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Q. Basic Definitions in Radiation —–Absorptivity, Reflectivity, transmissivity and Emissivity

  1.  Absorptivity, α

Absorptivity is the ratio of absorbed radiations to the incident radiations on the surface.

Symbol of absorptivity is ‘α’.

It has no units.

α ≤ 1.

2.  Reflectivity, ρ

Reflectivity is the ratio of reflected radiations to the incident radiations on the surface.

Its symbol is ρ.

It has no units.


3.  Transmissivity,  τ

Transmissivity is the ratio of transmitted  radiations to the incident radiations on the surface.

Its symbol is z.

It has no units.


Q incident= Q absorbed+ Q reflected+ Q transmitted

qi=qa+ qr+qt

divide each term by qi we get

1= qa/qi  + qr/qi  + qt/qi

1=α + ρ +τ

Black body has   α=1, ρ=0, τ=0 and Є=1

Opaque body has    α+ρ=1, τ=0 and Є<1

White object has   α=0, ρ=1, τ=0 and Є<1

For a transparent body α=0, ρ=0, τ=1 and Є<1  Transparent body is also called a Diathermanous body.


It is the ratio of emissive power of a grey body to the emissive power of a black body at the same temperature. Mathematically  Є=(Eg/Eb)T=C

Values of emitted energy or emittance vary greatly for different materials.

(i)  rough surfaces such as ceramics or oxidized metals, Є

(ii) polished metals or silvered reflectors, Є

Q. Why are microwave ovens suitable for cooking?

Microwave oven gives infra red radiations which are thermal (heat) radiations and hence are suitable for cooking.

Q. What do you mean by participating and non participating mediums?

Participating medium: is one which absorbs, emits and scatter radiations. For example, carbon dioxide, water vapor, carbon monoxide and ammonia, radiation shield etc.

Non-participating medium: is one which does not absorb, emits and scatters radiations. For example: air and vacuum.

Q . Give the wavelength range of various electromagnetic waves.

 Type of wave                                    Range of wavelength

γ radiations                                  10-16 to 10-12 m

x radiations                                  10-12 to 10-9 m

Ultra-violet radiations           10-9 to 4 x 10-7 m (1 nm to 400 nm)

Visible radiations                   4 x 10-7 to 7 x 10-7         (400 nm to 700 nm)

Infrared radiations               7 x 10-7 to 25 x 10-7 (700 nm to 2.5 microns)

Microwaves radiations          25 x 10-7 to 1 mm      (2.5 microns to 1000 microns)

Radio radiations                    1mm to 1000 m           (1000 µm to 106 µm)


Q.  Which radiations are radio waves?

Cell phone waves (30 cm to 3m), FM radio waves (3 m to 30 m), TV waves (30 m to 300 m) and radio AM (300 m to 3000 m)

Q. What is wavelength range for thermal radiations?

The range for thermal radiation is 1µm to 100 µm.



Boiler furnace

Billet furnace

Any furnace

Gas Chula

Heat exchangers

IC engines

Gas turbines

Steam turbines

Nuclear reactors

Air craft engines

Human body