Chemical Engineering - Heat Transfer - Discussion

Then how does the sunlight reach to the aquatic plants.

A body is considered transparent if it can transmit some of the radiation waves falling on its surface. If electromagnetic waves are not transmitted through the substance it is therefore called opaque. When radiation waves hit the surface of an opaque body, some of the waves are reflected back while the other waves are absorbed by a thin layer of the material close to the surface. For engineering purposes all materials are thick enough that they can be considered opaque.

Most solids are opaque to thermal radiation except glass, rock salt etc.

Many liquids and gases are transparent to thermal radiation.

Gases also absorbed heat. Should they not be considered opaque?

Then why liquid is opaque?

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

Refer introduction to heat transfer by incropera.

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

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

Why Liquid is not transparent?

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.