GENERAL PHYSICS III GENERAL PHYSICS III Optics & Quantum Physics What What does we learn in Gen. Phys. III? does we learn in Gen. Phys. III? Many physical phenomena of great practical interest to engineers, chemists, biologists, physicists, etc. were not in Gen. Phys. I & II  Wave phenomena of light:  Interference: what happens when two or more waves overlap? (Light passing through two slits give such kind of picture) Interference!  Quantum Physics: The development of experimental equiment and techniques  modern physics can go inside the microscopic world (atoms, electrons, nucleus, etc.)  New principles, new laws for the microscopic (subatomic) world were discoverved • Diffraction: The image of an object is not exact in fine details. For example, the image of a circular disk is diffused Interference & diffraction can be analyzed if we regard light as a wave  The basis concept: wave  particle duality Examples: light wave  photon electron  electron wave etc… • Subatomic objects obey new mechanics: quantum mechanics On the basis of quatum mechanics we study structure and properties of atoms, nucleus, solids, laser rays, etc… Chapter XVII Chapter XVII Interference of light Interference of light §1. Interference of coherent sources of light §2. Interference in thin films §3. Interferometer §1. Interference of light from coherent sources: We consider an overlap of light that comes from two sources. A remarkable phenomen takes place, if two sources satisfy some following conditions: The sources are monochromatic. It means that they emit light of a single color. A monochromatic light corresponds to a sinusoidal electromagnetic wave with a single frequency f and wave length  Two sources have the same frequency f (the same wave length ) Two souces are permanently in phase , or , at least, have any definite constant phase difference Then, two sources are called coherent sources. 1.1 Coherent sources of light:  Recall the formula for a sinusoidal e-m wave:         2 , cos cos 2 cos speed amplitude 2 wavelength wavenumber or wavevector frequency 2 angular frequency y x t A x vt A kx ft A kx t v A k f f                               Notes: Common sources of light do not emit monochromatic light (single-frequency light) → However one can produce approximately monochromatic light: • by using filters which block all but a very narrow range of wave length • by using light from a laser 1.2 Interference 1.2 Interference of light through narrow of light through narrow slits: slits: Monochromatic light source at a great distance, or a laser. Slit pattern Observation screen Young’s experiment on doubledouble slit interferenceslit interference (Thomas Young performed in 1800)(Thomas Young performed in 1800) Light (wavelength is incident on a two-slit (two narrow, rectangular openings) apparatus:  If either one of the slits is closed, a diffuse image of the other slit will appear on the screen. (The image will be “diffuse” due to diffraction. We will discuss this effect in more detail later.) Monochromatic light (wavelength ) S 1 S 2 screen Diffraction profile I 1  If both slits are now open, we see interference “fringes” (light and dark bands), corresponding to constructive and destructive interference of the electric-field amplitudes from both slits. I S 1 S 2 Dark fringes Light fringes  Important quantity: path difference = r 2 - r 1 The light density at the location of observer depends on the path difference  S 1 S 2 Observer Light d  r 1 r 2 A path difference corresponds to a phase difference of two waves at the observer’s point  . GENERAL PHYSICS III GENERAL PHYSICS III Optics & Quantum Physics What What does we learn in Gen. Phys. III? does. such kind of picture) Interference!  Quantum Physics: The development of experimental equiment and techniques  modern physics can go inside the microscopic