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Tamil Nadu Board of Secondary EducationHSC Science Class 12

Optical Instruments

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Estimated time: 5 minutes
CBSE: Class 12
Maharashtra State Board: Class 11

Introduction

Whether an object appears bigger or not does not necessarily depend upon its own size. Huge mountains far off may appear smaller than a small tree close to us because the angle subtended by the mountain at the eye from that distance is smaller than that subtended by the tree from its position. Hence, the apparent size of an object depends upon the visual angle subtended by the object from its position. For an object to appear bigger, we must bring it closer to us, or we must go closer to it.

Maharashtra State Board: Class 11

Definition: Least Distance of Distinct Vision

For a normal, unaided human eye, D = 25 cm. If an object is brought closer than this, we cannot see it clearly. The minimum distance from the eye at which an object can be seen clearly is called the least distance of distinct vision.

OR

Due to the limitation of focusing the eye lens, it is not possible to take an object closer than a certain distance. This distance is called the least distance of distinct vision.

Maharashtra State Board: Class 11

Definition: Angular Magnification or Magnifying Power

Angular magnification or magnifying power of an optical instrument is defined as the ratio of the visual angle made by the image formed by that optical instrument (β) to the visual angle subtended by the object when kept at the least distance of distinct vision (α).

Maharashtra State Board: Class 11

Need for Optical Instruments

If an object is too small, like the legs of an ant, the corresponding visual angle from 25 cm is not enough to see it, and if we bring it closer than that, its image on the retina is blurred. Also, the visual angle of cosmic objects far from us, such as stars, is too small to resolve minor details, and we cannot bring them any closer. In such cases, optical instruments are needed: a microscope in the former case and a telescope in the latter.

CBSE: Class 12

Examples of Optical Instruments

  • Periscope
  • Kaleidoscope
  • Binoculars
  • Telescope
  • Microscope
  • Human eye
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