# Nature of Light Not-For-Profit Public Foundation.

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Nature of Light Not-For-Profit Public Foundation

Light is a form of energy. Energy can be transported by waves. Use waves as a tool to understand light.

WAVE PARAMETERS Wavelength () is the length from crest to crest or trough to trough. Frequency (f) is the number of waves passing a point per second. Higher Frequency  Shorter Wavelength Lower Frequency  Longer Wavelength Speed of a wave (v) = Wavelength x Frequency

Light as a WAVE As a wave, Light can be described as having a Frequency Wavelength Speed Wave Speed = Wavelength x Frequency

What does a light wave look like? To answer this, must first know how light is produced. Start with an electron  has an electric field (example is static cling; or battery)has an electric field Also a moving electron produces a magnetic field.

Accelerate an electron and both electric and magnetic fields are produced  called an electromagnetic wave

The Nature of Light Light is an electromagnetic wave that travels at the speed of light Light is an electromagnetic wave that travels at the speed of light The speed of light is related to the wave’s frequency and wavelength The speed of light is related to the wave’s frequency and wavelength c = f = 300,000 km/sec (or 186,000 mi/sec)

As a wave, light may have any wavelength (and frequency), but its speed remains constant. As a wave, light may have any wavelength (and frequency), but its speed remains constant. An electromagnetic wave with a long wavelength has a low frequency and one with a short wavelength has a high frequency. An electromagnetic wave with a long wavelength has a low frequency and one with a short wavelength has a high frequency. Visible light is an electromagnetic wave Visible light is an electromagnetic wave that has wavelengths between 400 and 700 nanometers! (That is, 300 billionths to 700 billionths of a meter!)

When you have your teeth x-rayed, the dentist is shining an electromagnetic wave with a wavelength more than 1,000 times shorter than visible light When you listen to the radio, your radio is picking up electromagnetic waves almost a million times longer than visible light, or about 0.1 meters. The range wavelengths (or frequencies) of electromagnetic waves is called the electromagnetic spectrum.

Courtesy Lawrence Berkeley National Laboratory

The Spectrometer

Why does each element have its own fingerprint? Interaction between light and atoms… What are atoms?

Elementary Particles: Mass and Charge ParticleMassCharge Proton 1.67 x 10 -27 Kg +1.6 x 10 - 19 Coulomb Neutron 1.67 x 10 -27 Kg Neutral Electron 9.11 x 10 -31 Kg -1.6 x 10 -19 Coulomb

Build a Model of the Hydrogen Atom Proton Electron

Bohr Model of the Atom and Energy Levels

Bohr Model of the Hydrogen Atom 1.Electrons Orbit a Proton 2.Electrons exist only in discrete orbits and nowhere in between 3.When electrons absorb a photon of the correct energy, the electron jumps to a higher energy level 4.When the electron falls to a lower energy level, a photon of only one certain energy is emitted The emitted photons have a discrete energy, frequency or wavelength that we observe in the spectrum

Sir William Herschel More to Light than Meets the eye More to Light than Meets the eye Discovery of Infra-red (1800) Discovery of Infra-red (1800) (Also discovered Uranus in 1787) (Also discovered Uranus in 1787)

Herschel Experiment

Herschel Exp 14.6 o C 13.4 o C IR MOVIE

Spectra Continuous Spectrum Continuous Spectrum Thermal Radiation Thermal Radiation (black body radiation)(black body radiation) Atomic collisions result excitation of atoms and in emission of energy at multiple frequencies.Atomic collisions result excitation of atoms and in emission of energy at multiple frequencies. Emission Spectrum Emission Spectrum Excitation of electrons Excitation of electrons High energy levelHigh energy level Return to ground state produces emission of energy Return to ground state produces emission of energy Energy produced proportional to dropEnergy produced proportional to drop

Atomic and Molecular Mechanisms

Interpretation of Spectra

Atomic and Molecular Mechanisms

Formation of 21cm Radio waves (1420 MHz)

Milky Way Galaxy as Seen in Different Parts of the Spectrum

Multi-spectral Astronomy

w w w. p a r i. e d u

Infrared Movie…