Imaging Planets

FRS 120 Lecture 4

Light as a Wave

Light as a Particle

Light comes in individual “quanta” of energy.

How many photons does a 100 W light bulb emit in a second? About 3e20 photons/s

Thermal Radiation

• The radiation from most bodies can be approximated as “black body radiation” with a characteristic temperature.

• The luminosity emitted by a black body scales as its surface area:

Conservation of energy

How many photons does the puppy see from a 100 W bulb at a distance of 100 m?

Scattered light from a planet

Energy/time striking planet

AlbedoAlbedo is the fraction of light scattered off of a planet. Albedo varies from 0.04 for fresh asphalt to 0.8 for freshly fallen snow. The average albedo of the earth is 0.3. The average albedo of the moon is only 0.12. The moon

absorbs 88% of the light that strikes its surface.

Luminosity of Planet = (albedo) (Energy/time hitting planet)

What is the ratio of the luminosity of the earth in reflected light to the luminosity of the Sun?

Fin

aFin

r

Cooling rate of planet =

Heating rate of planet =

Heating = Cooling (Thermal Equilibrium)

Visible

Infrared

Blue stars are hotter than red stars

Resolving Power of a Telescope

Telescope Optics

The Diffraction Problem (Visible)

Focal plane

Diameter (D)

Wavelength

Entrance Pupil

0 5 10 1510 -12

10 -10

10 -8

10 -6

10 -4

10 -2

100

Angle (lambda/D)

1

1e-10

Size of Image is a function of Telescope Size and Wavelength

Coronagraphs

Space-based CoronagraphSOHO image of the solar corona

HR 8799

Occulters