Light. Light. Overview. In-class activity. What are waves? In this section: PSC 203. What is it? Your thoughts?

Transcription

1 Light PSC 203 Overview In this section: What is light? What is the EM Spectrum? How is light created? What can we learn from light? In-class activity Discuss your answers in groups of 2 Think of as many examples light sources that you can think of. List as many as you can on your paper. Hold onto this list until the end of lecture for the rest of the in-class assignment. What is it? Your thoughts? Light What is light? light is an electro-magnetic wave electric and magnetic fields oscillate carries energy as it travels What are waves? If light is a wave we need to understand what a wave is. 1

2 Waves all waves have three related properties speed: how fast the wave moves wavelength: the distance between peaks frequency: how often a peak passes by Speed the speed of light is very fast c = 300,000,000 m/s = 3 x 10 8 m/s c = 671,000,000 mi/hr Wavelength the distance between peaks You can see the wavelength of an ocean wave You can t see the wavelength of light but it works the same way Frequency how many peaks pass a fixed point during a set amount of time most commonly measured in Hz (Hertz) 1 Hz = 1 wave peak per second Visible Light We don t see the waves in light because they are very small Different wavelengths correspond to different colors We can see the colors if we use a tool In-class experiment 2

3 Visible Light the small portion of the light that our eyes can perceive Spectrum light has a range of energies, wavelengths, and frequencies the range is called the spectrum When we break light into its wavelengths, we see its spectrum Types of Spectra When we break light into its spectrum it doesn t always look the same It depends on how the light was generated Real Spectra Non-visible light The portion our eyes see is just a small part of the whole range Spectrum is divided into named bands What are they? 3

4 Spectrum Wavelength vs frequency Wavelength and frequency are related Speed = wavelength*frequency c c = λ f Frequency increases -> > wavelength decreases Frequency decreases -> > wavelength increases Energy vs frequency As frequency increases, so does the energy (takes more energy to create high frequencies) High frequency = high energy Low frequency = low energy Wavelength is opposite High energy = short wavelength Low energy = long wavelength Energy Span?? Radio waves have the lowest energy the frequency is very low (10 7 Hz) the wavelength is very large (30 m) Gamma rays have the highest energy the frequency is very high (10 18 Hz) the wavelength is very small (3 x m) Which of the following travels slowest through space? a) Radio waves. b) Visible light. c) X-rays. d) Infrared photons. e) They all travel at the same speed. 4

5 ?? A photon of which color of light carries the most energy? a. Red. b. Yellow. c. Green. d. Blue. e. All photons have the same energy. Appearance Objects look different depending on what type of light is used Need special tools to see different types of light We can get new information by looking at different types of light Object in various light Infrared Shows heat levels IR goes through dust and smoke X-rays Emitted only by high energy processes Can pass through lower density materials Ultraviolet Emitted by medium-high energy sources Flowers show highlighted portions near the pollen when viewed in UV (bees see UV light too) Old bruises on human skin can sometimes be seen in UV but not visible 5

6 Radio Emitted by cold hydrogen Can pass through solid objects (like walls) Types of Spectra Continuous, emission, absorption It depends on how the light was generated So how is light generated? Emission the release of light from any material light is a form of energy... energy has to be transformed into light from some other form sources are categorized by the original energy Incandescence light created from thermal energy object radiates its heat as light Light is radiated at all wavelengths Spectrum provides information about the temperature Temperature vs brightness Total amount of light generated depends on temperature Hotter object -> > more light Cooler object -> > less light Temperature vs color As an object heats, it appears to change color from red to white to blue Peak wavelength depends on Temperature T = (3 x 10 6 K nm)/λ m 6

7 Incandescent examples People radiate light because they have thermal energy Peak wavelength is in infrared Brightness is fairly low Incandescent examples Electric stoves radiate light because they have thermal energy Peak wavelength is in red orange yellow region of the visible spectrum Brightness is moderate Incandescent examples Blow torches radiate light because they have thermal energy Peak wavelength is in blue region of the visible spectrum Brightness is high Other processes Not all light is generated by heat Atomic Emission light created from atomic energy atom releases energy as light Light is radiated at specific wavelengths Spectrum provides information on the atoms 7

8 Atomic Excitation atoms have energy levels for each electron normally electrons in lowest levels possible can excite electrons into higher levels to move back into lower levels, need to release energy Emission spectra when a gas is excited it releases light at specific wavelengths, depending on the type of gas Hydrogen Iron Atomic examples Neon lamps High voltage electricity passes through gas Lasers Fluorescent materials Absorption spectra when a cold gas is in front of a continuous source only specific wavelength are absorbed same wavelength produced in emission spectra 8

9 Real Spectra Non-sources of light Some light we see appears to come from one source but it is really from another source Reflections or scattering Moon light Not generated by the moon Just reflected sunlight In-class activity Using the light source lists you created earlier Discuss as a group which are incandescent, atomic, non-sources 9