Characteristics	External Reflection	Internal Reflection
Refractive Index Condition	n_i < n_tni	n_i > n_tni>nt
Phase Change	Semi-wave loss present	No semi-wave loss
Critical Angle	Does not exist	Exists
Reflectivity	Increases gradually with increasing angle of incidence	Similar to external reflection below the critical angle; abrupt change occurs at the critical angle
Energy Distribution	Reflected and refracted light always coexist	Divided into two cases: partial reflection and total internal reflection
Common Applications	Flat mirrors, water surface reflections	Fiber optics, total internal reflection prisms, anti-reflective coating theory

1. Phase Shift (Half-Wave Loss)
This is the most fundamental physical distinction between the two.
External Reflection: When light travels from a less dense medium to a more dense medium, the reflected light undergoes a 180° phase shift, known as “half-wave loss.” This is equivalent to adding half a wavelength (λ/2) to the optical path of the reflected light.
Internal Reflection: During internal reflection, the reflected light does not undergo half-wave loss.
Why is this important? In thin-film interference phenomena (e.g., anti-reflective coatings, anti-glare films) and Newton's rings, half-wave loss is the key factor determining whether interference results in constructive or destructive interference.

2. Critical Angle and Total Internal Reflection
This is the most distinctive and crucial characteristic of internal reflection.
External Reflection: Since refracted light always enters the second medium, no critical angle exists.
Internal Reflection:
When the incident angle < critical angle: Similar to ordinary reflection, both reflected and refracted light exist. This is called partial internal reflection.
When the incident angle ≥ critical angle: Refracted light disappears, and all incident light is reflected back into the first medium. This is called total internal reflection.

Critical angle calculation formula:

Where ni is the refractive index of the denser medium, and nt is the refractive index of the less dense medium.

External reflection (red line): When the angle of incidence is less than approximately 50°, the reflectance is very low (about 4%). It then rises gradually until it reaches 100% at 90°.
Internal reflection (blue line): Its behavior is nearly identical to external reflection until the critical angle (approximately 41°). Upon reaching the critical angle, the reflectance instantly jumps to 100% and remains constant thereafter, perfectly demonstrating the phenomenon of total internal reflection.
External reflection is common but inefficient (unless the angle of incidence is very large). Mirrors utilize metallic reflection rather than the principle of external reflection.
Internal reflection, particularly total internal reflection, is a highly efficient optical process with negligible energy loss. It forms the foundation of modern optical communications (fiber optics) and serves as the core operating principle for numerous optical instruments, such as the total internal reflection prism in binoculars. Simply put, external reflection is the “standard mode,” while internal reflection possesses the remarkable capability to achieve “perfect reflection” at specific angles.