Module 1-1 Continued

1. Module 1-1 Continued Nature and Properties of Light

2. Basic Concepts Section 5 Absorption and scattering of light • When a light beam passes through an optical medium, some of the light is reflected at the two interfaces, some is absorbed inside the medium, some is scattered outside of the beam while in the medium, and what is left is transmitted. • See Figure 1-15

3. Basic Concepts Section 5 • Figure 1.15 Losses in light energy passing through an optical medium

4. Basic Concepts Section 5 • The intersection of light with matter is best understood by treating light made up of photons—more like wave packets. • The details of the interaction involve atoms, energy levels in atoms, and photons of electromagnetic energy. • Recall: All matter is made up of atoms. An atom is the smallest unit that retains the characteristics of a chemical element.

5. Basic Concepts Section 5 • Atoms consists of a positive nucleus surrounded by negative electrons arranged in distinct energy shells designated K through O, as shown in Figure 1-16. • The notation K(2) indicates that the K-shell is complete when it has 2 electrons. Similarly, L(8) indicates that 8 electrons complete the L-shell, and M(18) indicates that 18 electrons complete the M-shell.

6. Basic Concepts Section 5 • Figure 1.16 Atomic model

7. Basic Concepts Section 5 • When all the electrons are in an unexcited, orground state, the atom is assumed to be at its lowest energy level. • When the atom absorbs energy, electrons can be “excited” and moved into higher-energy shells.

8. Basic Concepts Section 5 • As electrons move from one shell to another, unique amounts, or quanta, of energy are absorbed or emitted. • A photon is such a quantum of energy. • This is how an atom can absorb or emit light.

9. Basic Concepts Section 5 • An atomic energy-level diagram shows the unique electron energies available in a given atom. • An energy-level diagram for hydrogen is shown in Figure 1-17. • The lowest level, E1, is the ground state. • Energy must be added to the atom for the electron to move to a higher level.

10. Basic Concepts Section 5 • Note that energy levels range from a value of −13.6eV (1eV = electron volts = 1.6×10−19 J) for the lowest energy level (n = 1) • to a value of 0eVfor the very highest energy level (n = ∞) • (n = ∞) is when the electron breaks free from the atom.

11. Basic Concepts Section 5 • Figure 1-17

12. Basic Concepts Section 5 • Suppose a hydrogen atom is in an excited state, say, the n = 3 level. • The atom can make a transition to the ground state by emitting a photon. • The energy of the emitted photon equals the decrease in energy of the atom, as illustrated on the next slide.

13. Basic Concepts Section 5 • Ephoton = E3 − E1 • Ephoton = − 1.51eV − (−13.6eV ) • Ephoton = 12.09eV

14. Basic Concepts Section 5 • The atom can also absorb photons. This happens when the energy of a photon exactly matches the difference between two electron energy levels. • For example, a hydrogen atom in the ground state can absorb a photon whose energy is 12.09eV. The electron in the atom will then move from energy level E1 to energy level E3.

15. Basic Concepts Section 5 Spectra of light sources • Usually sources are divided into two categories, natural and man-made. • Examples of natural sources of radiation include the sun, observable stars, radio stars, lightning, and, in fact, any living body. • Some of the man-made sources of radiation are incandescent and fluorescent lights, heaters, lasers, masers, radio and television antennas, radars, and X-ray tubes.

16. Basic Concepts Section 5 • Two types of spectra are important in photonics: emission and absorption spectra. • An emission spectrum is formed by light emitted from a source. • An absorption spectrum is formed when light that passes through an optical medium is partially absorbed.

17. Basic Concepts Section 5 • All materials with temperatures above absolute zero emit electromagnetic radiation. • Every atom and molecule has its own characteristic set of spectral lines. • The line spectra observed early in the scientific age led to a significant understanding of the structure of atoms and eventually to the development of modern quantum theory. • This theory holds that light emitted by an atom or molecule has a discrete wavelength, corresponding to a specific energy-level change within the atom or molecule.

18. Basic Concepts Section 5 • To observe a line or band spectrum, light is passed through a slit. • The image of this slit is then refracted by a prism or diffracted by a grating and recorded on film or a spectrograph. • Spectroscopy is the science that analyzes line spectra and identifies the different chemical elements that make up the material.

19. Basic Concepts Section 5 Emission Spectra • Figure 1-18 represents the typical emission spectrum of a monatomic gas, such as helium or neon. • This type of spectrum is produced by an electrical discharge passed through a gas sample contained at low pressure.

20. Basic Concepts Section 5 • Figure 1.18 Line spectrum of a monatomic element

21. Basic Concepts Section 5 • Source: Institute of Astronomy, University of Hawaii http://www.ifa.hawaii.edu/~barnes/ASTR110L_F05/spectralab.html

22. Basic Concepts Section 5 Absorption Spectra • Absorption is the transfer of energy from an electromagnetic wave to the atoms or molecules in a material. • Energy transferred to an atom, for example, excites electrons to higher energy states.

23. Basic Concepts Section 5 • The wavelength/intensity spectrum of light that passes through the material appears to have certain wavelengths removed (those of the absorbed light) and is called the absorption spectrum. • Figure 1-19 represents a typical absorption spectrum for a solid material. Unlike the comparatively narrow spikes of a gaseous spectrum seen in Figure 1-18, this spectrum consists of broad, irregularly spiked regions called absorption bands.

24. Basic Concepts Section 5 • Figure 1.19 Absorption spectrum of a solid