Chemical Engineering - Heat Transfer - Discussion

Solids and Liquids are generally considered opaque to radiation because their molecules are tightly packed, which leads to high absorption or reflection of radiation.

Gases, on the other hand, are typically transparent to radiation, allowing it to pass through with minimal interaction. This property is due to their sparse molecular arrangement.

When solar radiation hits a surface, the photons can be absorbed, reflected, or transmitted. In the case of opaque (not transparent) materials, none of the photons are transmitted. .

If the material is dark and dull (not reflective or shiny) very few of the photons are reflected.

Because both liquis and solids are reflect the radiation energy.

Why liquid are upaque?

Opaque: Opaque liquids have a high solids content, which prevents light from penetrating through. Examples of opaque beverages include coffee and milk. Opaque samples are best measured using a 45/0-degree reflectance tool.

Translucent: Translucent liquids have a medium solids content, which allows for light to pass through diffusely. Examples of translucent beverages include fruit juices and sodas. You can use reflectance and transmittance instruments to measure these types of samples.

Transparent: Transparent liquids have little to no solids content, which allows light to pass through with minimal distortion. Examples of transparent beverages include water, spirits, and beer. You can only use transmission instruments to measure transparent liquids. without doubt, it must be only solids.

Why Liquid is not transparent?

Opaque (means) : not able to be seen through; not transparent.

Well, Visible light is also an electromagnetic wave which can propagate through liquids. Then why liquid considered as opaque?

Yes. Liquids are transparent to radiation. But it is an engineering consideration that they are opaque.

Refer introduction to heat transfer by incropera.

Then why liquid is opaque?