1.6 Using radiation to investigate stars

Question 1

Black body

Answer 1

A black body is a body (or surface) which absorbs all the electromagnetic radiation that falls upon it. No body is a better emitter of radiation at any wavelength than a black body at the same temperature.

Question 2

Wien’s displacement law

Answer 2

The wavelength of peak emission from a black body is inversely proportional to the absolute (kelvin) temperature of the body.
λm = W / T [W=theWienconstant=2.90×10-3 mK]
(Wavelength is small temp is hot!)

Question 3

Absolute or kelvin temperature

Answer 3

The temperature, T in kelvin (K) is related to the temperature, θ, in celsius (°C) by:
T / K= θ / °C + 273.15
At 0 K (-273.15°C) the energy of particles in a body is the lowest it can possibly be.

Question 4

Stefan’s law [The Stefan- Boltzmann law]

Answer 4

The total electromagnetic radiation energy emitted per unit time by a black body is given by power = A σT^4 in which A is the body’s surface area and σ is a constant called the Stefan constant. [σ = 5.67 × 10-8 W m-2 K-4]

Question 5

Luminosity of a star

Answer 5

The luminosity of a star is the total energy it emits per unit time in the form of electromagnetic radiation. UNIT: W
[Thus we could have written luminosity instead of power in Stefan’s law (above).]

Question 6

Intensity

Answer 6

The intensity of radiation at a distance R from a source
is given by
I = P / 4πR^2
Unit = Wm^-2