Short question answers are extremely helpful in better

understanding of radiation. These also improve the depth of

knowledge and clarity. Thus, one can apply the fundamentals

in design of radiation equipment.

Fig. Absorption, Reflection & Transmission of Radiations incident on a Grey Body

Q. Define Absorptivity, Reflectivity, transmissivity and Emissivity

(a) 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.

(b).  Reflectivity, ρ

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

Its symbol is ρ.

It has no units.


(c).  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 a Diathermanous body.

(d). Emissivity, Є

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

Q2.   What is an opaque body?

A body which has zero transmissivity (τ=0). Or α + ρ=1. Solids normally do not transmit radiations and hence are opaque.

Q3.  Explain a white body.

A body which zero absorptivity as well as zero transmissivity. It reflects the entire radiations. Mathematically      ρ = 1 and α=0 and τ=0. For example a mirror.

Q4. Discuss a grey body.

A body which has absorptivity, reflectivity and transmissivity.  Further α, ρ and τ do not vary with different wavelengths. For example say α=0.4 for all wavelengths λ1, λ2 etc.

Mathematically for a grey body  α + ρ + τ =1

Q5. Give the definition of a colored body.

A colored body is one for which absorptivity; reflectivity and transmissivity vary with wavelength. Therefore if the absorptivity, reflectivity and transmissivity are to be changed, make it a colored body. Red color has the maximum absorption and violet has the least. Absorption increases with increase of wavelength.

Q6.  State a black body? Give examples of bodies which do not appear black but have high values of absorptivity.

A black body which absorbs all the radiations. It is a theoretical concept since there is no perfect black body. A black body is a reference body used for comparing real bodies for their radiation behavior. Snow, ice and white paper have absorptivity in the range of 0.97 to 0.98 and these do not look black and are not black bodies.

Q7. Main difference between emissivity and absorptivity.

Emissivity: Fraction of radiations given out by a body at a certain temperature to the radiation given by a black body at the same temperature. Its symbol is ε. It is < 1.

Absorptivity: Fraction of radiations absorbed by a body out of radiations incident on that body. Its symbol is α. It is < 1.

  Q10. Differentiate between a grey and colored body in terms of its wavelength.

Grey body: When the absorptivity of a body does vary with wavelength, it is called a grey body.

Colored body

When the absorptivity varies with wavelength, it is called a colored body.

Q11. Differentiate between emissive power and monochromatic power.

 Emissive power

 A body gives radiations of many wavelengths. Radiations due to all wavelengths are called total emissive power of a body. It is given by Stefan’s Boltzmann Law for a black and a grey body. Mathematically

Eb = σ T4

Eg = Єσ T4

Monochromatic emissive power: It is emissive power due to a particular one wavelength λ. It is given by Planck’s law of monochromatic radiations.

Eλ = C1 λ—5/ (eC2/λT  — 1)

C1 = 3.742 x 108    Wµm4/m2

C2 = 1.4388 x 10 4        µmK

 Q12.   Explain the main difference between absorptivity and emissivity.

Absorptivity is a fraction of radiations absorbed out of incident radiations. Suppose 100 radiations are falling on a body. Only 28 are absorbed. Then absorptivity =α= 28/100 = 0.28. It has no units.

Emissivity ‘Є’

ratio of radiations given out by a body to the radiations given out by a black body at the same absolute temperature. It is unit less.

Є = Eg/Eb

Q13. Why are microwave ovens suitable for cooking?

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

Q14. 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.

Q15. 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)


Q16.  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

Q18. Which color has the lowest and highest frequencies?

Red light is the lowest frequency of visible light. Violet has the highest frequency.

we see heated objects emit first red light as they get just hot enough to radiate in the visible range, and then orange, yellow, green, blue, and violet as they heat up further. Red light is the lowest frequency of visible light and violet is the highest, and there are also frequencies lower than those of light (infrared, microwaves, and radio waves) and higher (ultraviolet, X-rays, and gamma rays), and all of these were also observed in the appropriate intensities as black bodies were heated and their spectra were measured.

Q19. What is Planck’s black body function?

 The Planck’s function is a graph (Intensity along ‘y’ axis and wavelength along x-axis) which has a distinctive shape at any fixed temperature i.e.  It rises very sharply at short wavelengths (due to the exponential nature), reaches a peak value at some wavelength, and then falls gradually at longer wavelengths.

Q20. Which radiations are included in the thermal radiations (or heat radiations)?

  Small part of ultra-violet radiations + full visible radiations +full infra-red radiations.

Q21. What is type of radiations given out by the human body?

The human body gives only infra-red radiations (Heat radiations). The wavelength of these radiations is between 3 µm to 50 µm but most of these are of 9 µm.

Q22.  If a bulb emits 200 W at a temperature of T, how much energy will emitted when the temperature rises to 2T ?

 Since flux given is proportional to T4, the heat emitted at 2T will become 16 times i.e. 3200 W.

Q23. State the differences between diffusion and radiative heat transfer ?

Sr. no




It is due to RANDOM molecular motion

Radiation is the heat transfer by electromagnetic waves.


There is no bulk motion of the medium but a medium is required.

There is no medium at all.It is thus the only mode of heat transfer in space.


Neighboring molecules move randomly and transfer energy between one another.

All bodies emit thermal radiations at all temperatures above 0 Kelvin.


Diffusion is slow as compared to radiative heat transfer

Radiative heat transfer is fast.

What is the range for the emissivity of surfaces ?

The value of emissivity ranges between 0 and 1.

Q25. What are the basic assumptions in heat transfer?

(i). Heat transfer must be one-dimensional

(ii). It must be under steady state conditions

(ii) Material is homogeneous and isotropic i.e. k is constant.

Q26.  How does the wavelength of the maximum brightness depend upon the temperature?

Wien’s Law i.e. Absolute temperature is inversely proportional to the brightness.

λmT=2900 µm K

Q27. How do we determine the surface temperature of the Sun?

 Temperature of the Sun is found from solar constant.

Solar constant=1367= [(Eb) sun rsun­2]/(distance of sun from outer atmosphere) 2

Distance between the Sun and the earth = 1488 x 105 km

(Eb) sun =σTsun4

Q28. How does the surface flux emitted by a hot body depend upon the temperature?

Stefan-Boltzmann Law i.e. Surface flux is directly proportional to the fourth power of the absolute temperature.

Q29. Why did Planck introduce quantization of light into the blackbody formulation?

In 1900, the German physicist Max Planck introduced the idea that energy is quantized in order to derive a formula for a black body radiation as well to explain the photo-electric effect.


https://www.mesubjects.net/wp-admin/post.php?post=759&action=edit                       Radiation basics

https://www.mesubjects.net/wp-admin/post.php?post=264&action=edit                      Radiation Exchange


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