Wave & Electromagnetic Spectrum Notes

Transcription

1 Wave & Electromagnetic Spectrum Notes December 17, 2011 I.) Properties of Waves A) Wave: A periodic disturbance in a solid, liquid or gas as energy is transmitted through a medium ( Waves carry energy b/c they do work ) *Waves create wave fronts (circles) that have the same amount of total energy but as the circles get larger the energy spreads out over a larger area. 1) Mechanical Waves: waves that require a medium through which to travel a) Medium: physical environment in which phenomena occur 1) Example: water, air, Earth b) Almost all waves are mechanical waves, except EM waves 2) Electromagnetic Waves: waves that consist of oscillating electric and magnetic fields, which radiate outward at the speed of light (3.0 x 10 8 m/s) (Holy Gazoli! That s fast!) a) Example: visible light, microwaves, etc. b) DO NOT require a medium Wave Vocabulary A) Crest: section of the wave that rises above the equilibrium position B) Trough: section which lies below the equilibrium position C) Wavelength: distance between one point on the wave and the nearest point where the wave repeats itself 1) This can be measured from crest-tocrest, trough-to-trough, or from the start of a wave to the start of the next one. 2) Symbol for wavelength is λ 1

2 D) Amplitude: maximum displacement from the undisturbed position of the medium to the top of a crest. E) Period (T): time required for one wave to complete one full cycle of its motion i.e. The period measures how long it takes for a wave to pass by 1) Example: From its highest point to its lowest and back again 2) Measured in seconds (s) F) Frequency: the number of complete waves that pass a point in a given time. It has the symbol f. 1) SI Units: Hertz (Hz) 2) Hertz units measure the number of vibrations per second ex. 1 vibration per second is 1 Hz, 2 vibrations per second is 2 Hz 3) Period is related to frequency in that the frequency is the inverse of the period G) Wave speed 1) The frequency and wavelength are related to one another and to the speed of the wave by the following formula, where v is equal to the speed: Speed = Wavelength x Frequency v = λf II.) Traveling Through Mediums A) Mechanical waves require a medium in order to travel through 1) Waves are classified by the direction that they travel through the medium a) Transverse Wave: the particles in the medium move perpendicular to the direction wave is traveling 1) Ex: Light waves b) Longitudinal Wave: the particles in the medium move parallel to the direction the wave is traveling 1) Ex: Sound waves 2

3 Light as electromagnetic radiation The wave particle duality of light: Particle-like properties: photoelectric effect, Compton-effect. Wave-like properties: diffraction, polarisation, interference. Light wave λ = wavelength E = amplitude of electric field M = amplitude of magnetic field E B Propagation of electromagnetic waves x x Magnetic field strength vector Electronic field strength vector Period time (time) wavelength (space) distance Transversal wave Electronic and magnetic field vectors are perpendicular to each other, and to the directon of propagation also! Propagation of electromagnetic waves Laws of light propagation (geometric optics) The direction of light propagation is the direction of the light ray. The light ray is the parallel light beam limited to the smallest radius. Basic rules of geometric optics: Light propagates linearly. On bounderies of different media reflects refracts. Light rays are considered to be independent. Light rays are invertible (reversible). Fermat-principle: principle of minimal time / path. Reflection Refraction Can be regular (specular), diffuse (irregular) and mixed. In case of regular reflection: The incident ray, the normal and the reflected ray are included in the same plain, and the angle of incidence is eqal to the angle of reflection. On the boundary of different media, part of the light passes from one medium into another, while the direction of propagation alters (bends). The reason is the different propagation velocity of light in different media. Snell s law: 3

4 Refraction on prisms, light dispersion Total internal refraction When the light enters a less optically dense medium, it is refracted away from the normal. At a critical angle, the light is refracted along the interface of the media. At an angle greater than the critical engle, there is total internal reflection. Diffraction The direction of propagation of rays bends, when they propagate through a slit or when they collide. Phase delay occurs. Interference patterns can be seen. Interference Occurs when waves meet, and they are superponated. If they meet in the same phase, they amplify each other, in opposite phases they attenuate each other. Coherence of light waves is the most important condition of interference. max. amplification max. attenuation Optical grid Interference Interference 4

5 Polarization Polarization Polarized light: Vibration plain of electric field vectors are similar to all waves. Can be generated by the help of polarization (polar filters, dichroism), or reflection (optical grid). II.) Electromagnetic Spectrum A) The Waves of the Spectrum 1) EM spectrum: all of the frequencies or wavelengths of electromagnetic radiation 2) The EM spectrum includes radio waves, microwaves infrared rays, visible light, UV rays, X-rays, and gamma rays. E = hν c = λν 5

6 B) Radio Waves -Longest wavelength -Lowest Frequency 1) Radio waves are used in radio & TV technologies, as well as in radar. a) Radio 1) Amplitude modulation (AM) 2) Frequency modulation (FM) b) Radar 1) Send out short bursts of radio waves that bounce off objects & return to receiver C) Microwaves -Shorter wavelength than radio waves -Higher frequency than radio, but lower than infrared 1) Only penetrate food at the surface D) Infrared Waves -Shorter wavelength than microwaves -Higher frequency than microwaves, but lower than red light 1) Infrared rays are used as a source of heat & to discover areas of heat difference 2) You can t see infrared radiation, but your skin feels its warmth E) Visible Light -Each wavelength corresponds to a specific frequency ** Color determined by frequency 1) People use visible light to see, help keep them safe, and to communicate with one another. Spectrum of visible light F) UV Rays -Shorter wavelength and higher frequency than violet light 1) UV rays have applications in health and medicine, and in agriculture. a) Helps skin produce Vit. D, can cause sunburn, cancer, and used to kill microorganisms 400nm 500nm 600nm 700nm 6

7 G) X-Rays -Shorter wavelength and higher frequency than UV rays 1) X-rays are used in medicine, industry, and transportation to make pictures of the inside of solid objects. a) X-rays are absorbed by solid objects (teeth, bones) & so solid objects appear white H) Gamma Rays -Shortest wavelength -Highest frequency 1) Gamma rays are used in the medical field to kill cancer cells and to make pictures of the brain & in industrial situations as an inspection tool. 7