Light and Quantized Energy Chp 5

Light, a type of electromagnetic radiation, travels through space as a wave.

All EMR travels through space with the same speed. c=3.0x108 m/s Hz or s-1 m

Guided Practice Calculating Wavlength of an EM Wave 1. Microwaves are used to cook food and transmit information. What is the wavelength of a microwave that has a frequency of 3.44x109 Hz? Independent Practice 2. X-rays can penetrate body tissues and are widely used to diagnose and treat disorders of internal body structures. What is the frequency of an X-ray with a wavelength of 1.15x10-10m? 3. After careful analysis, an EMR wave is found to have a frequency of 7.8x108Hz. What is the speed of the wave?

Guided Practice Calculating Wavlength of an EM Wave 1. Microwaves are used to cook food and transmit information. What is the wavelength of a microwave that has a frequency of 3.44x109 Hz? Independent Practice 2. X-rays can penetrate body tissues and are widely used to diagnose and treat disorders of internal body structures. What is the frequency of an X-ray with a wavelength of 1.15x10-10m? 3. After careful analysis, an EMR wave is found to have a frequency of 7.8x108Hz. What is the speed of the wave?

The wave-like nature of light explains most of its properties: Waves reflect and refract Waves produce an interference pattern Motion produces the Doppler Effect on waves https://www.youtube.com/watch?v=9UkkKM1IkKg https://www.youtube.com/watch?v=y5tKC3nEx2I

Light as only a wave fails to adequately describe all of light's interactions with matter.

Max Planck Observation: heated matter emits and absorbs light at only certain frequencies (colors) Conclusion: matter can gain or lose energy only in small specific amounts called quanta. A quantum is the minimum amount of energy that can be gained or lost by atom.

Planck proposed that the energy emitted by hot objects was quantized. Equanta = hv For a given frequency, matter can emit or absorb energy only in whole number multiples-like 1hv, 2hv, 3hv, etc... Equanta= hc /λ Planck's Constant h=6.626x10-34 J*s

Photoelectric effect https://phet.colorado.edu/en/simulation/legacy/photoelectric Observation: electrons (photoelectrons) are emitted from a metal's surface only when light of a certain frequency or higher than a certain frequency, shines on the surface. Photoelectrons should have been emitted at any frequency. Given enough time, light (traveling as a wave) at low frequencies and energies would accumulate and supply enough energy to emit photoelectrons.

1905-Albert Einstein Conclusion: Light has a dual nature=wavelike and particlelike properties. Light is like a beam of bundles of energy called photons. Photon are massless particles that carry a quantum of energy. Extending Planck's idea of quantized energy, Einstein calculated that a photon's energy depends on its frequency. Einstein won the Nobel Prize in 1921 for this work. Planck won for quanta in 1919.  Ephoton= hv A photon has Energy while being massless

Example Problem 2 Every object gets its color by reflecting a certain portion of incident light. The color is determined by the wavlength of the reflected photons, thus by their energy. What is the energy of a photon from the violet portion of the Suns's light if it has a frequency of 7.230x1014 s-1? 5. Calculate the energy possessed by a single photon of each of the following types of EMR. 6.32x1020 s-1 b. 9.50x1013Hz c. 1.05x1016 s-1

Example Problem 2 Every object gets its color by reflecting a certain portion of incident light. The color is determined by the wavlength of the reflected photons, thus by their energy. What is the energy of a photon from the violet portion of the Suns's light if it has a frequency of 7.230x1014 s-1? (4.791 x 10-19 J) 5. Calculate the energy possessed by a single photon of each of the following types of EMR. a. 6.32x1020 s-1 (4.19x10-13 J) b. 9.50x1013Hz (6.29x10-20 J) c. 1.05x1016 s-1 (6.96x10-18 J)

Niels Bohr Observation: Hydrogen gas emits a unique emission spectra of frequencies (colors) All elements have unique emission lines and these can be used to identify elements. http://chemistry.bd.psu.edu/jircitano/periodic4.html

Conclusion: The Hydrogen atom has only certain Energy states where electrons orbit the nucleus The lowest state, closest to the nucleus is called the ground state. When an electron gains Energy it changes to an excited state, further from the nucleus. Energy is absorbed by the electron- jumps to the excited state. Electron immediately falls back to lower Energy state and emits a photon equal to the energy difference between two levels. http://science.sbcc.edu/physics/flash/siliconsolarcell/bohratom.swf

Energy levels are not evenly spaced. Balmer series=land in 2nd energy level Paschen series=land in 3rd energy level, can't see infrared radiation Lyman series=land in 1st energy level, can't see ultraviolet