Transcript Slide 1
Electromagnetic Radiation Topics • All about light; • Kirchhoff’s and Wien’s laws; • Doppler effect; • Demo. Motivation Learn about light. Learn about light as a tool. 1 What is Light? Light is a form of energy transport. A beam of light can be decomposed to individual components called photons. All light, no matter how much energy per photon, travels at the same speed. c=3×108 m/s An individual photon can be described as having a frequency (ν) and a wavelength (λ). These are related to each other by c=λν 2 Electromagnetic Spectrum Visible light • A prism produces a spectrum; • Visible colors (red, orange, yellow, green, blue, and violet) in roughly equal proportions gives white light. • Black is the lack of color. • Visible photons range in λ from 400 nm (0.004mm) to 700 nm (0.007 mm). Electromagnetic Spectrum • Radio • Microwave • Infrared • Visible • Ultraviolet • X-Ray • Gamma Ray 3 Light and Matter Emission An object creates and emits light (e.g., light bulb filament). Absorption When an object absorbs light, transmitting the energy to the internal energy of the object. Transmission Light simply passes through an object. Reflection/scattering Light bouncing off matter in the same general direction is reflection; when the bouncing is in random directions it is called scattering. 4 Kirchhoff’s Laws Kirchhoff’s laws summarize how the three types of spectra are produced. Without explaining the underlying physics… Continuum spectrum A hot, dense glowing object (a solid or dense gas) emits a continuous spectrum. Emission spectrum A hot, low-density gas emits light of only certain wavelengths - a bright line spectrum. Absorption spectrum When light having a continuous spectrum passes through a cool gas, dark lines appear in the continuous spectrum. 5 Wien’s Law An intensity/wavelength graph, a thermal spectrum, of an object emitting electromagnetic radiation can be used to determine its temperature. Therefore, the color of a star tells us about its surface temperature. A quantitative derivation is given by Wien’s Law: lmax= 2,900,000/T or T = 2,900,000/lmax where T is the temperature in Kelvin and lm is the wavelength where the thermal spectrum peaks in intensity in nanometers (nm). 6 Power Output How much thermal energy is being emitted (per square meter) from an object with at temperature T? E/m2= σT4 Where σ = Stefan-Boltzmann constant. While energy emitted by a sphere is: E=4πR2 σT4 7 The Doppler Effect: More Wave Behavior l l0 vr l0 l0 c l λo is the wavelength emitted by the object. λ is the wavelength we observe. vr is the radial speed of the emitting object. c is the speed of light, 300,000 km/s. vt vr v To Earth 8 Roll-up on waves!!!! What we can learn from light. • General conditions (Kirchhoff’s Laws); • Temperature (Wien’s Law); • Speed-overall (Doppler); • Speed-rotation (Doppler); • Speed-surface activity (Doppler); • Composition (spectral lines); • Density (excitations); • Magnetic fields (spectral lines splitting); • + more… 9