Reflectance, Absorptance & Transmittance-

 

When thermal radiation falls on a body, they are partly reflected, absorbed and rest get transmitted.

 

Let Q be the amount of thermal energy incident on a body. Suppose-

• The part R is reflected
• The part A is absorbed
• The part T is transmitted

 

 

By energy conservation for the incident radiation, we must have-

 

 

Here,

 

 

Thus,

 

From here, we can define the three terms as follows-

 

Reflectance (r)-

 

It is defined as the ratio of the amount of thermal energy reflected by a body in a certain time to the total amount of thermal energy falling upon the body in the same time.

 

 

Absorptance (a)-

 

It is defined as the ratio of the amount of thermal energy absorbed by a body in a certain time to the total amount of thermal energy falling upon the body in the same time.

 

 

Transmittance (t)-

 

It is defined as the ratio of the amount of thermal energy transmitted by a body in a certain time to the total amount of thermal energy falling upon the body in the same time.

 

 

Special Cases-

 

Case-01: If a body does not transmit radiation-

 

We know-

r + a + t =1

Here, t = 0

So, r + a = 1

Clearly, if ‘r’ is more, ‘a’ is less and vice-versa.

 

Important Note From here, we infer that- Good reflectors are bad absorbers. Bad reflectors are good absorbers of heat.

 

Case-02: If a body neither reflects nor transmits any radiation-

 

We know-

r + a + t =1

Here, r = t = 0

So, a = 1

Such a body which neither reflects nor transmits but absorbs whole of the heat radiation falling on it is called a black body.

 

Important Note No real body can absorb 100% of the radiation falling on it. Therefore, the value of absorptance (a) is always less than unity for a real body.

 

Factors Affecting Reflectance, Absorptance and Transmittance of A Body-

 

The reflectance, absorptance and transmittance of a body depends on-

1. Nature of the surface of the body
2. Wavelength of incident radiation

 

From here, we infer that for radiations of different wavelength, a given body may have different values of reflectance, absorptance and transmittance. It is more useful to define these quantities for a given wavelength λ.

 

For any specific wavelength λ, we can write-

 

 

Important Note It is important to note that these quantities do not depend on the nature of material of the body.