THE+NATURE+OF+EM+WAVES+AND+LIGHT+SOURCES

Back to IB PHYSICS > EM WAVES OPTION =THE NATURE OF EM WAVES AND LIGHT SOURCES= media type="custom" key="25285374" Outline the nature of electromagnetic (EM) waves. ELECTROMAGNETIC WAVE: A wave which consists of sinusoidally varying electric and magnetic fields which induce each other. It travels at 3 x 10 8 ms -1.
 * WAVES AND SOURCES || OPTICAL INSTRUMENTS || TWO-SOURCE INTERFERENCE || DIFFRACTION GRATING || X-RAYS || THIN-FILM INTERFERENCE ||

[|Walter Fendt animation] [|Cabrillo animation] Students should know that an oscillating electric charge produces varying electric and magnetic fields. Students should know that electromagnetic waves are transverse waves and all have the same speed in a vacuum.

Describe the different regions of the electromagnetic spectrum. Students should know the order of magnitude of the frequencies and wavelengths of different regions, and should also be able to identify a source for each region

EM SPECTRUM: EM waves form a spectrum of varying frequencies and wavelengths from radio waves through to gamma rays. PHOTONS: Particles of light energy which are emitted when the electrons in atoms fall from a higher to a lower energy level.

Describe what is meant by the dispersion of EM waves

DISPERSION: EM waves refract in transparent media at different angles depending on their wavelengths which separates them. eg dispersion of light in a rainbow

Describe the dispersion of EM waves in terms of the dependence of refractive index on wavelength. No quantitative discussion is required

Distinguish between transmission, absorption and scattering of radiation.

INTENSITY OF WAVE: Power per unit area which drops off in an inverse square relationship. ABSORPTION: When radiation interacts with a medium, its photons can excite electrons in the atoms and be absorbed. eg the ozone layer absorbs uv light; greenhouse gases absorb certain wavelengths of light and infrared. SCATTERING: After being absorbed, light can be re-emitted in all directions.

Discuss examples of the transmission, absorption and scattering of EM radiation. Students should study the effect of the Earth’s atmosphere on incident EM radiation. This will lead to simple explanations for the blue colour of the sky, red sunsets or sunrises, the effect of the ozone layers, and the effect of increased CO2 in the atmosphere.

ATMOSPHERIC EFFECTS: More blue light is scattered than other colours which is why the sky away from the Sun looks blue. At sunset, when the light travels through thicker atmosphere, more blue is scattered while red arrives directly from the Sun. GREENHOUSE GASES: Certain molecules such as water, methane and carbon dioxide absorb infrared radiation from the warmed earth and re-radiate it in all directions.



Video by the Engineer Guy - loads of excellent videos on youtube media type="custom" key="25345256"
 * Lasers **

[|LASERS SIMULATION FROM PHET]

It is quite difficult to get a result with this simulation: [|here are some settings which work] but try on your own first.

Explain the terms monochromatic and coherent

MONOCHROMATIC: EM waves of one frequency (and colour, if it is light).

COHERENT: Waves which have the same frequency and are in phase are coherent. They also must have similar amplitude to cause detectable interference.

Identify laser light as a source of coherent light.

LASER: In certain materials such as ruby, a very bright flash of light can cause most electrons in the ruby to be promoted to high energy level all at once. They can then simultaneously de-excite and induce more to do the same resulting in a coherent pulse of light.

Outline the mechanism for the production of laser light.



Students should be familiar with the term population inversion

POPULATION INVERSION: When the majority of electrons are in an excited meta-stable state, necessary for a laser.

Outline an application of the use of a laser. A laser diode (1) emits a beam (red) onto a combination of rotating (2) and fixed mirrors (3) that shine multiple beams onto the bar code (4). Although as many as 50 beams may hit the package at different angles, only one is shown in this illustration. The reflected light (yellow) is captured by the collector (5) and aimed at a sensor (6)

Students should appreciate that lasers have many different applications. These may include: • medical applications • communications • technology (bar-code scanners, laser disks) • industry (surveying, welding and machining metals, drilling tiny holes in metals) • production of CDs • reading and writing CDs, DVDs, etc.