Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Qns on Laser

  1. May 18, 2006 #1

    i have this Qns as follows :

    which of the following statements about laser is incorrect?

    a. Laser light can be focused to a tiny spot.
    b. Laser light must be visible.
    c. Laser light travels in vacuum at a speed of 3 X 10^8 m/s
    d. Laser light can be extremely powerful.
    e. Laser light is monochromatic.


    for (b.) must laser light neccessaryly must be visible? can't it be in a non- visible spectum.

    for (d.) the ans seems horrible unsciencetific, although i agree that laser can be powerful.

    for (e.), i checked up wikipedia can found this:
    "In the physical sense, no real source of electromagnetic radiation is purely monochromatic, since that would require a wave of infinite duration. Even sources such as lasers have some narrow range of wavelengths (known as the linewidth or bandwidth of the source) within which they operate."

    what does this whole chunk mean can anyone explain?? what is wave of infinite duration?? and what is monochromatic??
  2. jcsd
  3. May 18, 2006 #2

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    A monochromatic laser beam would contain light of only one frequency or wavelength.

    The amplitude of a monochromatic wave, which by definition consists of only one sine function, cannot vary in time. If it does, it must be comprised of wave functions of other frequencies. Fourier analysis shows that a wave packet whose shape (amplitude) varies in time - any stream of light having a beginning and end, for example - consists of the superposition of a series of sinusoidal functions having a range of frequencies. The range of frequencies depends on how frequently or quickly the amplitude changes.

  4. May 23, 2006 #3
    Yes, you are correct with these... so it would seem that the answer to the question is b.

    For e. what Andrew has said is correct, however lasers are quite close to monochromatic (read the wiki quote again.... narrow range of wavelengths....), so it really depends on how pedantic you want to be.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook