Light and Matter Chapter 5 PDF

5.1 Light Unable to reach far stars, too distant. Fast spaceship takes 100,000 years to reach. Study star's light for information. Produced by individual atoms A forms of light travels at the same speed (c) ○ C=3.0x10^8 m/s Light Energy Joule (J) = Energy Watt (...

5.1 Light Unable to reach far stars, too distant. Fast spaceship takes 100,000 years to reach. Study star's light for information. Produced by individual atoms A forms of light travels at the same speed (c) ○ C=3.0x10^8 m/s Light Energy Joule (J) = Energy Watt (W) = power = energy/time Energy measured in Joules 1 Watt = 1J/s Light and Color White light combines all colors, black is absence of light. Light particle has specific color, colors mixed in display. Interaction of Light and Matter Emission - “Hot” object release energy in the form of light Absorption - Object absorbs light. Gains energy Transmission - light passes through an object, no or limited absorption occurs  Reflection/Scattering - light bounces off an object. Reflection Light can bounce of a surface Appearance of light depends on the surface Smooth Surfaces For a smooth surface, incoming angle equals outgoing angle (Law of Reflection) Refraction Index of Refraction (n) = Ratio of speed of light in vacuum / Speed of light in material Light bending through materials. Speed change. Total Internal Reflection All light reflected at critical angle on surface. Prisms Light bent in prism, disperses into colors violet and red. Rainbows Light reflects inside of raindrops James Clerk Maxwell Created 4 equations depicting properties of particles, showing unified force of electricity and magnetism. Acts between charged particles. Oscillating charges produce electric and magnetic fields, forming electromagnetic radiation. “Seeing” When you see something, light strikes the object and is reflected back into your eye. The light waves stimulate nerve endings in the eye that then send a signal to your brain. 5.2 Light as a Wave Light travels as a wave, no medium required. Sound needs air, water waves need water. Aether theory debunked. Wavelength and Frequency Wavelength (λ) is the distance between wave crests, measured in meters (m) or nanometers (nm). Frequency (f) is the number of waves passing per second, measured in Hertz (Hz) or waves/sec (s-1). Heinrich Hertz named frequency, first to generate and detect radio waves in 1887. Color Each color we see is a different frequency (f) and wavelength (λ) of light. Wave equation ○ Speed = Wavelength x frequency ○ C=λf Calculate Photon Energy The energy of the photon emitted is proportional to the frequency of its light ○ E=hf E = Energy in Joules f = frequency (in Hz) h = Planck's Constant = 6.626 x 10 -34 J s Calculate the energy of a red photon of wavelength 630 nm (First find Frequency) Light as a Particle Sometimes light appears to behave like a particle (Called a photon) ○ Example: Reflection, light bounces off a surface like a ball would ○ Shadows have clean edges ○ Light can carry momentum and push an object. Wave- Particle Duality Light has wave and particle like properties. ○ Wave-particle duality Matter can also behave like a wave, especially electrons in an atom ○ Studied in quantum mechanics Propagation of Light Light waves spread out in all directions growing dimmer as you get further away from the light source Intensity measured in Watts/Meter2 or W/m2 For a spherical wave front ○ Intensity = Power of light source / Area = Power / 4 π R2 Electromagnetic Spectrum All possible frequencies (colors) of light. From longest wavelengths (smallest frequencies) to shortest (highest frequency) ○ Radio waves, Microwaves, Infrared, Visible Light, Ultraviolet, X-rays, and gamma rays Visible Light 400 nm (violet) to 740 nm (red) Can be observed with our eye ball. Easily passes through atmosphere ○ Infrared Discovered by William Herschel (Uranus) We sense these as heat. Absorbed by Carbon dioxide and water vapor in atmosphere ○ Ultraviolet (Beyond Violet) 1801, John Ritter, German Used prism to expose silver chloride (changes color when exposed to light) Saw region beyond the violet range still exposed the silver chloride Some insects and birds can “See” it ○ X-rays 1895, Wilhelm Roentgen (German) Used a “Crookes tube” Showed when operating it caused phosphorous nearby to glow. Covered tube with cardboard and phosphorous still glowed. Can penetrate soft tissue but not hard bone or metal ○ Gamma Rays 1900, Paul Villard (French) Third form of radiation discovered from radioactive materials Alpha - 1st type (Helium nuclei) Beta - 2nd type (high energy electrons) Gamma - 3 type (High energy photons) Not deflected by a magnet as are the other two. Mostly pass right through you ○ Radio Waves First discovered by Heinrich Hertz Connected a high voltage to a spark gap and induction coil Found that a secondary spark gap across the room will get a similar spark across the gap. Called them Hertzian Waves  Saw no practical purpose for them. Atmospheric Absorption FM and TV bands not absorbed by atmosphere ○ FM - Frequency modulation ○ TV (VHF - very high frequency, UHF - ultra-high frequency) AM band absorbed by ionosphere (Upper atmosphere) ○ AM - Amplitude modulation Microwaves Predicted by Maxwell, discovered by Hertz Easily absorbed by water Percy Spencer working at Raytheon on microwave radar dishes noticed his chocolate bar melted when he stood next to one. Radiation and Temperature All objects are in constant motion (Kinetic Theory of Matter) Temperature is a measure of the average kinetic energy of all the particles in a substance Higher temperature = greater motion of particles Solids vibrate in place Liquids have enough energy to move relative to each other Gases have enough energy to break free from interaction amongst particles Blackbody Radiation All objects emit a continuous spectrum of colors. Peak radiation emitted depends on temperature (Wien’s Law) Wien’s Law Peak wavelength is related to temperature ○ b = 3.0 x 106 ○ Wavelength in nanometers ○ T in Kelvin Can use to measure temperature of stars 1nm = 1x10^-9m Stefan-Boltzmann’s Law Power output of a star is related to its surface temperature and size Luminosity (L) - Light output of a star σ = 5.67 x 10-8 W / m2 K4 5.3 Properties of Matter Atoms Smallest pieces of stable matter Everything is made of atoms (~118 different ones) Element - Contains only 1 type of atoms Atomic Structure Atoms made of 3 particles ○ Proton - positive charge, in nucleus ○ Neutron - neutral charge, in nucleus ○ Electron - negative charge, orbits nucleus, roughly 1/2000th the mass of neutron or proton Atomic and Mass Number Atomic Number - the number of protons, gives position on Periodic Table  Mass Number - the number of protons plus number of neutrons For a neutral atom (no net charge) the number of protons equals the number of electrons Isotopes Atoms with the same number of protons but different numbers of neutrons. Molecules Two or more atoms bonded together in set ratios Water - 2 Hydrogen, 1 Oxygen Carbon Dioxide - 1 carbon, 2 Oxygen Phases of Matter Solid - particles tightly packed, set volume and shape, low energy Liquid - particles held together loosely, set volume and shape of container, higher energy than solid Gas - little attraction between particles, takes volume and shape of container, high enough energy to fly freely Plasma - like a gas but higher energy, enough to rip electrons away from atoms Bose-Einstein Condensate - Very cold, all atoms vibrate like a single large atom. Phase Changes Solid → Liquid → Gas → Plasma ○ Takes energy (Endothermic) Plasma → Gas → Liquid → Solid ○ Loses energy to surroundings (Exothermic) Molecular Dissociation If molecule given enough energy, it will break down into its atomic components. It may break into charged ions (atoms with extra charge) or individual atoms Phases and Pressure Temperature usually causes a phase change Pressure can as well ○ Gas under extreme pressure will turn into a liquid or solid 5.4 Learning From Light Spectroscope Splits light into a rainbow of colors Invented in 1859 by Gustav Kirchhoff and Robert Bunsen Continuous Spectrum Light from any object will emit a continuous spectrum colors based on temperature. Absorption Spectrum As light passes through any material, some of the colors will be absorbed by the material Leaves dark bands in the spectrum Solar Spectrum (right) Emission Spectrum A low pressure gas will emit specific color bands of light The fingerprint is unique to each atom. Rutherford Model Placed positive charge in the center of atom Suggested electrons orbit like planets around the sun Could not explain why electrons don’t just spiral into the nucleus (opposite charges attract) Bohr Model (1913) Niels Bohr (Danish) Proposed that electrons must have discrete orbits with definite energies. The difference in energy between the levels is a “Quantum” of energy Electron can only be on these steps of the ladder but not in between. As an electron jumps from one rung of the ladder to another, it either absorbs or emits a single  quantum of light (photon) Explained Balmer calculations E=hf H= F= Electron Can move between energy levels when it gains or loses a precise amount of energy. Energy corresponds to colors in the light. Different color carries different amounts of energy. Ground State - Lowest possible energy for an electron in an atom Ionization Ionization Energy - If enough energy given to atom, electron will be completely ripped from atom and an ion will form Removing more electrons takes even more energy ○ Second ionization Energy > First ionization energy Often see ionization in gas around young, very hot stars Ionized atoms have different sets of spectral lines. Object in Motion - Doppler Effect When an object moves, it compresses the waves released in the direction of motion In the opposite direction waves are stretched Called the Doppler Effect (Christian Doppler, 1842) Color Shift Color of light appears to shift if object moving toward (Blue shift) or away from (Red shift) observer Calculation A stationary source produces a red light at 650 nm. If the object is moving towards an observer a shift in the wavelength of 0.42 nm is observed. How fast is the object moving? A star produces the same red light. Its Doppler shift is observed to be 4.2 nm less than the original color. How fast is the star moving relative to the Earth?