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Guiding Questions1. How fast does light travel? How can this speed be
measured?2. Why do we think light is a wave? What kind of
wave is it?3. How is the light from an ordinary light bulb
different from the light emitted by a neon sign?4. What is a photon? How does an understanding of
photons help explain why ultraviolet light causes sunburns?
5. How can astronomers tell what distant celestial objects are made of?
6.What are atoms made of?7.How does the structure of atoms explain what
kind of light those atoms can emit or absorb?
Speed of Light
The speed of light in the vacuum
C = 299,792.458 km/s, or C = 3.00 X 105 km/s C = 3.00 X 108 m/s
Light: spectrum and color• Newton found that the white light from the Sun is composed
of light of different color, or spectrum (1670).
Light has wavelike propertyYoung’s Double-Slit Experiment indicated light
behaved as a wave (1801)The alternating black and bright bands appearing
on the screen is analogous to the water waves that pass through a barrier with two openings
The nature of light is electromagnetic radiation In the 1860s, James Clerk Maxwell succeeded in describing all
the basic properties of electricity and magnetism in four equations: the Maxwell equations of electromagnetism.
Maxwell showed that electric and magnetic fields should travel space perpendicular to each other.
Light is Electromagnetic Radiation
Light: Wavelength and Frequency
• Example– FM radio, e.g., 103.5 MHz (radio station) => λ = 2.90 m– Visible light, e.g., red 700 nm => f = 4.29 X 1014 Hz
Electromagnetic SpectrumVisible light falls in the 400 to
700 nm rangeIn the order of decreasing
wavelength Radio waves: 1 mMicrowave: 1 mmInfrared radiation: 1 μmVisible light: 500 nmUltraviolet radiation: 100 nmX-rays: 1 nmGamma rays: 10-3 nm
Dual properties of Light: (1) waves and (2) particlesLight is an electromagnetic radiation waveLight is also a particle-like packet of energy - photon
Light particle is called a photonThe energy of a photon is related to the frequency of lightThe higher the frequency the more energy the photon has.
E = hf E- energy, J; h- Planck’s constant, 6.626 X 10-34-34 J*s; f – frequency, J*s; f – frequency,
HzHzLight has a dual personality; it behaves as a stream of
particles like photons, but each photon has wavelike properties
Planck’s law relates the energy of a photon to its wavelength or frequencyE = energy of a photonh = Planck’s constant
= 6.626 x 10–34 J sc = speed of lightλ= wavelength of light
Energy of photon is inversely proportional to the wavelength of light
Example: 633-nm red-light photonE = 3.14 x 10–19 J
Dual properties of Light: Planck’s Law
Structure of AtomAn atom consists of a small, dense nucleus at the center,
surrounded by electrons which orbit the nucleus.The nucleus contains more than 99% of the mass of an atom,
but concentrates in an extremely small volume
• A nucleus contains two types of particles: protons and neutrons
• A proton has a positive electric change, equal and opposite to that of an electron.
• A neutron, about the same mass of a proton, has no electric charge.
• An atom has no net electric charge
Electrons occupy only certain orbits or energy levels
When an electron jumps from one orbit to another, it emits or absorbs a photon of appropriate energy.
The energy of the photon equals the difference in energy between the two orbits.
Bohr’s Model of Atom
Bohr’s Model of Hydrogen
Bohr’s Model of Atom • Absorption is produced when electron absorbs incoming
photon and jumps from a lower orbit to a higher orbit
• Emission is produced when electron jumps from a higher orbit to a lower orbit and emits a photon of the same energy
Atomic Emission Spectra or Spectral Lines
• Bright spectrum lines can be seen when a chemical substance is heated and valoprized (Kirchhoff, ~1850)
Each chemical element has its own unique set of spectral lines.
