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Total internal reflection (TIR) definition
When all light is reflected inside a medium instead of refracting out
Conditions for TIR
Light must travel from denser → less dense medium AND angle of incidence > critical angle
Critical angle definition
Angle of incidence that produces a 90° refracted ray
Critical angle formula
sin(c) = 1 ÷ n
n meaning
Refractive index of the denser medium
If i < critical angle
Light refracts out of the medium
If i = critical angle
Refracted ray travels along the boundary
If i > critical angle
Total internal reflection occurs
Reason for TIR
Light speeds up when leaving dense medium, bending away from normal
Optical fibres use
TIR keeps light trapped inside fibre for communication
Endoscopes use
TIR allows light to travel through fibre bundles inside the body
Prisms use
TIR used in periscopes, binoculars, cameras
Advantage of TIR
No energy loss; reflection is 100% efficient
Glass to air example
Common situation where TIR occurs
Water to air example
TIR can occur when looking up from underwater
Refracted ray behaviour
Only appears when i ≤ critical angle
Key idea
TIR happens only when angle is large enough and medium change is dense → less dense
The angles of refraction, critical and total internal reflection DIAGRAM
