Colours, Temperatures and Blackbodies

Question 1

What is a black body?

Answer 1

Something which absorbs all radiation that falls on it.

Question 2

What does the graph of black body emission against wavelength look like for higher/lower temperatures?

Answer 2

For higher temperatures, the curve has high black body emission but shorter wavelength. How lower temperatures, the curve has low black body emission but higher wavelengths.

Question 3

What is Wien’s law?

Answer 3

λmax T = 2.9*10^-3

Question 4

What is the Stefan-Boltzmann law?

Answer 4

F = σ T^4, where F is the flux and σ is 5.67*10^-8 Wm^-2 K^-4

Question 5

What is the equation of luminosity for a spherical source with radius R?

Answer 5

L = 4piR^2 * F = 4piR^2σT^4, where F is the flux (or intensity)

Question 6

What do we get when we measure the light in a few specific intervals and compare the measured fluxes?

Answer 6

Filters.

Question 7

How can we define a colour using filters?

Answer 7

Define magnitudes within each filter (mB - mBref = -2.5*log(fB/fBref)), then can define a colour as the difference between two magnitudes.

Question 8

What difference in magnitude (colour) do hotter objects have?

Answer 8

They have a smaller colour than cooler objects.