# River Dell Regional High School Unit 3 – Electron Configurations Part A – Electromagnetic Waves.

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River Dell Regional High School Unit 3 – Electron Configurations Part A – Electromagnetic Waves

What had we learned so far? Atomic Structure – Nucleus Electrons Essential question: how are those electrons surrounding nucleus arranged?

Experimental Evidence Discharging Tubes The Flame Test The light coming out of the excited atomic entities is very specific to particular element! Results are quite reproducible.

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 Note: great link to an online simulation of waves. http://phet.colorado.edu/sims/wave-on-a-string/wave-on-a-string_en.html - Wavelength - distance from crest to crest abbreviated Greek letter,  pronounced “lambda”. Also can be defined as how far the wave travels in a cycle.

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,  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 Wavelength - defined as how far the wave travels in a cycle. Frequency – the number of complete waves passing any given point per second. Wavelength x Frequency = how far the wave travels in a second (speed of the wave) s = s: speed of wave : wavelength : frequency

Exercise I 1.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? 2.What is the speed of the sound wave where the wavelength is 4.5 m and the frequency is 36 kHz. (1kHz = 10 3 Hz) 3.What is the wavelength of a sound wave that travels at 2300 m/s and at a frequency of 150 Hz? Pause and complete the following exercise before proceeding. s = s: speed of wave : wavelength : frequency

Exercise I 1.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] 2.What is the speed of the sound wave where the wavelength is 4.5 m and the frequency is 36 kHz. (1kHz = 10 3 Hz) [1.62 x 10 5 m/s] 3.What is the wavelength of a sound wave that travels at 2300 m/s and at a frequency of 150 Hz? [15.3 m] Answers to the Questions. s = s: speed of wave : wavelength : frequency

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 x10 10 cm/second (or 3.00 x 10 8 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) c = c: speed of light (3.00 x 10 8 m/s) : wavelength : frequency Since the speed of light in vacuum does not change with frequency nor wavelength, frequency and wavelength are inversely proportional. For an electromagnetic wave, frequency goes up, then wavelength has to come down proportionally and vies versa.

Electromagnetic Waves (Lights) E = h E: energy of the photon h: Planck’s constant, 6.626 x 10 -34 J.s : frequency 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 = c: speed of light (3.00 x 10 8 m/s) : wavelength : frequency http://www.colorado.edu/physics/2000/quantumz one/photoelectric2.html

Exercise II – Electromagnetic Waves 1.What is the frequency of an electromagnetic wave with wavelength of 3.4m? 2.What is the wavelength of the light where the frequency is 36 MHz. (1MHz = 10 6 Hz) 3.What is the energy of a photon that has a frequency of 4.23 x 10 7 Hz? 4.What is the wavelength of a photon that carries 2.56 eV energy? (1 eV=1.602 x 10 -19 J) c = c: speed of light (3.00 x 10 8 m/s) : wavelength : frequency E = h E: energy of the photon h: Planck’s constant, 6.626 x 10 -34 J.s : frequency

Exercise II – Electromagnetic Waves 1.What is the frequency of an electromagnetic wave with wavelength of 3.4m? [8.82 x 10 7 m/s] 2.What is the wavelength of the light where the frequency is 36 MHz. (1MHz = 10 6 Hz) [8.33 m] 3.What is the energy of a photon that has a frequency of 4.23 x 10 7 Hz? [2.80 x 10 -26 J] 4.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] Answers

Exercise II – Electromagnetic Waves 2. What is the energy of a photon that has a frequency of 4.23 x 10 7 Hz? 3.What is the wavelength of a photon that carries 2.56 eV energy? (1 eV=1.602 x 10 -19 J) c = c: speed of light (3.00 x 10 8 m/s) : wavelength : frequency E = h E: energy of the photon h: Planck’s constant, 6.626 x 10 -34 J.s : frequency

Electromagnetic Waves (Radiations) Examples: radio waves microwaves infrared white light (visible spectrum) ultraviolet light X-rays gamma radiation

Radio waves Micro waves Infrared. Ultra- violet X- Rays Gamma Rays Low Energy High Energy Low Frequency High Frequency Long Wavelength Short Wavelength Visible Light

---------------- > decreasing energy ---------------------  ----------------> decreasing frequency ----------------> ---------------> increasing wavelength ---------------->

Diagram Showing Wavelength and Frequency

Types of Spectra Continuous – all wavelengths within a given range are included. Electromagnetic – all electromagnetic radiation arranged according to increasing or decreasing wavelength. a. unit for wavelength ranges from meters to nanometers b. unit for frequency is hertz (Hz) (# waves per second)

Types of Spectra Visible spectrum - light you can see (ROY-G-BIV) a. Red has the longest wavelength and the smallest frequency. b. 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 1. Emission spectra of a substance is studied to determine its identity. 2. Spectroscope – instrument that separates light into a spectrum. 3. Spectral lines – represent wavelength of light emitted when excited electrons fall back to the ground state.

How Does a Spectroscope Work?

Emission Spectrum (Line Spectrum)

Emission Spectrum

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