The Nature of Light

1. The Nature of Light

2. Guiding Questions • How fast does light travel? How can this speed be measured? • Why do we think light is a wave? What kind of wave is it? • 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?

3. 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

4. Light: spectrum and color • Newton found that the white light from the Sun is composed of light of different color, or spectrum (1670).

5. Light has wavelike property • Young’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

6. Light is Electromagnetic Radiation • 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.

7. 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

8. Electromagnetic Spectrum • Visible light falls in the 400 to 700 nm range • In the order of decreasing wavelength • Radio waves: 1 m • Microwave: 1 mm • Infrared radiation: 1 μm • Visible light: 500 nm • Ultraviolet radiation: 100 nm • X-rays: 1 nm • Gamma rays: 10-3 nm

9. Dual properties of Light: (1) waves and (2) particles • Light is an electromagnetic radiation wave • Light is also a particle-like packet of energy - photon • Light particle is called a photon • The energy of a photon is related to the frequency of light • The higher the frequency the more energy the photon has. • E = hf • E- energy, J; h- Planck’s constant, 6.626 X 10-34 J*s; f – frequency, Hz • Light has a dual personality; it behaves as a stream of particles like photons, but each photon has wavelike properties

10. Dual properties of Light: Planck’s Law • Planck’s law relates the energy of a photon to its wavelength or frequency • E = energy of a photon • h = Planck’s constant = 6.626 x 10–34 J s • c = speed of light • λ= wavelength of light • Energy of photon is inversely proportional to the wavelength of light • Example: 633-nm red-light photon • E = 3.14 x 10–19 J

11. Structure of Atom • 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 • An 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

12. Bohr’s Model of Atom • 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 Hydrogen

13. 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

14. Atomic Emission Spectra or Spectral Lines • Bright spectrum lines can be seen when a chemical substance is heated and valoprized (Kirchhoff, ~1850)

15. Each chemical element has its own unique set of spectral lines.