12 - Fundamentals of Thermal Radiation

Question 1

How does radiation differ from conduction and convection?

Answer 1

It does not require the presence of a material medium to take place

Question 2

How are EM waves or EM radiation formed?

Answer 2

Accelerating charges or changing electric currents gives rise to electric and magnetic fields
Represent the energy emitted by matter as a result of the changes in electronic configurations of atoms or molecules

Question 3

What is thermal radiation emitted by?

Answer 3

All matter with a temperature above absolute zero

Question 4

Wavelength range of visible light

Answer 4

0.40 - 0.76 micrometers

Question 5

Wavelength range of solar radiation

Answer 5

0.3 - 3 micrometers

Almost half is light, with the remaining being ultraviolet and infrared

Question 6

Which bodies emit infrared radiation?

Answer 6

Bodies at room temperature

0.76 - 100 micrometers

Question 7

What type of phenomenon is radiation?

Answer 7

Volumetric/3D

Question 8

Radiation from a blackbody

Answer 8

Blackbody will emit the maximum amount of radiation by a surface at a given temperature
Idealised body to serve as standard against which radiative properties of real surfaces may be observed
Perfect emitter and absorber of radiation
Absorbs all incident radiation, regardless of wavelength and direction

Question 9

Define spectral blackbody emissive power

Answer 9

Amount of radiation energy emitted by a blackbody at a thermodynamic temperature T per unit time, per unit surface area, and per unit wavelength

Question 10

Wien’s displacement law

Answer 10

Gives the wavelength at which the peak occurs for a specified temperature

Question 11

Define radiation intensity

Answer 11

A quantity which describes the magnitude of radiation emitted/incident in a specified direction in space

Question 12

How is radiation considered for opaque or semitransparent materials?

Answer 12

To be a surface phenomenon

Question 13

Define emissivity

Answer 13

The ratio of the radiation emitted by the surface at a given temperature to the radiation emitted by a blackbody at the same temperature
(measures how closely a surface approximates a blackbody)

Question 14

What does the emissivity of a real surface vary with?

Answer 14

Surface temperature
Wavelength of emitted radiation
Direction of emitted radiation

Question 15

What is spectral emissivity?

Answer 15

Emissivity of a surface at a specified wavelength

Question 16

What is directional emissivity?

Answer 16

Emissivity of a surface in a specified direction

Question 17

When can a surface be said to be ‘diffuse’?

Answer 17

If its properties are independent of direction

Question 18

When can a surface be said to be ‘grey’?

Answer 18

If its properties are independent of wavelength

Question 19

Effect of diffuse approximations on a surface

Answer 19

Directional emissivity is constant

Question 20

Effect of grey approximations on a surface

Answer 20

Spectral emissivity is constant

Question 21

Effect of diffuse and grey approximations on a surface

Answer 21

Spectral emissivity = directional emissivity = constant

Question 22

Common assumption for radiation analysis

Answer 22

Surfaces are diffuse emitters with an emissivity equal to the value in the normal direction

Question 23

Define irradiation

Answer 23

Radiation flux incident on a surface

Question 24

Define specular reflection

Answer 24

Angle of reflection equals angle of incidence of the radiation beam

Question 25

Define diffuse reflection

Answer 25

Radiation is reflected equally in all directions

Question 26

Kirchhoff’s Law

Answer 26

Total hemispherical emissivity of a surface at temperature T is equal to its total hemispherical absorptivity for radiation coming from a blackbody at the same temperature