How a Polarimeter works: A more detailed explanation

Light waves as it travels. As shown in Figure 1, light may seem to travel unidirectionally. In actuality light travels in all directions as shown in Figure 2.


When light, which waves in all directions, goes through a grating placed in its course of travel, only the light wave that oscillates in the direction parallel to the bars of the grating passes through, Light waves that oscillate in other directions get blocked by the bars of the grating. ( Figure 3 ) Such light, which waves in one particular direction, is called polarized light, and the grating is called a polarizing plate.

When polarized light travels through in a polarimeter an observation tube filled with a sample solution that does not make light rotate (water, for example), the light continues to wave in the same direction even after passing through the solution. ( Figure 4 )


In contrast, when it travels through in a polarimeter an observation tube filled with a sample solution that makes light rotate (sucrose solution, for example), the light wave begins to rotate as it passes through the solution. (Figure 5) This is called optical rotation.


Those samples that make light rotate have a molecular formula that contains asymmetric carbon ( indicated by "C" ) . Sugar is the most common. The explanation of the asymmetric carbon can be highly technical. Imagine making a light path by placing a polarizing plate, an observation tube, another polarizing plate, and a sensor one after another. (Figure 6 and 7). The path in Figure 6 has an observation tube filled with water, in Figure 7 a sample solution, such as sucrose solution, that makes light rotate, such as you would find in a polarimeter.



In Figure 6 a certain amount of light reaches the sensor.

In Figure 7 the light does not reach the sensor. (Technically speaking, in terms of a vector an imperceptible amount of light does reach the sensor, but let's assume that the light does not reach the sensor here. )

When the second polarizing plate is rotated as shown in Figure 8, the same amount of light as in Figure 6 now reaches the sensor.


Conducting Zero-Setting on a Polarimeter
Conduct zero-setting in the step shown in Figure 6. In the actual adjustment procedure, the observation tube filled with water is not necessary and zero-setting is conducted by letting light travel through the air. Next, place an observation tube filled with a sample solution that makes light rotate as shown in Figure 8. Rotate the second polarizing plate so that the equal amount of light reaches the sensor as it did when zero-setting was conducted. The measured angle of the rotated polarizing plate is the angle of rotation of the sample solution.

Author Name: Kathy Brasch : Nationalmicroscope.com

How a Polarimeter works: A more detailed explanation

How A Polarimeter Works: The Simple Explanation

Imagine tying a piece of thick rope to a hook in a wall, and then shaking the rope vigorously. The rope will be vibrating in all possible directions - up-and-down, side-to-side, and all the directions in-between - giving it a really complex overall motion. Now, suppose you passed the rope through a vertical rectangular hole, like this: []. The rope has a really tight fit in the hole. The only vibrations still happening at the other side of the hole will be vertical ones. All the others will have been prevented by the hole.

What emerges from the hole could be described as "plane polarized rope", because the vibrations are only in a single (vertical) plane. Now look at the possibility of putting a second hole on the rope. If it is aligned the same way as the first one, the vibrations will still get through. But if the second hole is at 90° to the first one (so horizontally), the rope will stop vibrating entirely to the right of the second hole. The second hole will only let through horizontal vibrations - and there aren't any.

Light is also made up of vibrations - this time, electromagnetic ones. Some materials have the ability to screen out all the vibrations apart from those in one plane and so produce plane polarized light. The most familiar example of this is the material that Polaroid sunglasses are made of. If you wear one pair of Polaroid sunglasses and hold another pair up in front of them so that the glasses are held vertically rather than horizontally, you'll find that no light gets through - you will just see darkness. This is equivalent to the two holes at right angles in the rope analogy. The polaroids are "crossed". (This not exactly the way Polaroid glasses work, but it gives a good idea).

A polarimeter works the same way: You have two polaroid glasses, like the two holes with the rope, one glass is the polarizer, the other glass is the analyzer. The polarizer ensures that only a beam of polarized monochromatic light (light of only a single frequency - in other words a single color) is passed through the solution behind the polarizer in the polarimeter. After the tube with the solution is the analyzer. The polarimeter is originally set up with water in the tube. Water isn't optically active - it has no effect on the plane of polarization. The analyzer is rotated until you can't see any light coming through the polarimeter. The polaroids are then "crossed".

An optically active substance is a substance which can rotate the plane of polarization of plane polarized light. If you shine the polarized monochromatic light through a solution with an optically active substance, then light emerges: its plane of polarization is found to have rotated. The substance rotates the plane of polarization of the light, and so the analyzer won't be at right-angles to it any longer and some light will get through. You would have to rotate the analyzer in order to cut the light off again.

The rotation may be either clockwise or anti-clockwise. Assuming the original plane of polarization was vertical, you can easily tell whether the plane of polarization has been rotated clockwise or anti-clockwise, and by how much.


Author Name: Kathy Brasch : Nationalmicroscope.com

How A Polarimeter Works: The Simple Explanation

Principals of Refractometers

Water is placed in a reservoir. When a pencil is dipped into the water, the tip appears bent. Now put concentrated sugar water into a cup and try the same thing. The tip of the pencil should appear even more bent. This is the phenomenon of light refraction. Refractometers are measuring instruments in which this phenomenon of light refraction is put to practical use. They are based on the principal that as the density of a substance (e.g. when sugar is dissolved in water), it's refractive index rises proportionately.

