Isotropic substances do not change the vibration direction of light as the light passes through the substance. So, if we place a mineral grain on a glass slide glass is also isotropic , and view the grain through the ocular lens with the analyzer not inserted in the light path, we will be able to clearly see the grain. If the grain selectively absorbs light of certain wavelengths, then the grain will show its absorption color. If we now insert the analyzer in the light path, the light coming out of the grain will still be polarized in an E-W direction, since isotropic substances do not change the polarization direction, and the analyzer will cut out all of this light.
Thus, no light coming out of the mineral grain will pass through the analyzer. The mineral is thus said to be extinct in this position.
Similarly, if we rotate the stage of the microscope, and thus rotate the grain, it will remain extinct for all rotation positions. This is the primary means to determine whether or not a substance is isotropic. That is, rotate the grain on the microscope stage with the analyzer inserted. If the grain remains extinct throughout a o rotation of the stage, then the mineral or substance on the microscope stage is probably isotropic. In isotropic substances, there are only two optical properties that can be determined.
One of these is the absorption color, as discussed above. The other is the refractive index. Tables of refractive indices for isotropic minerals, list only the refractive index for one wavelength of light. The wavelength chosen is nm, which corresponds to a yellow color.
Such a wavelength would be given off of a sodium vapor lamp. Since these are expensive and generate much heat, sodium vapor lamps are not generally used in optical mineralogy. Instead we use white light. Still, as we shall see later, we can determine the refractive index for nm.
The determination of the refractive index of an isotropic substance is made by making a comparison with a substance of known refractive index. The comparison materials used are called refractive index oils. These are smelly organic oils that are calibrated over a range of refractive indices from 1. As you will see in lab, grains of the unknown substance are placed on a glass slide, a cover glass is placed over the grains, and a refractive index oil is introduced to completely surround the grains.
This is called the immersion method. The grains are then observed with the analyzer not inserted. If the grain has a refractive index that is very much different from the refractive index of the oil, then the grain boundaries will stand out strongly next to the surrounding oil. The grain will then be said to show high relief relative to the oil.
High relief indicates that the refractive index of the grain is very much different from the refractive index of the oil. It does not tell us if the refractive index of the grain is less than or greater than the oil. If the refractive index of the grain and the oil are closer, then the outline of the grain will not stand out as much from the oil. In this case, the grain is said to low relief relative to the oil. Again, low relief only indicates that the grain and oil have similar refractive indices, and does not indicate that the grain as a lower or higher refractive index than the oil.
If the refractive index of the grain is exactly the same as the refractive index of the oil, the boundaries of the grain will not be visible. That is to say that the grain will completely disappear in the oil. In this case the grain is said to have no relief relative to the oil. In order to determine whether the grain or the oil has a higher refractive index, a method called the Becke Line Method is used.
A grain surrounded by oil when viewed through the microscope focused slightly above the position of sharpest focus will display two lines, one dark and one bright that concentric with the border of the grain.
The brighter of these lines is called the Becke line and will always occur closest to the substance with a higher refractive index. This can be used to determine if the grain or the oil has the higher refractive index. To use this method, one first focuses the microscope as sharply as possible on the grain of interest.
It is also useful to use the iris diaphragm to cut down the incoming light as much as possible. This will make the Becke line stand out better. Then using the fine focus dial adjustment the microscope stage is lowered or the objective lens is raised slightly out of focus.
During this increase in focal distance one observes a moving bright Becke line. If the Becke line moves inward, the refractive index of the grain is greater than the refractive index of the oil. It is important to remember that the Becke line test is performed by increasing the distance between the grain and the objective lens.
Thus, you should not memorize which way to turn the focusing knob, because it may be different with different brands of microscopes. Note also that if the focal distance is decreased, rather than increased, then the opposite results will be obtained, that is with decreasing focal distance the bright Becke line will move into the substance with lower refractive index. For a grain with a refractive index less than that of the oil, the opposite effect will be observed.
When raising the objective lens or lowering the stage so that the grain goes slightly out of focus, if the bright Becke line moves into the oil, then the oil has a refractive index greater than that of the grain.
For example, what happens when a pebble is thrown into a pond? As shown in the image above, where the pebble enters, the water starts to oscillate up and down. The disturbance in the water moves outward as more pieces of water start to move up and down. The water in each place only moved up and down , but a wave moved outward from where the pebble entered the water.
No water moved outward—what moved outward is the disturbance in the pond's surface. The outward motion of the disturbance transports energy from one place the location where the pebble entered the water to another all points outward from the pebble entry point. This example illustrates that a wave is really a mechanism by which energy gets transported from one location to another. Electric fields and magnetic fields can be disturbed in a similar way to the surface of a pond.
When a stationary charged particle begins to vibrate or more generally, if it is accelerated , the electric field that surrounds the particle becomes disturbed. Changing electric fields create magnetic fields, so a moving charge creates a disturbance in both the electric field and magnetic field near the charged particle. For each medium, there is a characteristic velocity at which the disturbance travels.
There are three measurable properties of wave motion: amplitude, wavelength, and frequency the number of vibrations per second. When utilizing these equations to determine wavelength, frequency, or velocity by manipulation of the equation, it is important to note that wavelengths are expressed in units of length, such as meters, centimeters, nanometers, etc; and frequency is typically expressed as megahertz or hertz s —1.
If light consisted strictly of ordinary or classical particles, and these particles were fired in a straight line through a slit and allowed to strike a screen on the other side, we would expect to see a pattern corresponding to the size and shape of the slit. However, when this single-slit experiment is actually performed, the pattern on the screen is a diffraction pattern in which the light is spread out.
The smaller the slit, the greater the angle of spread. Since light behaves like a wave, one would have good reason to believe that it might be a wave. In Lesson 1, we will investigate the variety of behaviors, properties and characteristics of light that seem to support the wave model of light. On this page, we will focus on three specific behaviors - reflection, refraction and diffraction. A wave doesn't just stop when it reaches the end of the medium.
Rather, a wave will undergo certain behaviors when it encounters the end of the medium. Specifically, there will be some reflection off the boundary and some transmission into the new medium. The transmitted wave undergoes refraction or bending if it approaches the boundary at an angle.
If the boundary is merely an obstacle implanted within the medium, and if the dimensions of the obstacle are smaller than the wavelength of the wave, then there will be very noticeable diffraction of the wave around the object.
Each one of these behaviors - reflection, refraction and diffraction - is characterized by specific conceptual principles and mathematical equations. The reflection, refraction, and diffraction of waves were first introduced in Unit 10 of The Physics Classroom Tutorial. In Unit 11 of The Physics Classroom Tutorial , the reflection, refraction, and diffraction of sound waves was discussed.
Now we will see how light waves demonstrate their wave nature by reflection, refraction and diffraction. All waves are known to undergo reflection or the bouncing off of an obstacle.
Most people are very accustomed to the fact that light waves also undergo reflection. The reflection of light waves off of a mirrored surface results in the formation of an image. One characteristic of wave reflection is that the angle at which the wave approaches a flat reflecting surface is equal to the angle at which the wave leaves the surface. This characteristic is observed for water waves and sound waves.
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