I. Answer the questions:

1. What kind of motion characterizes light? 2. What waves are called transverse? 3. What experiment can make the phenomenon of polarization more evident? 4. In what way must the tourmaline crystal be cut for this experiment? 5. How many crystals of tourmaline must be used for this experiment? 6. What will we rotate the second crystal about? 7. When will the light be almost completely transmitted by both crystals? 8. When will the intensity of the transmitted light decrease? 9. At what moment will no light at all pass through the second crystal? 10. What will happen when the axes of the two crystals become perpendicular to each other? 11. What rays does the first crystal of tourmaline transmit? 12. What is the property of the tourmaline crystal that makes the light passing through it different from ordinary light? l3. What do we call that kind of light? 14. Is tourmaline the only mineral that has the property of selective reflection? 15. How can polarized light be obtained? 16. What must the respective directions of the incident and the polarized rays be for the reflected ray to be totally polarized? 17. Can you tell the value of i for ordinary glass? 18. Why is it customary to use a pile of glass plates instead of a single plate when an attempt at polarizing light by this technique is made? 19. What does the word "technique" mean?

II. Translate and memorize:

a) 1. This photo of the Moscow Kremlin is like the one on that wall except that it is much smaller. 2. You are allowed to use every instrument in our laboratory except that on the small table.

b) 1. After the lessons we will go together as far as the comer- of Chekov Street. 2. As far as I can remember, tourmaline is not the only crystal that polarizes light: so do, for example, Iceland spar and quartz, also crystalline substances

III. Speak about the phenomenon of polarization.

Double reflection

In studying refraction we formerly assumed the media under consideration to have the same physical properties in all directions whereas in crystalline substances like Iceland spar and quartz, the physical properties are quite different in different directions. When a beam of light is incident on such a crystal it is found to be split up into two beams, or components, one of which obeys the ordinary laws of refraction and is undeviated, having a constant velocity regardless of the direction in which it is travelling. With the other component the ordinary laws of refraction do not hold and it is generally found to be deviated for its velocity varies with the direction of propagation in the crystal: in a certain direction, known as the optic axis, its velocity equals that of the first component, in other directions it does not. These two components are appropriately called the ordinary ray and the extraordinary ray, respectively. In positive crystals the velocity of the extraordinary rays is, for directions other than that of the optic axis, greater than that of the ordinary ray. For negative crystals the reverse holds.

The types of crystal used most extensively in optical instruments for producing polarized light, such as calcite, have a single optic axis and are thus called uniaxial crystals. Many other crystals have two optical axes and are accordingly called biaxial. On examination, both the ordinary ray and the extraordinary ray are found to be polarized, and their planes of polarization are perpendicular to each other, as Fig. 16 shows by the dots and crosslines on the rays to indicate the directions of the vibrations. Crystals that produce polarized light in this fashion are known as birefringent, or doubly refracting.

It is obvious that provided one of the two beams could be removed, leaving only the other, the crystal could be used as a source of plane polarized light. Tourmaline is birefringent, while having the additional unusual property of absorbing the ordinary ray. unless the thickness of the crystal is under 2 mm, and transmitting the extraordinary ray almost undiminished in intensity. However, the absorption is not uniformed for all wave lengths, which results in the emergent beam being rather strongly colored.

Calcite is birefringent to a great extent and is widely used for production of polarized light.

Double refraction is easily observed by putting a piece of calcite on a sheet of paper having some printed matter on it. On looking through the crystal at the printed matter, two images are seen, one produced by the ordinary ray and the other by the extraordinary ray, the image from the ordinary ray appearing to be nearer to the observer than that from the extraordinary ray.

Fig. 16 Double refraction of calcite.

The incident ray is split into two rays, the ordinary ray and the extraordinary ray.

I. Answer the following questions:

1. What is the difference between the physical properties of ordinary crystals and those of such crystalline substances as Iceland spar and quartz? 2. What is the effect of such a crystal on a transmitted beam of light? 3. Are the two components similar in every respect? 4. Why is one of the rays deviated while the path of the other is the same as before? 5. What do we call the two components? 6. What is the Russian for "the ordinary ray" and "the extraordinary ray"? 7. What is the difference between uniaxial and biaxial crystals? 8. Do birefringent (i.e. doubly refracting) crystals cause one or both beams to be polarized? 9. Does tourmaline have any unusual property and if it does, what is it? 10. In what case will the ordinary ray transmitted by a doubly refracting crystal fail to be absorbed? 11. Why is the emerging beam found to be rather strongly colored? 12. How many images will we see if we look through a piece of calcite? 13. What do these images result from? 14. Which of the images seems to be nearer to the observer? 15. What can one do by rotating the piece of calcite?