Understanding Transmissivity of Opaque Surfaces

The question asks about the transmissivity of opaque surfaces. To answer this, we need to understand what an opaque surface is and what transmissivity means.

What is an Opaque Surface?

An opaque surface or material is one that does not allow electromagnetic radiation, such as light, to pass through it. When radiation strikes an opaque surface, it is either absorbed by the surface or reflected off it.

What is Transmissivity?

Transmissivity ($\tau$) is a property of a material that describes the fraction of incident electromagnetic radiation that passes through it. It is a value between 0 and 1 (or 0% and 100%).

• A transmissivity of 1 means all radiation passes through.
• A transmissivity of 0 means no radiation passes through.

Transmissivity of Opaque Surfaces

Since an opaque surface, by definition, does not allow any radiation to pass through, the fraction of radiation transmitted through it is zero.

Therefore, the transmissivity of an opaque surface is always $\tau = 0$.

Relationship between Radiative Properties

For any surface, the incident radiation is either absorbed, reflected, or transmitted. These fractions are represented by absorptivity ($\alpha$), reflectivity ($\rho$), and transmissivity ($\tau$), respectively. These properties are related by the equation:

\begin{equation*} \alpha + \rho + \tau = 1 \end{equation*}

For an opaque surface, we know that $\tau = 0$. The equation then simplifies to:

\begin{equation*} \alpha + \rho = 1 \end{equation*}

This shows that for an opaque surface, the incident radiation is only absorbed or reflected.

Analyzing the Options

Let's look at the given options for the transmissivity of opaque surfaces:

• 100: This implies 100% transmission, which is incorrect for an opaque surface.
• 0: This implies 0% transmission, which is correct for an opaque surface.
• 10: This implies 10% transmission, which is incorrect for an opaque surface.
• 1: In the context of fractions (0 to 1), this would mean 100% transmission, which is incorrect.

Based on the definition and properties of opaque surfaces, their transmissivity is zero.

The correct answer is 0.