Electromagnetic Radiation

Electromagnetic Radiation • • • • • Is light a wave or a particle?

Yes It’s both, and neither At atomic scales, we have no exact analogs for phenomena For some purposes light is best treated as a wave, for others as particles (photons)

Light Waves • • • Light consists of oscillating electric and magnetic fields Changing electric field creates changing magnetic field …..

Mostly we visualize the electric field – it’s simpler

Waves and Energy • • • • Frequency * Wavelength = c (speed of light) Speed of light in materials < c – In air, the difference is tiny, but critical in surveying – When light changes speed, it changes direction – Refraction of light causes distortion Energy of a photon = h*frequency – h = Planck’s Constant High Frequency = Short Wavelength = High Energy

Fig. 18-2, p.430

Light and Materials • • • • • Transmission Absorption Reflectance Scattering – Related to roughness and particle size Emission – Mostly infrared

Sources of Radiation • Black body – Hot objects glow visibly – Warm objects emit infrared and radio – Useful for remote temperature sensing – Thermal emission also useful for characterizing surfaces

Black-Body Radiation • • • • Objects Emit Radiation Because They Are Hot Why “Black”? Because None of the Radiation is Reflected from Some Other Source The Sun Emits Black-Body Radiation, Mars Does Not Close Example of pure Black-Body radiation: Peephole in a pottery kiln

Black Body Radiation

What’s The Source of the Light?

Color = Temperature

Why Black-Body Radiation is so Important • • • Color is directly related to temperature Temperature is the only determinant of color Energy per unit area is the same if temperature is the same – If two stars have the same color and distance, difference in brightness is due to difference in size – Dwarf and giant stars are literally dwarfs or giants

Sources of Radiation • • Molecular vibrations – Mostly infrared – Create absorption bands in atmosphere – Active sensing with tuned lasers Atomic excitation – Largely visible light – Many mechanisms at work – How we see

Atoms and Radiation

Spectroscopy • • • Different atoms absorb or emit specific wavelengths of light When light spread into a spectrum, the absorbed wavelengths show up as dark (missing) bands These spectral lines are indicators of: – Chemical composition – Physical conditions

The Solar Spectrum

Remote Sensing and EM • • • • Microwaves: Used in radar imaging Infrared – Absorption by molecules – Emission by warm materials – Reflectance Visible Light Ultraviolet – – Absorbed by ozone in atmosphere Most rocks absorb UV strongly – Mostly useful in astronomy for high energy phenomena