No, total internal reflection cannot occur when light travels from air into water.
Understanding Total Internal Reflection
Total internal reflection (TIR) is an optical phenomenon that occurs when light traveling through a medium encounters a boundary with another medium having a lower refractive index, and the angle of incidence exceeds a specific value called the critical angle. Instead of passing into the second medium or refracting, the light is entirely reflected back into the first medium.
The Essential Conditions for Total Internal Reflection
For total internal reflection to happen, two critical conditions must be met:
- Condition 1: Light must be traveling from a medium with a higher refractive index to a medium with a lower refractive index.
- Condition 2: The angle of incidence (the angle between the incoming ray and the normal to the boundary) must be greater than the critical angle for the two media.
Why Air to Water Doesn't Qualify
Let's consider light moving from air to water. We need to examine the refractive indices of these two common substances:
- The approximate refractive index of air is 1.00.
- The approximate refractive index of water is 1.33.
When light travels from air to water, it is moving from a medium with a lower refractive index (air, ~1.00) to a medium with a higher refractive index (water, ~1.33).
As stated in the reference, "There is no total reflection for rays going in the other direction—for example, from air to water—since the condition that the second medium must have a smaller index of refraction is not satisfied."
Since water (the second medium) has a higher refractive index than air (the first medium), the fundamental requirement for total internal reflection (traveling from a higher index to a lower index) is not fulfilled.
You can see this relationship clearly in the table below:
| Medium | Approximate Refractive Index (n) |
|---|---|
| Air | ~1.00 |
| Water | ~1.33 |
When light moves from Air to Water, the refractive index increases (1.00 -> 1.33), which is the opposite of what is needed for TIR.
In Summary
Because light traveling from air to water moves from a medium of lower refractive index to a medium of higher refractive index, total internal reflection cannot occur. Refraction will occur instead, bending the light ray towards the normal as it enters the water.
