Unit 3 – Electron ConfigurationsPart A – Electromagnetic Waves Dr. Chin Chu River Dell Regional High School
What had we learned so far? • Atomic Structure – • Nucleus • Electrons • Essential question: how are those electrons surrounding nucleus arranged?
Experimental Evidence Discharging Tubes The light coming out of the excited atomic entities is very specific to particular element! Results are quite reproducible. The Flame Test
Experimental Evidence There has been no radioactive decay going on. Hence the nucleus does not change when the atomic entity gets excited either by electricity or heat. So the colored light must have come from those electrons. Light emitted from excited atomic entities is the tool used to probe how electrons are arranged.
What is light? Lights, both visible and invisible to human eyes, are electromagnetic waves. Time Out! Before we go any further, what is a Wave?
What is a wave? A wave is a means to transfer energy from point A to point B. Waves in water Sound waves Typical mechanical waves such as those in water and sound waves DO need medium in which they propagate. Water and air are the prerequisites for waves to travel.
Waves – in more the abstract form Wavelength - distance from crest to crest abbreviated Greek letter, l, pronounced “lambda”. Also can be defined as how far the wave travels in a cycle. Note: great link to an online simulation of waves. http://phet.colorado.edu/sims/wave-on-a-string/wave-on-a-string_en.html -
Waves – in more the abstract form • Frequency – the number of complete waves passing any given point per second. • SI unit for frequency is Hertz (Hz), or cycles/sec. • Abbreviated Greek letter, n, pronounced “nu”. The graph shows that the top wave passes any given point 4 complete wave forms every second; the middle one 2 complete wave forms; and the bottom one 1 complete wave form.
Waves – in more the abstract form Frequency – the number of complete waves passing any given point per second. Wavelength - defined as how far the wave travels in a cycle. Wavelength x Frequency = how far the wave travels in a second (speed of the wave) s = ln s: speed of wave l: wavelength n: frequency
Exercise I Pause and complete the following exercise before proceeding. What is the frequency of a wave in water where the speed of the wave is 3.4m/s and the wavelength is 0.5 m? What is the speed of the sound wave where the wavelength is 4.5 m and the frequency is 36 kHz. (1kHz = 103 Hz) What is the wavelength of a sound wave that travels at 2300 m/s and at a frequency of 150 Hz?
Exercise I Answers to the Questions. What is the frequency of a wave in water where the speed of the wave is 3.4m/s and the wavelength is 0.5 m? [6.8 Hz] What is the speed of the sound wave where the wavelength is 4.5 m and the frequency is 36 kHz. (1kHz = 103 Hz) [1.62 x 105 m/s] What is the wavelength of a sound wave that travels at 2300 m/s and at a frequency of 150 Hz? [15.3 m]
Electromagnetic Waves (Lights) Disturbance in a magnetic field is perpendicular to a disturbance in an electric field. • Can travel in vacuum. No need for medium! • Travels at 3 x1010 cm/second (or 3.00 x 108 m/s)in vacuum. Known as the “Speed of Light”, which does not vary with frequency nor wavelength • Varying in wavelength and frequency.
Electromagnetic Waves (Lights) Since the speed of light in vacuum does not change with frequency nor wavelength, frequency and wavelength are inversely proportional. c = ln c: speed of light (3.00 x 108 m/s) l: wavelength n: frequency • For an electromagnetic wave, frequency goes up, then wavelength has to come down proportionally and vies versa.
Electromagnetic Waves (Lights) Now you have all the parameters and two relationships (see boxes below). Then you should be able to solve problems related to lights. Keep in mind how to manipulate parameters and variables. Good luck! c = ln c: speed of light (3.00 x 108 m/s) l: wavelength n: frequency E = hn c: energy of the photon h: Planck’s constant, 6.626 x 10-34 J.s n: frequency
Exercise II – Electromagnetic Waves Pause and complete the following exercise before proceeding. What is the frequency of an electromagnetic wave with wavelength of 3.4m? What is the wavelength of the light where the frequency is 36 MHz. (1MHz = 106 Hz) What is the energy of a photon that has a frequency of 4.23 x 107 Hz? What is the wavelength of a photon that carries 2.56 eV energy? (1 eV=1.602 x 10-19 J)
Exercise II – Electromagnetic Waves Answers What is the frequency of an electromagnetic wave with wavelength of 3.4m? [8.82 x 107 m/s] What is the wavelength of the light where the frequency is 36 MHz. (1MHz = 106 Hz) [8.33 m] What is the energy of a photon that has a frequency of 4.23 x 107 Hz? [2.80 x 10-26 J] What is the wavelength of a photon that carries 2.56 eV energy? (1 eV=1.602 x 10-19 J) [4.85 x 10-7 m]
Electromagnetic Waves (Radiations) Examples: • radio waves • microwaves • infrared • white light (visible spectrum) • ultraviolet light • X-rays • gamma radiation
High Energy Low Energy Low Frequency High Frequency X-Rays Radiowaves Microwaves Ultra-violet GammaRays Infrared . Long Wavelength Short Wavelength Visible Light
---------------- > decreasing energy --------------------- ----------------> decreasing frequency ----------------> ---------------> increasing wavelength ---------------->
Types of Spectra • Continuous– all wavelengths within a given range are included. • Electromagnetic– all electromagnetic radiation arranged according to increasing or decreasing wavelength. • unit for wavelength ranges from meters to nanometers • unit for frequency is hertz (Hz) (# waves per second)
Types of Spectra • Visible spectrum - light you can see (ROY-G-BIV) • Red has the longest wavelength and the smallest frequency. • Violet has the shortest wavelength and the greatest frequency. • Bright Line spectrum (emission spectrum) • Bands of colored light emitted by excited electrons when they return to the ground state.
Passing Light Through a Prism • White light is made up of all the colors of the visible spectrum. • Passing it through a prism separates the colors in white light.
If the light is not white, • By heating a gas with electricity we can get it to give off colors. • Passing this light through a prism does something different.
If the light is not white, • Each element gives off its own characteristic colors. • Can be used to identify the atom. • This is how we know what stars are made of.
Spectroscopy • Emission spectraof a substance is studied to determine its identity. • Spectroscope– instrument that separates light into a spectrum. • Spectral lines – represent wavelength of light emitted when excited electrons fall back to the ground state.