

During refraction, the frequency (f) of light remains completely unchanged. Therefore, the speed of light inside the prism depends directly on its wavelength. As the wavelength of light decreases, its speed inside the glass medium also decreases. Consequently, a colour of light with a shorter wavelength will travel slower in the glass and bend more, resulting in a larger angle of deviation compared to a light with a longer wavelength.

Example (Violet vs. Red Light):

In the visible spectrum, different colours have different wavelengths:

• Violet Light: It has a shorter wavelength (≈ 400 nm). Because of this, it travels slower inside the prism and undergoes the maximum deviation (bends the most).
• Red Light: It has a longer wavelength (≈ 800 nm). Therefore, it travels faster inside the prism and undergoes the minimum deviation (bends the least).