Equations Paper 3 Explained

Question 1

What is the equation relating intensity and luminosity in astrophysics?

Answer 1

Intensity = luminosity / 4πr²

Intensity - how bright it is (Wm^-2)
Luminosity - total power output (W)
r - distance from observer to the centre of the light source (m)

This isn’t on the formula sheet

Question 2

What is the equation for intensity and luminosity in astrophysics?

Answer 2

Intensity = power produced by star / area of the sphere that is from centre of star to observer
Luminosity = power produced by star

This isn’t on the formula sheet

Question 3

What is the equation for the length of a refracting telescope in terms of focal length and eyepiece length?

Answer 3

length of telescope = f₀ + f_e

f₀ - focal length (m)
f_e - eyepiece length (m)

This isn’t on the formula sheet

Question 4

What three formulas can be used for magnification (M) ?

Answer 4

M = angle made with the eye / angle made with unaided eye
M = f₀ / f_e
(objective length compared to the eyepiece length)

M = α / β where alpha is the angle made by the ray to the focal plane after passing through the eyepiece lens and beta is the angle made by the ray to the focal plane before passing through the eyepiece lens

The second one isn’t on the formula sheet

Question 5

What is the Rayleigh criterion formula and what do the symbols mean?

Answer 5

θ = λ / D

θ - Minimum angle which can resolve
λ - wavelength of the light (m)
D - the diameter of the objective lens (m)

This only applies if the limiting factor of the telescope is its ability to resolve maximas