When a straw is placed into a glass of water, the straw appears bent. Now if a straw is placed in a glass with water containing dissolved sugar, the straw should appear even more bent (see illustrations). This phenomenon is known as the principle of light refraction. Refractometers are measuring instruments which put this phenomenon of light refraction to practical use. They are based on the principle that as the density of a substance increases (e.g. when sugar is dissolved in water), its refractive index (how much the straw appears bent) rises proportionately. Refractometers were devised by Dr. Ernst Abbe, a German/Austrian scientist in the early 20th century.

The prism in refractometers has a greater refractive index than the sample solution. Measurements are read at the point where the prism and solution meet. With a low concentration solution, the refractive index of the prism is much greater than that of the sample, causing a large refraction angle and a low reading. The reverse (lower refraction angle and higher reading) would happen with a highly concentrated solution.
There are two detection systems for refractive index: transparent systems and reflection systems. Hand-held refractometers and Abbe refractometers use transparent systems, while digital refractometers use reflection systems.

Transparent Systems
The detection system for hand-held refractometers (transparent system) is summarized below.
1. In the figure below the detection is done by utilizing the refractive phenomenon produced on the boundary of the prism and sample. The refractive index of the prism is much larger than that of the sample
2. If the sample is thin, the angle of refraction is large (see "a") because of the large difference in refractive index between the prism and the sample.
3. If the sample is thick, the angle of refraction is small (see "b") because of the small difference in refractive index between the prism and the sample.

Reflection Systems

In the figure below, Light A, being incident from the lower left of the prism, is not reflected back by the boundary, but exits through the sample. Light B is reflected by the boundary face to the right, directly along the prism boundary. Light C, having an incident angle too large to be let through to the sample side, is totally reflected toward the lower right of the prism.

As a result, a boundary line is produced dividing light and dark fields on either side of the dotted line "B' " in the figure. Since the angle of reflection of this boundary line is proportional to refractive index, the position of the boundary line between light and dark fields is caught by a sensor and converted into refractive index.

Author Name: Kathy Brasch : Nationalmicroscope.com

Principals of Refractometers

Choosing A Microscope: Compound or Stereo microscope?

Your application is the most important factor in choosing a microscope. What you need to see and what you want to do with that image will determine what kind of microscope you need. Microscopes typically come in two types: compound or stereo microscope.

The most common is the compound microscope. It is the one most people visualize when they think about microscopes. A microscope with one eyepiece is called a monocular microscope; with two eyepieces it is called a binocular microscope, or it might have an additional camera tube and is called a trinocular microscope. The compound microscope has a number of objectives (the lens closest to the object being viewed) of varying magnification mounted in a rotatable nosepiece. It uses a light source beneath the stage to illuminate slides. These microscopes are generally used to view very small objects such as cells or bacterium. Magnification of compound microscope scopes range from 40X up to 1000X. Actual magnification can be figured by multiplying the power of the eyepiece by the power of the objective lens.

The other type of microscope is called a stereo microscope or dissecting microscope. It uses two eyepieces and two paired objectives. Stereo microscopes come in models that have full zooming capability and models that just have only two magnification settings. Stereo microscopes are particularly useful for biologists performing dissections, technicians building or repairing circuit boards, paleontologists cleaning and examining fossils or any one who needs to work with their hands on small objects. You can find stereo microscopes that have a built in light source from above, below, or none at all. Magnification is usually much less than that of a compound microscope, but is figured in the same way by multiplying the power of the eyepiece by the power of the objective lens.

Author Name: Kathy Brasch : Nationalmicroscope.com

Choosing A Microscope: Compound or Stereo microscope?

Digital Microscope

A digital microscope is real 21st century advancement on microscope technology. It comes with software for your computer that allows you to see real-time images on your monitor of what you're observing with the microscope. Take a step into the modern age by learning how to use a digital microscope.

With the advent of computers and the digital era things have improved a lot. You can now buy a handheld digital microscope from the market at affordable prices which will plug straight into the USB port of almost any computer, and displays and records the image in real-time. A digital microscope still uses optics much the same way as a traditional microscope, but also has a built-in digital camera, which works just like a webcam but with magnification. The software that comes with these cameras will let you take still or video pictures while magnifying the image by 200 times or more. You can then use your regular image software to manipulate and use the picture in many ways.

Although these digital microscopes are obviously great in a science classroom environment where a teacher can present and discuss a rapid sequence of images, don't neglect their home use. Digital microscopes offer an amazing insight into the world around us from a rarely seen perspective.

Digital microscopes were brought to a new level of excellence with the introduction of Olympus' MIC-D. The MIC-D uses a USB connection to the computer for live, full-color images to be displayed on a monitor screen. The design of the MIC-D is inverted, which means that the lens is tilted up at the stage instead of positioned down at the specimen. This feature allows large objects and dishes of water to be magnified with amazing clarity. A further innovation of the MIC-D's design is that it uses one master lens instead of a series of fixed lenses.

Author Name: Kathy Brasch : Nationalmicroscope.com

Digital Microscope