Birefringence / photoelasticity (5)
Here is a quick comparison image of a few CA bricks. The explanation below is not really proper science, but it might still be interesting to see what we can say about them....
First, some of the (simplified) physics:
Polarized light
A ray of light behaves as a wave. It has a direction in which it travels, but the wave also has an orientation perpendicular to that direction. The wave can be oriented horizontally, vertically or anything in between. (Imagine drawing an arrow on a sheet of paper. The paper can lie flat on the table, or you can lift it up and rotate it while the arrow keeps pointing in the same direction.)
Normal light has a mix of all these orientations. That orientation can be changed when light is reflected on a surface or when it is refracted in a material.
A polarizer filter blocks light waves in one of the orientations and lets the light waves in the perpendicular orientation go through. As a result, the background of my photos can be black or white. The polarised light is blocked (black) or allowed to pass through (white).
Refraction and dispersion
When we talk about “the speed of light” we generally refer to the speed of light in a vacuum. The speed of light travelling through a medium (any material) is different. Light travelling through anything other than a perfect vacuum will be slowed down by an interaction with whatever particles it encounters. The amount by which light slows in a given material is described by the refractive index. The value of that refractive index depends on the material properties (molecules) and the frequency (color) of the light.
If a ray of light enters a medium at an angle, the change of refractive index will cause a change in the direction in which the light travels. Because the refractive index depends on the frequency/wavelength, not all colors change their direction over the same angle (dispersion). The most common example of this is a prism. A ray of white light enters on one side, and a rainbow spectrum exits on the other side.
Birefringence (or double refraction) and photoelasticity
Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. The light is refracted into two rays, each polarized with the vibration directions oriented at right angles (mutually perpendicular) to one another and traveling at different velocities. As a result, the two rays of light take a different path through the material, with a different speed and a different color dispersion.
Photoelasticity is the phenomenon of birefringence of polarized light by a transparent material under elastic stress. For these materials, the size of the refractive indices at each point in the birefringent material is directly proportional to the internal stresses at that point. Photoelasticity can be used as a non-destructive test to show internal stresses in transparent plastics. Areas with high levels of stress will show more colourful light fringes close to each other than the other areas.
Many crystals are naturally birefringent, but isotropic materials such as plastics and glass can also often be made birefringent by introducing a preferred direction through (for example by applying an external load, creating elastic stress). The injection molding process aligns the molecules of the plastic in the direction of the flow, which creates such a preferred direction through and creates the birefringent property of the material. As a result, it is not surprising that the effect occurs in transparent LEGO bricks. (Because colored bricks act as a color filter, the effect is only visible in the transparent-clear bricks. We cannot make a full rainbow using only red light)
Internal stresses in molded parts
When bricks are molded, liquid material is pushed into the mold. The material starts to cool down and solidify on the outside first. As the material cools it shrinks.
As long as there is an open connection to the mold pip, new material can flow in and fill up the space created by that shrinkage. When there is no open connection however, there is no inflow of new material. In such locations, the material in the center cannot shrink any more because it has to fill the volume between the walls that have already become solid. This causes internal stress. The material inside is “permanently stretched” to fill that void. Places where this is likely to occur are places where there are sudden jumps in the material thickness. On older bricks we find solid studs. When the walls of the bricks had hardened, the inside of the studs would still be warmer and liquid. As a result the studs experienced internal stresses. Often, they collapsed creating “pinholes” on top. In later molds the studs were made hollow to avoid this problem.
The setup of my accidental experiment
In everyday situations, normal mixed light passes through the transparent brick and gets refracted in mixed directions. The end result is that we usually do not see anything odd. The orientation has changed, but it is still a mix of all orientations.
By using a polarized light source (in this case the screen of my phone) we make sure that all light enters in the same orientation.
The light passes through the brick. Because the material is birefringent, the light rays are split into two rays with a different (perpendicular) polarisation. The two rays travel their different paths and have their colours dispersed differently. On leaving, their wave crests can be in phase and combine to give a bright colour. They can be out of phase giving less or no light. The phase condition depends on the wavelength (colour) and the viewing angle. This constructive and destructive interference between the rays of light causes the color bands.
The effects of this are not very apparent until we view the brick through a polarising filter. That filter lets only half of the rays through, and blocks rays with the perpendicular orientation. That makes the color bands visible. By rotating the filter we can change what we see and optimise our result for a nice photo.
So, what can we say about these bricks?
All the bricks in this photo are made from Cellulose Acetate (CA). CA is known to warp. This deformation is a result of internal stresses, so we can expect to get more spectacular results when looking at this material.
On the left there are two slotted bricks (Dogbone logo). On the second brick (number 8), the internal stress around the solid studs is clearly visible in the many fringes around the studs. The brick on the left of that (number 5) has collapsed studs. Bricks number 10 and 18 are hollow bricks (no internal tubes) and they feature the newer hollow stud design. The clear pattern around the studs is gone. The bricks still show a lot of internal stresses, and especially number 18 is very chaotic. As a result of the internal stress brick 18 (and 5) cracked. The internal stress became larger than the molecular binding.
A final thought...
Because LEGO bricks are birefringent, I now have to think about the possibility of making a Viking sunstone out of them… If they are birefringent enough, you can use them to find the sun in any weather and use that for navigation … I wonder if that works with these thin-walled parts… Probably not, but it should be fun to try :-P
Birefringence / photoelasticity (5)
Here is a quick comparison image of a few CA bricks. The explanation below is not really proper science, but it might still be interesting to see what we can say about them....
