Human Eye and Colorful World
Wavelength of VIBGYOR colors
| Color | Wavelength | Frequency | Photon Energy |
|---|---|---|---|
| Red | 625-700 nm | 400-480 THz | 1.65-1.98 eV |
| Orange | 590-625 nm | 480-510 THz | 1.98-2.10 eV |
| Yellow | 565-590 nm | 510-530 THz | 2.10-2.19 eV |
| Green | 500-565 nm | 530-600 THz | 2.19-2.48 THz |
| Light Blue | 484-500 nm | 600-620 THz | 2.48-2.56 eV |
| Deep Blue | 450-484 nm | 620-670 THz | 2.56-2.75 eV |
| Violet | 380-450 nm | 670-790 THz | 2.75-3.26eV |
| [Science notes](Visible Light Spectrum Wavelengths and Colors) |
Dispersion in a Prism
Definition:
Dispersion is the separation of white light into its constituent colors (VIBGYOR) due to refraction (bending of light) in a medium (e.g., a glass prism).
Why does it happen?
- The refractive index of a material (like glass) varies with the wavelength of light.
- Shorter wavelengths (violet/blue) travel slower in the medium compared to longer wavelengths (red/orange).
- This causes shorter wavelengths to bend more than longer ones when entering/leaving the prism.
Wavelength Dependence:
- Violet light (400–440 nm) bends the most.
- Red light (620–720 nm) bends the least.
Result:
A spectrum of colors (VIBGYOR) emerges, with violet at the top and red at the bottom.
Example:
A rainbow is formed when sunlight undergoes dispersion in water droplets (acting like tiny prisms).
Scattering of Light
Definition:
Scattering is the redirection of light as it interacts with particles or molecules in a medium (e.g., air molecules in the atmosphere).
Why does it happen?
- The intensity of scattering depends on the wavelength of light and the size of the scattering particles.
- For small particles (like air molecules), Rayleigh scattering dominates.
- The scattering intensity is inversely proportional to the fourth power of the wavelength - I ∝ 1 / λ^4
Wavelength Dependence:
Shorter wavelengths (violet/blue) scatter much more than longer wavelengths (red/yellow).
Example:
The sky appears blue during the day because blue light scatters more in Earth's atmosphere.
Exceptions:
- At sunrise/sunset, light travels through more atmosphere.
- Blue light scatters away, leaving longer wavelengths (red/orange) to dominate our view.
- Violet light scatters the most, followed by blue, green, etc.
- However, the sky appears blue (not violet) because:
- Human eyes are less sensitive to violet.
- The sun emits more blue light than violet.
- Some violet light is absorbed by the upper atmosphere.
Example:
The Tyndall effect (visible beam in fog) occurs when light scatters off larger particles (e.g., water droplets).
Key Differences Between Dispersion and Scattering
| Aspect | Dispersion | Scattering |
|---|---|---|
| Cause | Refractive index variation with wavelength | Interaction with particles or molecules |
| Dependence | n∝1/λ (shorter λ bends more) | I∝1/λ^4 (shorter λ scatters more) |
| Result | Separation of colors (spectrum) | Redirection of light (e.g., blue sky) |
| Medium | Transparent materials (e.g., glass) | Atmosphere, colloids, or particles |
