Bellwork Envision the way the water molecules on a lake’s surface behave as a water wave travels across the lake. What does it look like? What happens when the water wave reaches the lake’s shore?

Sections 1-2

History Continues  After discovering the 3 subatomic particles scientists continued to search for a better understanding of atomic structure and the arrangement of electrons within atoms.  We already knew:  Rutherford had figured out that the nucleus had a positive charge, virtually all of an atom’s mass was in the nucleus and that fast-moving electrons surround the nucleus.  What we didn’t know:  How the electrons were arranged  Why the electrons were not pulled into the nucleus by the protons.  Rutherford’s nuclear atomic model didn’t account for similarities or differences in chemical behavior among the various elements

Scientists Unravel the Mystery  Scientists saw that certain elements emitted visible light when heated in a flame.  Further analysis of the light showed that an elements chemical behavior was related to the arrangement of the electrons in its atoms  Decided that Light had a wave-like behavior

The Wave-Nature of Light  Visible light is a type of electromagnetic radiation.  Electromagnetic radiation- a form of energy that exhibits wave-like behavior as it travels through space. (Examples beyond visible light: microwaves, x-rays, radio waves, etc.)

Electromagnetic Spectrum

Characteristics of Waves  Label: origin, crest, trough

Characteristics of Waves Cont.  Wavelength ( )-shortest distance between two equal points on a continuous wave (crest to crest, or trough to trough)  Usually measured in meters, cm, or nm  Short wavelength means moving really fast  Frequency ( )- tells how fast a wave is going  The number of waves that pass a given point/sec  1 Hertz (Hz) is the SI base unit for frequency  36 Hz = 36 wave/sec = 36/sec = 36s -1

Characteristics of Waves Cont.  Amplitude ( )-wave’s height from origin to crest or origin to trough.  It is the power of the object  Higher the amplitude, the brigher the light  Intensity (energy) of a wave is related to its amplitude  Wavelength and frequency do not affect the amplitude of a wave.  Diffraction  Wave experiences an interference when striking the edge of an object.  Particles continue in straight lines and collide with objects

Characteristic of all Waves  All electromagnetic waves travel at the speed of light (c), in a vacuum.  C = 3.00x10 8 m/s  The speed of light is the product of frequency and wavelength. c = ____ ____  Different waves will have different frequencies and wavelengths.  As frequency increases, wavelength decreases  Inversely related

Go to page 138  Look at Figure 5.3 and answer the following 3 questions:  Count the number of wavelengths shown in each of the two waves. How many are there?  How does the wavelength of the higher frequency wave compare to that of the lower frequency wave?  How does the frequency of the higher frequency wave compare with that of the lower frequency wave?

Example Problem  The red colored light in a fireworks display might be produced when strontium salts are heated. What is the frequency of such red light with a wavelength of 6.50 x m? Answer: 4.2 x s -1

 Sunlight has a nearly continuous range of wavelengths and frequencies due to its ‘white light’  When white light passes through a prism it is separated into a continuous spectrum of its components

 A rainbow allows you to see all the visible colors at once  Rainbows form when tiny drops of water in the air disperse the white light from the sun into its component colors.

Visible Light Spectrum  A small fraction of the entire electromagnetic spectrum  Differences between the types are due to their frequencies and wavelengths  Energy increases with increasing frequency  Violet has a higher frequency than red which means its energy is higher

Electromagnetic Radiation surrounds us  Sunlight, human activities (radio, tv, cell phones, light bulbs, medical x-ray equipment), lightening, and even glowing fireflies contribute to the EM radiation around us  Our view of the universe is based on the electromagnetic radiation emitted by distant objects and detected with instruments here on Earth

Homework:  Worksheet page 7. Use the diagram on page 139 for guidance.  Practice Problems 1-4 on page 140  Extra Practice 1-2 on page 978

Particle Nature of Light  Light as a wave explains much of its everyday behavior, but if fails to adequately describe aspects of light’s interactions with matter.  Can’t explain why heated objects emit only certain frequencies of light at a given temperature OR why some metals emit electrons when light of a specific frequency shines on them.  When objects are heated, they emit glowing light. As it gets hotter it possesses more energy and emits different colors of light.  Different colors correspond to different frequencies and wavelengths  WAVE MODEL COULD NOT EXPLAIN THESE DIFFERENT WAVELENGTHS

Max Plank  He went searching for an explanation for why different wavelengths occurred with different amounts of energy  His Conclusions: matter can gain or lose energy only in small, specific amounts called quanta  Quantum – the minimum amount of energy that can be gained or lost by an atom  Some scientists felt the idea was revolutionary…others found it to be disturbing

Originally scientists thought…  Energy could be absorbed and emitted in continually varying quantities with no minimum limit to the amount  Take a microwave oven for example…it appears as if you change the temperature of water by regulating the power and duration of the microwaves BUT really the temperature is increasing in small steps  Individual molecules absorb quanta of energy, and because the steps are so small…the temperature appears to be rising continuously

Planck Proposed:  The energy emitted by hot objects was quantized  A relationship exists between the energy of a quantum and the frequency of the emitted radiation.

Energy of a Quantum  E quantum = h v  E quantum = energy  h = Planck’s constant, x J-S  (J) Joule is the SI unit for energy  v = frequency of emitted radiation  As Energy increases, frequency increases

Plank’s theory  For a given frequency, matter can emit or absorb energy only in whole-number multiples of h v (1h v, 2h v, etc.) –why is this?  Matter can only have certain amounts of energy  Analogy: Its like a child building a block tower. It can only be built up or taken apart 1 whole block at a time.

Photoelectric Effect  Electrons are emitted from a metal’s surface when light of a certain frequency or higher than a certain frequency, shines on the surface.  Occurs when light of a certain frequency strikes a metal surface and ejects electrons  As the intensity of light increases, so does the number of ejected electrons.  Wave model predicts that given enough time, low Energy, low frequency light would accumulate and supply enough energy to eject photoelectrons from a metal-the reality is that metal won’t eject photoelectrons below a specific frequency.

Light’s Dual Nature  Albert Einstein proposed that light had a dual nature  Light has both wave and particle-like properties  Can be thought of as a beam of bundles of energy called photons.  Photons are mass-less particles that carry a quantum of energy  How much energy a photon has depends on its frequency.  E photon = hv  Energy of a photon must have a certain threshold value to cause ejection of a photo electron from the surface of the metal.  Won the Nobel prize for his work.

Homework:  Page 143: 5-7  Page 145: 8,  Page 978: 3-4

 How is light produced in the glowing tubes of a neon sign?  Pass electricity through a tube filled with neon gas, neon atoms in the tube absorb energy and become excited, excited atoms release that energy in order to become stable again in the form of light.

Atomic Emission Spectra  It’s the set of frequencies of the electromagnetic waves emitted by atoms of the element  Usually consists of several individual lines of color corresponding to the frequencies of the radiation emitted by the atoms.

Characteristics of Atomic Emission Spectra  Different elements give off a different color of gas  Each element emits its own unique spectrum  Each line tells you specific information about the element.  Can identify the element by its spectrum  If only certain colors appear in an element’s atomic emission spectrum that means only certain frequencies of light are emitted  Elements absorb the same frequencies that they emit  Absorbed frequencies appear black  By comparing the black lines to the emission spectrum of the elements, scientists can determine the composition of the outer layers of the stars.