First, some of the (simplified) physics:
Polarized light
A ray of light behaves as a wave. It has a direction in which it travels, but the wave also has an orientation perpendicular to that direction. The wave can be oriented horizontally, vertically or anything in between. (Imagine drawing an arrow on a sheet of paper. The paper can lie flat on the table, or you can lift it up and rotate it while the arrow keeps pointing in the same direction.)
Normal light has a mix of all these orientations. That orientation can be changed when light is reflected on a surface or when it is refracted in a material.
A polarizer filter blocks light waves in one of the orientations and lets the light waves in the perpendicular orientation go through. As a result, the background of my photos can be black or white. The polarised light is blocked (black) or allowed to pass through (white).
Refraction and dispersion
When we talk about “the speed of light” we generally refer to the speed of light in a vacuum. The speed of light travelling through a medium (any material) is different. Light travelling through anything other than a perfect vacuum will be slowed down by an interaction with whatever particles it encounters. The amount by which light slows in a given material is described by the refractive index. The value of that refractive index depends on the material properties (molecules) and the frequency (color) of the light.
If a ray of light enters a medium at an angle, the change of refractive index will cause a change in the direction in which the light travels. Because the refractive index depends on the frequency/wavelength, not all colors change their direction over the same angle (dispersion). The most common example of this is a prism. A ray of white light enters on one side, and a rainbow spectrum exits on the other side.
Birefringence (or double refraction) and photoelasticity
Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. The light is refracted into two rays, each polarized with the vibration directions oriented at right angles (mutually perpendicular) to one another and traveling at different velocities. As a result, the two rays of light take a different path through the material, with a different speed and a different color dispersion.
Photoelasticity is the phenomenon of birefringence of polarized light by a transparent material under elastic stress. For these materials, the size of the refractive indices at each point in the birefringent material is directly proportional to the internal stresses at that point. Photoelasticity can be used as a non-destructive test to show internal stresses in transparent plastics. Areas with high levels of stress will show more colourful light fringes close to each other than the other areas.
Many crystals are naturally birefringent, but isotropic materials such as plastics and glass can also often be made birefringent by introducing a preferred direction through (for example by applying an external load, creating elastic stress). The injection molding process aligns the molecules of the plastic in the direction of the flow, which creates such a preferred direction through and creates the birefringent property of the material. As a result, it is not surprising that the effect occurs in transparent LEGO bricks. (Because colored bricks act as a color filter, the effect is only visible in the transparent-clear bricks. We cannot make a full rainbow using only red light)
Internal stresses in molded parts
When bricks are molded, liquid material is pushed into the mold. The material starts to cool down and solidify on the outside first. As the material cools it shrinks.
As long as there is an open connection to the mold pip, new material can flow in and fill up the space created by that shrinkage. When there is no open connection however, there is no inflow of new material. In such locations, the material in the center cannot shrink any more because it has to fill the volume between the walls that have already become solid. This causes internal stress. The material inside is “permanently stretched” to fill that void. Places where this is likely to occur are places where there are sudden jumps in the material thickness. On older bricks we find solid studs. When the walls of the bricks had hardened, the inside of the studs would still be warmer and liquid. As a result the studs experienced internal stresses. Often, they collapsed creating “pinholes” on top. In later molds the studs were made hollow to avoid this problem.
The setup of my accidental experiment
In everyday situations, normal mixed light passes through the transparent brick and gets refracted in mixed directions. The end result is that we usually do not see anything odd. The orientation has changed, but it is still a mix of all orientations.
By using a polarized light source (in this case the screen of my phone) we make sure that all light enters in the same orientation.
The light passes through the brick. Because the material is birefringent, the light rays are split into two rays with a different (perpendicular) polarisation. The two rays travel their different paths and have their colours dispersed differently. On leaving, their wave crests can be in phase and combine to give a bright colour. They can be out of phase giving less or no light. The phase condition depends on the wavelength (colour) and the viewing angle. This constructive and destructive interference between the rays of light causes the color bands.
The effects of this are not very apparent until we view the brick through a polarising filter. That filter lets only half of the rays through, and blocks rays with the perpendicular orientation. That makes the color bands visible. By rotating the filter we can change what we see and optimise our result for a nice photo.
So, what can we say about these bricks?
All the bricks in this photo are made from Cellulose Acetate (CA). CA is known to warp. This deformation is a result of internal stresses, so we can expect to get more spectacular results when looking at this material.
On the left there are two slotted bricks (Dogbone logo). On the second brick (number 8), the internal stress around the solid studs is clearly visible in the many fringes around the studs. The brick on the left of that (number 5) has collapsed studs. Bricks number 10 and 18 are hollow bricks (no internal tubes) and they feature the newer hollow stud design. The clear pattern around the studs is gone. The bricks still show a lot of internal stresses, and especially number 18 is very chaotic. As a result of the internal stress brick 18 (and 5) cracked. The internal stress became larger than the molecular binding.
A final thought...
Because LEGO bricks are birefringent, I now have to think about the possibility of making a Viking sunstone out of them… If they are birefringent enough, you can use them to find the sun in any weather and use that for navigation … I wonder if that works with these thin-walled parts… Probably not, but it should be fun to try :-P