How Things Work - Fall 2018

Optics questions for review Exam 2 on 11/12/18 1.  Review the concept of reflection, particularly the law of reflection.  Draw what happens when a light ray hits a mirror at various angles. 2.  Review the concept of refraction:  what it is, what causes it, what happens during it, under what circumstances does light bend, etc.  Draw what happens when a light ray hits a block of transparent plastic at various angles.   3.  Show how to calculate the wavelength of WTMD's signal (89.7 MHz). 4.  This problem has to do with the Doppler Effect. a.     Write out a sentence that explains the Doppler Effect. b.     If an ambulance is driving toward you with its siren on (frequency equals 1000 Hz), what would be true about the frequency YOU hear (compared to the one that is emitted by the ambulance)? c.     If an ambulance has driven past you with its siren on (frequency ...

Basics of how things fly; Bernoulli Effect   Energy Waves - wavelength - frequency - speed - amplitude - crests and troughs wave speed = frequency x wavelength (Note that the wave speed is the speed of light when you are talking about electromagnetic waves.) mechanical vs. electromagnetic waves music - octaves (doubling the frequency); the next note on the piano (1.0594) Doppler effect - red shift, blue shift electromagnetic spectrum - radio, micro, IR, visible (ROYGBV), UV, X, gamma light reflection light refraction lenses and mirrors (convex and concave) focal length/point predicting light paths (when light is reflected or refracted)

Lenses As shown and discussed in class, light refracts TOWARD a normal line (dotted line on the left image, perpendicular to surface of lens) when entering a more dense medium. Note in this  convex  lens that this direction of bend changes from down (with the top ray) to up with the bottom ray. This is due to the geometry of the lens. Look at the picture to make sure that this makes sense.  As a result, the rays will intersect after leaving the lens.  An image can form! The FOCAL LENGTH (f) of a lens (or curved mirror) where the light rays would intersect, but ONLY IF THEY WERE INITIALLY PARALLEL to each other. Otherwise, they intersect at some other point, or maybe not at all (if the object is too close to be focused on)! Note that your (human) eye lenses are convex - slightly thicker in the middle.  Thus, your eyes form "real" images on the retina - upside-down!  Unless, of course, the object is too close. If an image is proj...

Refraction : Consider a wave hitting a new medium - one in which is travels more slowly. This would be like light going from air into water. The light has a certain frequency (which is unchangeable, since its set by whatever atomic process causes it to be emitted). The wavelength has a certain amount set by the equation, c = f l, where l is the wavelength (Greek symbol, lambda). When the wave enters the new medium it is slowed - the speed becomes lower, but the frequency is fixed. Therefore, the wavelength becomes smaller (in a more dense medium). Note also that the wave becomes "bent." Look at the image above: in order for the wave front to stay together, part of the wave front is slowed before the remaining part of it hits the surface. This necessarily results in a bend. MORE DETAIL: The general rule - if a wave is going from a lower density medium to one of higher density, the wave is refracted TOWARD the normal (perpendicular to surfac...

Reflection - light "bouncing" off a reflective surface. This obeys a simple law, the law of reflection! The incident (incoming) angle equals the reflected angle. Angles are generally measured with respect to a "normal" line (line perpendicular to the surface). https://ricktu288.github.io/ray-optics/simulator/ Note that this works for curved mirrors as well, though we must think of a the surface as a series of flat surfaces - in this way, we can see that the light can reflect in a different direction, depending on where it hits the surface of the curved mirror. So - light reflects from mirrors, according to the law of reflection.  However, if the mirrors is curved, light still obeys this rule - it just looks a bit different.  You have to visualize the curved mirror as a series of little flat mirrors. A convex mirror (top) acts reflects light rays "outward" - the light rays  seem  as though they are coming from inside the conv...

Recall that waves can be categorized into two major divisions: Mechanical waves, which require a medium. These include sound, water and waves on a (guitar, etc.) string Electromagnetic waves, which travel best where there is NO medium (vacuum), though they can typically travel through a medium as well. All electromagnetic waves can be represented on a chart, usually going from low frequency (radio waves) to high frequency (gamma rays). This translates to: long wavelength to short wavelength. All of these EM waves travel at the same speed in a vacuum: the speed of light (c). The standard wave velocity equation is still: But for light, where c is the speed of light (3 x 10^8 m/s ): c = f  l Still, f is frequency (in Hz) and  l  is wavelength (in m).  Note that, depending on your browser, the lambda (Greek symbol for wavelength) may show as an l. (We will see during the next class that the speed of light, while constant i...

Doppler! The Doppler Effect First, some animation: http://www.lon-capa.org/~mmp/applist/doppler/d.htm http://falstad.com/ripple/ So above, the blue dot is emitting sound and moving to the right.  Since it is moving AND emitting sound at the same time, the waves are getting closer on the right - resulting in a shorter wavelength (or higher frequency).  And it is the complete opposite on the left. http://falstad.com/ripple/ Play with this and choose the "Doppler Effect 1" example. And for some more visuals: https://highered.mheducation.com/olcweb/cgi/pluginpop.cgi?it=swf::800::600::/sites/dl/free/0072482621/78778/Doppler_Nav.swf::Doppler%20Shift%20Interactive The key in the Doppler effect is that motion makes the "detected" or "perceived" frequencies higher or lower.   It's worth noting that the effect also works in reverse. If you (the detector) move toward a sound-emitter, you'll detect a higher frequency. If you...