Question

Define the coefficients of absorption, reflection and transmission. Obtain the relation between them.

Answer 1

1. The ratio of the amount of radiant energy absorbed by the body to the amount incident on the body simultaneously is known as the body's coefficient of absorption, absorptive power, or absorptivity.
2. The ratio of the amount of radiant energy reflected by a surface to the amount of radiant energy incident on it simultaneously is known as the surface of a body's coefficient of reflection, reflectance, or reflecting power.
3. The ratio of the amount of radiant energy that a body transmits to the amount of radiant energy that is incident on it simultaneously is known as the body's coefficient of transmission, transmittance, or transmissive power.

Relation between the coefficients: Let Q represent the amount of radiant energy incident on a body, and let Q_a, Q_r, and Q_t represent the amounts of radiant energy the body simultaneously absorbs, reflects, and transmits. Since there is a conservation of energy overall, we have,

Q_a + Q_r +  Q_t = Q

∴ `Q_a/Q + Q_r/Q + Q_t/Q` = 1

By definition,

`Q_a/Q = a`   ... (coefficient of absorption),

`Q_r/Q = r`   ... (coefficient of reflection) and

`Q_t/Q = t`   ... (coefficient of transmission)

Hence, a + r + t = 1

This is the required relation.