Reflection & Refraction
Mechanism of Bending: Refraction is the change in direction of a wave as it crosses a boundary between two media. This occurs because the speed of light changes depending on the optical density of the material it is traveling through.
Optical Density: When light enters a more optically dense medium (e.g., from air to glass), it slows down and bends towards the normal. Conversely, when entering a less dense medium, it speeds up and bends away from the normal.
Mathematical Relationship: The relationship between the angles and the refractive indices of the media is given by Snell's Law: . Here, represents the refractive index, which is the ratio of the speed of light in a vacuum to its speed in the medium ().
The Critical Angle: As the angle of incidence increases when light travels from a denser to a less dense medium, the angle of refraction also increases. The critical angle () is the specific angle of incidence that results in an angle of refraction of exactly , where the light travels along the boundary.
Conditions for TIR: Total Internal Reflection occurs when two conditions are met: the light must be traveling from a more dense to a less dense medium, and the angle of incidence must be greater than the critical angle ().
Result of TIR: Under these conditions, no light is refracted into the second medium; instead, all the light is reflected back into the original denser medium, following the standard law of reflection.
| Feature | Reflection | Refraction |
|---|---|---|
| Medium | Stays in the original medium | Passes into a new medium |
| Speed | Remains constant | Changes based on optical density |
| Direction | Bounces off at the same angle | Bends toward or away from normal |
| Frequency | Remains constant | Remains constant |
Always Draw the Normal: Before attempting any calculation or ray diagram, draw a dashed line perpendicular to the surface. This ensures you measure the correct angles ( and ) from the normal, not the surface.
Check the Bending Direction: Use the mnemonic 'Fast to Slow, Towards the Normal' (FST) and 'Slow to Fast, Away from Normal' (SFA) to quickly verify if your refracted ray is drawn in the correct direction.
TIR Verification: If a question asks about light moving from air to glass, Total Internal Reflection is impossible. Always check that the light is moving from a higher refractive index to a lower one before considering TIR.