This question involves the beam of a laser, expanded into a horizontal line using a cylindrical lens, as shown in the picture at the upper right below. A side view of the apparatus is seen in the photograph at the upper left below.

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The picture at the upper right above is a photograph of the apparatus taken from behind the apparatus, showing the laser beam expanded into a horizontal line by the cylindrical lens. The picture at the lower left above, and the close-up (with the laser turned off) at the lower right, show the apparatus from the front, looking backwards; you can see the laser, the cylindrical focusing lens in front of the laser, and a thin, taut heating wire running along the optic axis of the system immediately below the laser beam. The two posts holding the taut wire can be identified by the laser light skimming over them, rendering them visible with a slight red laser glow.


These two photographs below show the laser beam striking a screen about ten feet (three meters) in front of the laser without the cylindrical lens, and the expanded laser beam striking the screen after the cylindrical lens (seen mounted on the laser beam exit port) has been inserted.

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So, what will happen when the wire gets hot?

An electric current is than passed through the wire, having enough magnitude to make the wire very hot to the touch but not red hot to the eye. The question this week involves how this affects the expanded laser beam.

 Shown below are the original expanded laser beam, labelled (a), and four other possible photographs of the laser beam after the wire has been heated by passing a large electrical current, labelled (b), (c), (d), and (e). (The original expanded laser beam is shown for comparison with the possible laser beams affected by the heated wire.) Surveying the possibilities, heating the wire may cause the entire bright line to move higher (b) or to move lower (c), it might cause only a small part of the bright line to move higher (d) or to move lower (e), or it might have no effect, so the line stays the same (a).

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After March 20, click Read More for the answer.

 he answer is (d), as seen in this mpeg video showing what happens to the straight line of laser light (without the heated wire) when current is run through the wire, causing it to heat up.

In the video, you see that the hot wire heats the air adjacent to the wire, and the heat rises, yielding an air temperature above the wire that decreases with distance from the wire. The index of refraction of the air, which decreases with increasing temperature, thus creates a type of lens, bending the laser beam upward to cause the small "bubble" in the line produced by the laser beam when it is expanded horizontally by the vertically oriented cylindrical lens.