Why is the sky blue? Apollo astronaut, Neil Armstrong told Patrick Moore in a 1970 BBC interview, "THE SKY IS A DEEP BLACK WHEN VIEWED FROM THE MOON. THE EARTH IS THE ONLY OBJECT OTHER THAN THE SUN THAT CAN BE SEEN, ALTHOUGH THERE HAVE BEEN REPORTS OF SEEING PLANETS. I MYSELF DID NOT SEE PLANETS FROM THE SURFACE OF THE MOON, BUT I SUSPECT THEY MIGHT BE VISIBLE". Apollo 11, July 1969 Rayleigh Scattering We have three types of colour receptors, or cones, in our retina. They are called red, blue and green because they respond most strongly to light at those wavelengths. As they are stimulated in different proportions, our visual system constructs the colours we see. Response curves for the three types of cone in the human eye Electric field Electron oscillator Electric field Electron oscillator 2 i Dipole moment (t) E y (t) = E 0 cos ( t) (t) (q 2 /m) i EyEy (t) Induced dipole moment, = E = polarizability 2 i Dipole moment (t) E y (t) = E 0 cos ( t) (t) (q 2 /m) i EyEy (t) Induced dipole moment, = E = polarizability Absorption ii rr Dipole moment in the excited state is larger compared to ground state (q 2 /m) i EyEy (t) (t) Rotation of plane-polarized light * * Chirality Circular polarization Rotation of plane-polarized light Absorption ii rr Dipole moment in the excited state is larger compared to ground state (q 2 /m) i EyEy (t) (t) Circular BirefringenceCircular Dichroism Rotation of plane of polarization Rotation of plane-polarized light Molecular Vibrations 1595 cm cm cm -1 Layout of Light Absorption Measurement I0I0 I Absorbance: A = log (I 0 /I) = c L t 10 ns/ 100 MHz Mode-locked laser Cavity Ring Down Spectroscopy Reflectivity = 99.99% 0 = 200 s Path = 100 km James Clerk Maxwell Born: 13 June 1831 Edinburgh, Scotland Died: 5 November 1879 (aged 48) Cambridge, England