Frame a question for the "Interferences of different colours.
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As you have probably noticed by now, viewing an anisotropic crystal under crossed polars (analyzer inserted) the crystal is extinct when either of the two privileged directions in the crystal are lined up parallel to the polarizing direction of the microscope. This is because when the privileged directions are parallel to the polarizer, the crystal does not change the polarization direction and the light will thus be vibrating perpendicular to the analyzer. When the privileged directions are not parallel to the polarizer some light is transmitted by the analyzer and this light shows a color, called the interference color. In this lecture we will discover what causes this interference color and how it can be used to determine some of the optical properties of the crystal.
The Interference of Light
Waves Polarized in the Same Plane
As we discussed in the lecture on X-rays, when electromagnetic waves emerge from a substance, they can interfere with each other and either become enhanced, partially destroyed, or completely destroyed. The same is true for polarized light. In the upper diagram below, labeled A, two waves polarized in the vertical plane are emerging from the polarizer. These waves are in phase, and thus we only see one wave. The difference between points on the same wave is called the path difference, symbolized as Δ. If the path difference is an integral number of wavelengths, nλ, the waves are in phase. For points that are completely out of phase, for example between crests and troughs of the wave, Δ will be equal to 1/2λ , 3/2λ, 5/2λ, etc. or [(2n+1)/2]λ. For path differences where the waves are completely out of phase, complete destructive interference will occur and no wave will emerge from the polarizer.
For path differences that are neither in phase or completely out of phase, the waves will be summed together to produce a resultant wave with crests and troughs at different positions than the original waves.
This is shown in the part of the diagram labeled B, and the blow-up to the right.
Furthermore, the vector sum of the two waves will produce a resultant wave with a different amplitude than the two original waves.
Answer:
similarin naturaland both are due to interference of light.