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Light. — Science STD 8 — Question

Tamilnadu BoardEnglish MediumSTD 8ScienceLight.7 Marks
Question
Explain the working of a periscope.
✓

Answer

Periscope:
  1. It is an instrument used for viewing bodies or ships, which are over and around another body or a submarine.
  2. It is based on the principle of the law of reflection of light.
  3. It consists of a long outer case and inside this case mirrors or prisms are kept at each end, inclined at an angle of 45°.
  4. Light coming from the distant body falls on the mirror at the top end of the periscope and gets reflected vertically downward.
  5. This light is reflected again by the second mirror kept at the bottom, so as to travel horizontally and reach the eye of the observer.
  6. In some complex periscopes, optic fiber is used instead of mirrors for obtaining a higher resolution.
  7. The distance between the mirrors also varies depending on the purpose of using

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Cross - word puzzle :
Clues :
Across :
2. The bouncing back of sound wave when it strikes a hard surface.
4. Form of matter of fixed shape other than liquid or gas.
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1. A unit used to measure the loudness or intensity of sound.
3. Sound wave whose frequency is too high to be heard by humans.
5. Sound wave whose frequency is too low to be heard by humans.
7. A reflection of sound.
9. The branch of science deals with the study of seismic waves.
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Crossword puzzle :
Across:
1. The acid found in apples.
3. The ion formed when an acid is dissolved in water.
5. An indicator that remains colourless in an acid.
7. The gas liberated when dilute hydrochloric acid reacts with limestone.
9. A gas that dissolved in water to form ammonium hydroxide.

Down:
2. An acid used for cooking.
4. An acid that has three hydrogen atoms in its moelcule.
6. A reaction in which an acid reacts w’ith a base to form salt and water only.
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