Q.Explain what is optical path length. How is it different from actual path length?

[!TLDR] Optical path ($n\Delta x$) is the vacuum-equivalent distance for the SAME phase change; actual path is the raw physical distance travelled -- they're equal only in vacuum/air ($n=1$), and optical path is always the larger of the two in any denser medium. [!ANSWER] Optical path = $n \times$ (actual physical distance); …

The ACTUAL (physical) path is simply the raw geometric distance a light ray travels through a medium -- an ordinary length, measured in metres, with no reference to the medium's optical properties at all. The OPTICAL path, by contrast, is defined as $n\,\Delta x$, where $\Delta x$ is that same actual physical distance and n is the medium's refractive index -- it represents the distance light would need to travel in VACUUM to accumulate the identical amount of phase change that it actually accumulates crossing the real distance $\Delta x$ in the medium (equivalently, it is the distance light in vacuum would cover in the same TIME the medium-crossing genuinely takes). Since a medium's wave vector is $k'=\omega/v=\omega n/c$, a physical distance $\Delta x$ produces phase change $\Delta\phi'=(\omega/c)(n\Delta x)$ -- exactly the same phase change a distance $n\Delta x$ would produce travelling in vacuum, which is the precise sense in which $n\Delta x$ is the 'equivalent' vacuum path. Since $n\geq1$ for any real medium (n=1 exactly for vacuum, and very close to 1 for air), the optical path is always GREATER THAN OR EQUAL TO the actual physical …

State the defining relation $\text{optical path} = n\,\Delta x$, explain it as the vacuum-equivalent distance for the same phase change (or same travel time), and contrast it with …

Treating 'optical path' and 'actual path' as simply two different names for the same physical distance, rather than recognising optical path is a PHASE-EQUIVALENT quantit …