Chapter 2C: Radiation

Question 1

What is radiation?

Answer 1

Radiation is when energy propagates as electromagnetic waves or photons.

Question 2

Which bodies emit electromagnetic radiation?

Answer 2

All bodies warmer than absolute zero

(T > 0 K) emit electromagnetic radiation.

Question 3

How does radiation differ from sensible heat transfer?

Answer 3

Radiation does not require a medium to propagate through.

Question 4

What is the relation between the wavelength and the energy emitted by a wave/photon?

Answer 4

The shorter the wavelength, the more energy is emitted.

The longer the wavelength, the less energy is emitted.

Question 5

What is a black body?

Answer 5

A black body is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. Perfect black bodies do not exist in nature.

Question 6

What is emissivity?

Answer 6

Emissivity
ε = emitted radiation per unit area from an object / emitted radiation per unit area from a black body

Perfect black body: ε=1
Most solid materials have emissivity ε≈1 but never >1

Question 7

What does Planck’s law describe?

Answer 7

Planck’s law describes the intensity of radiation emitted by a black body at a constant temperature for different wavelengths or frequencies.

Question 8

What does Planck’s function describe?

the graph

Answer 8

• Planck’s function is expressed in terms of wavelength and temperature.
• It shows for a black body at a certain temperature how much energy is emitted depending on the wavelength.
• There is a certain wavelength at which the black body emits its maximum amount of radiation.
• The higher the temperature, the higher the peak (max emissivity)
• **A black body at a higher temperature than another will emit more energy than the second black body at ALL wavelenghts.

Question 9

What is Wien’s law?

Answer 9

Wien’s displacement law states that the wavelength at which an object (black body) radiates with maximum intensity depends on its absolute temperature (only)

λmax=2897 μmK / T (K)

Question 10

In what range of wavelengths is the maximum intensity emitted by the Sun in?

Answer 10

Visual light ; green light ; ≈0.5μm≈500nm

Question 11

In what range of wavelengths is the maximum intensity emitted by the Earth in?

Answer 11

Infrared; ≈10μm≈10 000nm

Question 12

What type of radiation do the Sun and the Earth emit (shortwave or longwave)?

Answer 12

The Sun’s radiation is shortwave radiation.

The Earth’s radiation is longwave radiation.

Question 13

99% of the radiant energy emitted by a black body is between what limits?

Answer 13

1000μmK ≤ (λ*T) ≤ 2.4 x 10^4μmK

Question 14

Shortwave radiation (or solar radiation) are wavelengths shorter than how many μm?

Answer 14

shorter than 4 μm.

Question 15

Longwave radiation (or terrestrial radiation) are wavelengths longer than how many μm?

Answer 15

longer than 4 μm.

Question 16

Why is the perceived color of the flame white? (photo)

Answer 16

1. The flame is hot (>1000K) so it emits in the visible; what we see is a mixture of visible wavelengths, perceived as white because white is the even mix of all colors.
2. The flame is hot, the actual wavelength of light emitted is likely in the yellow color range, but the camera taking the photo was compensating for the high intensity emission of light in the flame and reports it as such.

Question 17

What is the color black?

Answer 17

Black is the uneven mix of all colours.

Question 18

What is Stefan-Boltzmann law?

Answer 18

The radiant energy flux is proportional to the temperature to the power of 4.
E=σ*T^4
→ this means that a small increase/decrease in temperature leads to a large increase/decrease in total electromagnetic radiation emitted by an object.

Question 19

What is Kirchhoff’s law?

Answer 19

Kirchhoff’s law states that good absorbers at a particular wavelength are good emitters at that wavelength and vice versa.

It does not mean that an object radiates the same amount of energy that it absorbs.

Question 20

Why does the Sun emit more energy at its 550nm peak than a black body at T=5777K does (at the same λ) ?

Answer 20

Because a black body is perfect it emits the maximum amount of radiation at a certain temperature (here 5777K). So for the Sun to emit more energy at its peak than the black body, it has to be emitting at other temperatures as well.

Question 21

What is irridiance?

Answer 21

Irradiance is radiant energy per unit time per unit area perpendicular to incoming rays. (W/m^2)

Question 22

What is the solar constant (the solar irradiance)?

Answer 22

The solar constant is the solar power received at the top of the atmosphere on a unit surface area perpendicular to the Sun’s rays.

The (total) solar irradiance (as a constant) at the top of the atmosphere varies is ≈ 1367 W/m^2.

Question 23

What is intercepted solar irradiance?

Answer 23

The intercepted solar irradiance is the solar irradiance intercepted by the area of a disk with the Earth’s radius. It is equal to the solar irradiance divided by 4 (1367W/m^2 ÷ 4 ≈ 342W/m^2.

Question 24

What are the four possible outcomes when an electromagnetic wave (photon) passes through the atmosphere?

Answer 24

• transmission
• scattering
• reflection
• absorption

Question 25

What is transmission?

Answer 25

It is when the electromagnetic wave passes straight through the medium (atmosphere).

Question 26

What is scattering?

Answer 26

Scattering is the deflection of light in all directions by air molecules, small particles/droplets and/or irregularities in fluid medium.