Cambridge IGCSE Physics 0625 – Refraction of Light
3.2.2 Refraction of Light
Internal Reflection
When a light ray travels from a medium of higher refractive index to a medium of lower refractive index, it bends away from the normal. If the angle of incidence is large enough, the ray may be reflected back into the original medium rather than refracted out. This phenomenon is called internal reflection
The reflected ray lies in the same medium as the incident ray and obeys the law of reflection:
Total internal reflection occurs when the angle of incidence exceeds a specific value called the critical angle. At angles greater than the critical angle, the refracted ray disappears and all the light is reflected back into the original medium.
where \$n1\$ is the refractive index of the medium the light is coming from (higher index) and \$n2\$ is the refractive index of the second medium (lower index). The condition for TIR is \$n1 > n2\$ and \$\theta{\text{incident}} > \thetac\$.
Direct the ray from the ray box onto one face of the glass block at a small angle of incidence.
Increase the angle of incidence gradually and observe the refracted ray emerging from the opposite face.
When the incident ray reaches the second internal face (glass–air interface), continue increasing the angle.
At a certain angle the emergent ray disappears and a bright reflected ray is seen inside the block – this is the critical angle.
Measure this angle with the protractor; compare with the theoretical value using the refractive indices of glass (\$n\approx1.50\$) and air (\$n\approx1.00\$).
Everyday Examples
Optical fibres: Light is guided along the fibre core by repeated total internal reflection at the core‑cladding boundary.
Diamond sparkle: Diamonds have a high refractive index (\$n\approx2.42\$); light entering a diamond undergoes many internal reflections, giving the stone its brilliance.
Mirage: In a hot desert, light traveling from the ground (hot, less dense air) to cooler air above is refracted away from the normal. When the angle exceeds the critical angle, the light is internally reflected, creating the illusion of water.
Periscopes in submarines: Mirrors are used, but the principle of reflecting light within a confined medium is analogous to TIR.
Critical Angle Table for Common Materials
Medium (light from)
Medium (light to)
Refractive Index \$n_1\$
Refractive Index \$n_2\$
Critical Angle \$\theta_c\$ (degrees)
Glass
Air
1.50
1.00
\$\sin^{-1}(1/1.5)=41.8^\circ\$
Water
Air
1.33
1.00
\$\sin^{-1}(1/1.33)=48.8^\circ\$
Diamond
Air
2.42
1.00
\$\sin^{-1}(1/2.42)=24.4^\circ\$
Glass (core)
Cladding (n≈1.40)
1.50
1.40
\$\sin^{-1}(1.40/1.50)=66.4^\circ\$
Suggested diagram: Ray diagram showing an incident ray striking the glass–air interface at an angle greater than the critical angle, resulting in total internal reflection back into the glass.
Key Points to Remember
Internal reflection occurs when light moves from a denser to a rarer medium.
Total internal reflection requires \$n1 > n2\$ and \$\theta{\text{incident}} > \thetac\$.
The critical angle depends only on the ratio of refractive indices.
Applications of TIR are fundamental in modern technology (fibre optics, sensors, medical endoscopes).