Red light has a lower refractive index because it travels faster through a given medium compared to other colors in the visible spectrum, such as blue light. This difference in speed directly influences how much light bends when it passes from one medium to another.
Understanding Refractive Index
The refractive index ($n$) of a material is a fundamental optical property that describes how fast light travels through it compared to its speed in a vacuum. It is calculated by dividing the speed of light in a vacuum ($c$) by the speed of light in the medium ($v$):
$n = c/v$
Since the speed of light in a vacuum ($c$) is constant, any variation in the speed of light ($v$) within a medium directly affects its refractive index. A higher speed ($v$) in the medium results in a lower refractive index ($n$), and conversely, a slower speed leads to a higher refractive index.
The Role of Wavelength and Speed
Light is an electromagnetic wave, and different colors correspond to different wavelengths. Red light has the longest wavelength in the visible spectrum, while blue and violet light have shorter wavelengths.
When light enters a medium like glass or water, its speed changes. This change in speed is not uniform across all wavelengths due to a phenomenon called dispersion. In most transparent materials:
- Longer wavelengths (like red light) interact less frequently or less strongly with the electrons in the medium. This allows them to pass through more quickly.
- Shorter wavelengths (like blue or violet light) interact more frequently or more strongly with the medium's electrons, causing them to slow down more significantly.
Because red light waves travel at a faster velocity through a medium than bluer light waves do, they experience less of a reduction in speed. This higher velocity through the medium directly translates to a lower refractive index for red light.
How Lower Refractive Index Affects Refraction
Light bends, or refracts, when it passes from one medium to another at an angle (not straight on at 90°). The amount of bending is governed by Snell's Law and the refractive indices of the two media. A lower refractive index means that light bends less when entering a denser medium.
Key Relationships:
- Faster Speed in Medium $\Rightarrow$ Lower Refractive Index $\Rightarrow$ Less Bending (Refraction)
This explains why red light is refracted less than blue light. When a beam of white light, which contains all colors, passes through a prism, red light deviates the least, and blue/violet light deviates the most, creating a spectrum.
Comparison: Red Light vs. Blue Light in a Dispersive Medium
| Property | Red Light | Blue Light |
|---|---|---|
| Wavelength | Longest in visible spectrum | Shorter than red, longer than violet |
| Speed in Medium | Faster velocity | Slower velocity |
| Refractive Index ($n$) | Lower | Higher |
| Amount of Refraction | Less (bends less) | More (bends more) |
Practical Observations
This principle is evident in several natural and artificial phenomena:
- Prisms: Prisms separate white light into its constituent colors (the spectrum) because each color refracts at a slightly different angle based on its wavelength and speed through the glass. Red light is always at one end of the spectrum, deviating the least.
- Rainbows: Rainbows are formed when sunlight passes through raindrops, which act like tiny prisms. The different colors refract and reflect at slightly different angles, separating the light into the familiar arc of colors, with red on the outer edge.
- Chromatic Aberration: In lenses, the fact that different colors refract differently can lead to chromatic aberration, where colors are not focused to the same point, resulting in color fringes around images. Lens designers often use combinations of lenses to correct for this.
In essence, the lower refractive index of red light is a direct consequence of its longer wavelength allowing it to travel faster through transparent materials, causing it to bend less than shorter-wavelength colors like blue light.
