Today I was demonstrating Young's double slit experiment using an old =
(26 yrs) He-Ne laser. The laser stayer lit for about 15 min, the time =
to show the diffraction pattern to the whole class by groups of four =
students. We then set out to measure the wavelength of the emitted =
radiation. I had two students count and measure the fringes when =
suddenly one of the student exclaimed..."Sir, why is the pattern getting =
dimmer with time ?" I first thought the student was seeing things, =
verified the class light setting, etc... but he was right. The pattern =
was definitely getting dimmer and within a few minutes completely gone. =
Is this the usual way for a laser to go ? =20
<!DOCTYPE HTML PUBLIC "-//W3C//DTD W3 HTML//EN">
<HTML>
<HEAD>
<META content=3Dtext/html;charset=3Diso-8859-1 =
http-equiv=3DContent-Type>
<META content=3D'"MSHTML 4.71.1712.3"' name=3DGENERATOR>
</HEAD>
<BODY bgColor=3D#ffffff>
<DIV><FONT color=3D#000000 face=3DArial size=3D2>Today I was =
demonstrating Young's=20
double slit experiment using an old (26 yrs) He-Ne laser. The =
laser stayer=20
lit for about 15 min, the time to show the diffraction pattern to the =
whole=20
class by groups of four students. We then set out to measure the=20
wavelength of the emitted radiation. I had two students count and =
measure=20
the fringes when suddenly one of the student exclaimed..."Sir, why =
is the=20
pattern getting dimmer with time ?" I first thought the =
student was=20
seeing things, verified the class light setting, etc... but he was =
right. =20
The pattern was definitely getting dimmer and within a few minutes =
completely=20
gone. Is this the usual way for a laser to go ? =20
</FONT></DIV></BODY></HTML>