1. What refractive index really tells me
The refractive index of a medium tells me how much light slows down inside that medium. Light travels fastest in vacuum, a bit slower in air, and much slower in glass or water. The greater the slowing, the higher the refractive index.
I think of refractive index as a measure of how strongly a medium bends or "controls" the path of light. A higher value means stronger bending at the boundary.
2. Basic definition
The refractive index of a medium is defined as:
\( n = \dfrac{c}{v} \)
where:
- c is the speed of light in vacuum.
- v is the speed of light in the medium.
If light slows down a lot inside a material, the value of \(n\) becomes high.
3. Absolute and relative refractive index
I often come across two kinds of refractive index values:
3.1. Absolute refractive index
This is the refractive index of a medium with respect to vacuum. Most tables give this value. For example:
- Air ≈ 1.0003 (almost 1)
- Water ≈ 1.33
- Glass ≈ 1.5
- Diamond ≈ 2.42
3.2. Relative refractive index
This compares one medium with another. If light goes from medium 1 to medium 2, the relative refractive index is:
\( n_{21} = \dfrac{v_1}{v_2} \)
It tells me how much the ray bends when entering the second medium.
4. Connection with bending of light
A medium with higher refractive index slows down light more. This causes stronger bending at the boundary.
Some rules I remember:
- If light goes to a medium with higher n, it bends towards the normal.
- If it goes to a medium with lower n, it bends away from the normal.
5. Refractive index and wavelength
In many transparent materials, refractive index depends on wavelength. Shorter wavelengths (blue light) slow down slightly more than longer wavelengths (red light). Because of this, a prism spreads white light into colours.
5.1. Dispersion idea in one line
Higher n for blue → bends more. Lower n for red → bends less.
6. Snell’s law in refractive-index form
Snell’s law describes how light bends at a boundary. In terms of refractive index, it is written as:
\( n_1 \sin i = n_2 \sin r \)
This tells me that the ratio of the sines of angles depends on the refractive indices of the two media.
7. A quick physical feel of refractive index
Whenever I compare two materials:
- Higher refractive index → light travels slower → stronger bending.
- Lower refractive index → light travels faster → weaker bending.
Even without calculation, I can guess the direction of bending by thinking which medium has the higher refractive index.
8. Small example
If light enters water (n ≈ 1.33) from air (n ≈ 1.00), it bends towards the normal. If it exits water into air, it bends away from the normal.
This simple reasoning helps me sketch ray diagrams quickly.