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Step 2A Little Theory
The idea behind all of this is that the laser light is split into two separate paths. The beams travel along these paths and then recombine.
In most situations, when you add one thing to another, well you simply add the amplitude of the 'things'.
Put a cheese infront of a cheese-o-meter, and it would read '1'.
Put two cheeses infront of a cheese-o-meter, and it should read '2'.
(yeah yeah - squish the two cheeses together and you have one cheese - it's a bad analogy but it makes me smile)
But, critically, with two cheeses infront of it, the cheese-o-meter never should display zero. It doesn't matter how you arrange them, side-by-side, one on another, the meter should read '2'.
But light can demonstrate wave-like properties. And anything that oscillates can, at any given time, can be said to have a phase with respect to some other thing. I'll explain.
The phase simply describes how far the oscillating thing is along its path, with respect to some other point. Consider two perfectly bouncy balls.
A bouncing ball may be said to be 'in phase' with another similarly excited ball if the two both reach the apex of their bounces at the same time. A fancy way of saying that is that their phase difference is zero.
If the two balls are dropped from the same height at different times, then they will strike the ground at different times, but that difference will not change for subsequent bounces. One might say that their phase difference will be a constant.
Clearly, the balls could be dropped so that one is at the top of its path when the other hits the ground. The balls' motions are then in anti-phase: when one is doing one thing, the other is doing its opposite.
So - back to the two light beams alluded to earlier. If one light beam takes a slightly longer path than another light beam, then when the two are brought back to the same point, there will be a phase difference between the two. If it helps to think of something associated with each beam wiggling back and forth while it travels, well, good for you, but don't imagine that it's the full truth.
Because the wavelength (ie, distance between wiggles) is very very small for light, it doesn't take very much displacement for two beams to end up completely in anti-phase with each other.
And that's where the cheese come in.
See, because cheese hasn't got a phase, it always adds in a simple way.
1+1 = 2
But the electromagnetic fields that make up light have a phase (with respect to other light fields). So if one light beam is in anti-phase with another, when I add them the sum is zero.
So two light particles can be combined to give '0', and '2', and any number in between when shone into a light-o-meter (such as an eye, or a camera).
The fields in a light beam are always oscillating - they wiggle back and forth while the light propagates forward.
Thus, if one light beam is split into two rays, and if the rays cover different distances before recombining, the two rays will have different phases. And unlike cheese, they can cancel each other out, or they can add together.
(phew!)
In most situations, when you add one thing to another, well you simply add the amplitude of the 'things'.
Put a cheese infront of a cheese-o-meter, and it would read '1'.
Put two cheeses infront of a cheese-o-meter, and it should read '2'.
(yeah yeah - squish the two cheeses together and you have one cheese - it's a bad analogy but it makes me smile)
But, critically, with two cheeses infront of it, the cheese-o-meter never should display zero. It doesn't matter how you arrange them, side-by-side, one on another, the meter should read '2'.
But light can demonstrate wave-like properties. And anything that oscillates can, at any given time, can be said to have a phase with respect to some other thing. I'll explain.
The phase simply describes how far the oscillating thing is along its path, with respect to some other point. Consider two perfectly bouncy balls.
A bouncing ball may be said to be 'in phase' with another similarly excited ball if the two both reach the apex of their bounces at the same time. A fancy way of saying that is that their phase difference is zero.
If the two balls are dropped from the same height at different times, then they will strike the ground at different times, but that difference will not change for subsequent bounces. One might say that their phase difference will be a constant.
Clearly, the balls could be dropped so that one is at the top of its path when the other hits the ground. The balls' motions are then in anti-phase: when one is doing one thing, the other is doing its opposite.
So - back to the two light beams alluded to earlier. If one light beam takes a slightly longer path than another light beam, then when the two are brought back to the same point, there will be a phase difference between the two. If it helps to think of something associated with each beam wiggling back and forth while it travels, well, good for you, but don't imagine that it's the full truth.
Because the wavelength (ie, distance between wiggles) is very very small for light, it doesn't take very much displacement for two beams to end up completely in anti-phase with each other.
And that's where the cheese come in.
See, because cheese hasn't got a phase, it always adds in a simple way.
1+1 = 2
But the electromagnetic fields that make up light have a phase (with respect to other light fields). So if one light beam is in anti-phase with another, when I add them the sum is zero.
So two light particles can be combined to give '0', and '2', and any number in between when shone into a light-o-meter (such as an eye, or a camera).
The fields in a light beam are always oscillating - they wiggle back and forth while the light propagates forward.
Thus, if one light beam is split into two rays, and if the rays cover different distances before recombining, the two rays will have different phases. And unlike cheese, they can cancel each other out, or they can add together.
(phew!)
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Jun 4, 2010. 9:04 PMaamenabar
says:
Wow, fabulous explanation, finally I understand how distance is measured with light. Thanks a lot.
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Jun 5, 2010. 12:14 PMjrcgarry (author)
says:
Well, in many applications that use optical methods for measuring distance, a simple geometric approach is used with *no* use of interferometry For example, in a CD, the focusing lens is continually moving up and down to track the 'bumpy' surface of the reflective layer in the disc. The distances involved can be as large as 1mm (warped disc) but the lens is moved exceedingly rapidly using a voice-coil arrangement with an accuracy of a few microns. All of that is controlled simply with geometric optics. http://electronicdesign.com/article/components/measure_micro_displacements_using_a_cd_s_optical_pickup_head.aspx Is the reflected spot small and tight or big and blurry?
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