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Question
1.How does LED emits light different colors?
2.And why does a standard bulb show only yellow light?
3.Can human body conduct electricity if they are connected in an electric circuit?
4.How does the bulb glows at an instant just when we switch it on? Do they electrons move so fast?

Answer
Here is a good explanation:

http://en.wikipedia.org/wiki/LED

The led emits light according to the electron gap of the photoluminescence of the materials employed. The wavelength of the emitted light determines the color.

The standard incandescent lamp emits light from a different concept; the filament inside the bulb becomes very hot; the heat or temperature of the filament material determines the light given off.

Conduction of the flow of electricity in the human body is primarily on the skin surface unless the voltage becomes very high.  The resistance of the surface of human skin is quite high ordinarily but depends on many things such as the moisture of the surface of the skin such as perspiration and the surface area of the electrical contacts to the skin surface.  So, yes the human body can carry electric current but the resistance is high and only small currents are plausible.  You might think of the human conductivity more as an insulator than a conductor.

When turning on an incandescent lamp or bulb the filament heats up very rapidly as the temperature rise is not instant but less than one second!  If you look carefully in most cases you can see a slight delay before it is full illuminated; but it is only a fraction of a second in time.

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Expertise

Electronics questions about AC, DC and digital theory.

Experience

Graduate electrical engineer with over 40 years in electronic design, manufacturing, project organization and patent review. Experience in fields of industrial and consumer electronics (audio, video, acoustics, etc.)

Organizations
IEEE (Institute of Electrical and Electronics Engineers); Senior Life member AES (Audio Engineering Society), Fellow Life member

Education/Credentials
BSEE University of North Dakota

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