L i g h t !

1. Light!

2. Light is a Wave? Objectives: Describe the relationship between the wavelength and frequency of light. Calculate the wavelength and frequency of light using light equations.

3. Light • The study of light led to the development of the quantum mechanical model • Light is a type of electromagnetic radiation (energy) • Electromagnetic radiation includes many types: gamma rays, x-rays, ultraviolet light, visible light, infrared, microwaves, and radio waves • Speed of light = 2.998 x 108 m/s; abbreviated “c” • All electromagnetic radiation travels at this same rate when measured in a vacuum

4. The Electromagnetic Spectrum • Visible light is a very small portion of the entire spectrum

6. Waves • Electromagnetic radiation travels in waves (analogous to water waves) • Wavelength = distance from peak to peak • Amplitude = height of the peak (distance from axis to crest or trough) • Frequency = the number of wave peaks that pass in a given time; usually measured per second (1/s or s-1 or Hertz (Hz)) • Speed = rate the waves travel

7. Wavelength and Frequency • Are inversely related • As one goes up the other goes down • Different frequencies of visible light are different colors • There is a wide variety of frequencies • The whole range is called a spectrum

8. Wavelength and Frequency • Electromagnetic radiation travels through space as a wave moving at the speed of light Equation: c = c = speed of light, a constant (2.998 x 108 m/s) (lambda) = wavelength, in meters n (nu) = frequency, in units of hertz (Hz or s-1)

9. Light Problem Solving • Calculate the wavelength of yellow light emitted by a sodium lamp if the frequency of radiation is 5.10 x 1014 Hz. • How far (in kilometers) does starlight travel in 10. minutes? • During a flame test, strontium emits a strong band at 663 nm. What frequency is this light? What is its color? 5-Step Problem Solving/Dimensional Analysis

10. Light is a Particle? Objectives: Describe the relationship between frequency and energy of light. Calculate wavelength, frequency, and/or energy using light equations.

11. Light is a particle? • Energy is quantized. • Light is a form of energy. • Therefore, light must be quantized. • These smallest pieces of light are called photons- particles of electromagnetic energy

12. Light is a particle? • The energy of electromagnetic radiation is directly related to the frequency of the radiation. Equation: E = h E = Energy, units of Joules (kg.m2/s2) h(Planck’s constant) = 6.626x10-34J.s n (nu) = frequency, in units of hertz (Hz or s-1)

13. More Light Problem Solving • What is the wavelength of blue light with a frequency of 8.3 x 1015 Hz? • What is the frequency of red light with a wavelength of 4.2 x 10-5 m? • What is the energy of a photon of each of the above? 5-Step Problem Solving/SF’s/Dimensional Analysis

14. Spectra Objectives: • How do continuous emission spectra and atomic emission spectra differ? • Identify the source of atomic emission spectra. • Explain how the frequencies of emitted light are related to changes in electron energies.

15. Continuous Spectra & Light Continuous Spectra • Sunlight consists of light with a continuous range of wavelengths and frequencies • When sunlight passes through a prism, the different frequencies separate into a continuous spectrum of colors. • Each color blends into the next in order red, orange, yellow, green, blue, and violet (ROYGBV)

16. Light • The color of light depends on its energy, frequency, and wavelength. • Red has the longest wavelength, the lowest frequency, and the lowest energy

17. Light LONG  LOW  LOW E Dec.  Inc.  Inc. E SHORT  HIGH  HIGH E

18. White Light If 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

19. Atomic Spectra • These are called atomic emission spectra • They are unique to each element, like a fingerprint • Very useful for identifying elements

20. Atomic Emission So how does light relate to electrons??? • When atoms absorb energy, electrons move into higher energy levels. These electrons then lose energy by emitting light when they return to lower energy levels. • The light emitted by an electron moving from a higher to lower energy level has a frequency directly related to the energy change of the electron. • Each electron “jump” produces a distinct brightly colored line in the spectrum.

21. Atomic Emission • This spectrum is called an atomic emission spectrum and each element’s spectrum is unique

22. Atomic Emission • Absorption of Energy • Atom absorbs energy from some outside source (e.g. electricity, heat, etc.) • Electron has too much energy to stay in the energy level it is in • Electron moves from lower EL (GROUND STATE) to higher EL (EXCITED STATE)

23. Atomic Emission • Emission of Energy • Electron in an excited state is now UNSTABLE • It loses its excess energy and moves back to a lower energy level • Each “leap” is equal to a an exact frequency

24. Atomic Emission

25. Atomic Emission • The greater the energy transition (“leap”), • the higher the energy released, • the higher the frequency, • the shorter the wavelength • Color in the “visible region” is dependent on this transition. n 6 5 4 3 2 Energy H Violet Teal Red 410nm 486nm 656nm 434nm