Understanding Electromagnetic Waves: Spectrum, Energy, Radiation Pressure, and Doppler Eff, Study notes of Physics

Download Understanding Electromagnetic Waves: Spectrum, Energy, Radiation Pressure, and Doppler Eff and more Study notes Physics in PDF only on Docsity! 10/24/05 U. Wisconsin, Physics 104, Fall 2005 1 The EM Spectrum • Note the overlap between types of waves • Visible light is a small portion of the spectrum • Types are distinguished by frequency or wavelength 10/24/05 U. Wisconsin, Physics 104, Fall 2005 2 The Spectrum of EM Waves • Forms of electromagnetic waves exist that are distinguished by their frequencies and wavelengths c = ƒλ • Wavelengths for visible light range from 400 nm to 700 nm • There is no sharp division between one kind of em wave and the next 10/24/05 U. Wisconsin, Physics 104, Fall 2005 5 Determining Radiation Pressure • This is an apparatus for measuring radiation pressure • In practice, the system is contained in a vacuum • The pressure is determined by the angle at which equilibrium occurs 10/24/05 U. Wisconsin, Physics 104, Fall 2005 6 Question? If the Black disk is replaced with another which has larger diameter, which of the following is true? 1. Radiation pressure on the bigger disk is higher 2. Momentum transferred to the bigger disk is higher The radiation pressure does not change because pressure is force per unit area. Larger disk absorbs more radiation, resulting in a larger force and momentum transfer per unit time. 10/24/05 U. Wisconsin, Physics 104, Fall 2005 7 Doppler Effect and EM Waves • A Doppler Effect occurs for em waves, but differs from that of sound waves – For sound waves, motion relative to a medium is most important • For light waves, the medium plays no role since the light waves do not require a medium for propagation – The speed of sound depends on its frame of reference • The speed of em waves is the same in all coordinate systems that are at rest or moving with a constant velocity with respect to each other 10/24/05 U. Wisconsin, Physics 104, Fall 2005 10 Red shift • If a star is moving away from us, the signature “spectrum” of light from that star will be shifted to lower frequencies - the Red Shift. • All astronomical objects are found to be retreating from each other - the Universe is expanding. • Extrapolating back in time, the Universe must have begun from a single point in space and time - the Big Bang. 10/24/05 U. Wisconsin, Physics 104, Fall 2005 11 Preflight 1 • If beam 1 is the incoming beam in the figure below, which of the other four beams are reflected and which are refracted? – 2 and 5 are reflected + 3 and 4 are refracted – 2 and 4 are reflected + 3 and 5 are refracted – 2, 4 and 5 are reflected + 3 is refracted – 2 is reflected + 3, 4, and 5 are refracted 0 5 10 15 20 25 30 35 40 Score (%) A B C D 10/24/05 U. Wisconsin, Physics 104, Fall 2005 12 Light incident on an object • Absorbed Everything true for wavelengths << object size • Reflects (bounces) – See it – Mirrors • Refraction (bends) – Lenses 10/24/05 U. Wisconsin, Physics 104, Fall 2005 15 Flat Mirror Lines appear to intersect a distance d behind mirror. This is the image location. Virtual: No light actually gets here d d Draw first ray perpendicular to mirror 0 = θi = θf Draw second ray at angle. θi = θf Virtual image since light rays don’t really converge. All rays originating from peak will appear to converge at same point behind mirror! θi θf 10/24/05 U. Wisconsin, Physics 104, Fall 2005 16 Plane Mirror Summary Virtual: No light actually gets here d d • Image appears: – Upright (Inverted) – Same size (magnified) – Located same distance from, but behind, mirror (image distance) – Facing opposite direction: Left/Right reversed – Virtual image: (real image) 10/24/05 U. Wisconsin, Physics 104, Fall 2005 17 Left-Right Reversal • Why is the word 'AMBULANCE' written backwards on the front hood of many ambulances? – Mirror reflection is upright • Several preflight responses said that reflection is upside-down or inverted - but all reflected images are erect. – However, mirror reflection reverses left-right – Drivers looking in their rear-view mirrors see the reflected word, ‘AMBULANCE’, properly. 10/24/05 U. Wisconsin, Physics 104, Fall 2005 20 1 Two Mirrors: Question? Is the central image (#3) 1. Upside down and left-right reversed 2. Upside down and not left-right reversed 3. Erect and not left-right reversed 2 3 10/24/05 U. Wisconsin, Physics 104, Fall 2005 21 v = c n Speed of light in medium Index of refraction Speed of light in vacuum n > 1 v < c so Index of Refraction 186,000 miles/second: it’s not just a good idea, it’s the law! 10/24/05 U. Wisconsin, Physics 104, Fall 2005 22 Snell’s Law n1 n2 When light travels from one medium to another the speed changes v=c/n, but the frequency is constant. So the light bends: n1 sin(θ1)= n2 sin(θ2) θ1 θ2 1) n1 > n2 2) n1 = n2 3) n1 < n2 θ1 < θ2 sinθ1 < sinθ2 n1 > n2 Preflight 4: Which is true? 57% 7% 36% 0% 20% 40% 60% 10/24/05 U. Wisconsin, Physics 104, Fall 2005 25 actual fish d n2 n1 ! d = d n 2 n 1 apparent fish d′ eye Apparent Depth 10/24/05 U. Wisconsin, Physics 104, Fall 2005 26 Preflight 3 • Which of the following statements about the angles shown in the figure is true? – θ1 < θ3 – θ1 = θ3 – θ1 > θ3 0 10 20 30 40 50 60 70 80 Score (%) A B C