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Question

Which one of the following is the use of a concave lens?


A

Magnifying glass

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B

Optical instruments.

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C

In theatre spotlights

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D

all of the above

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Solution

The correct option is B

Optical instruments.


The explanation for the correct option:

Concave lens:

  1. A concave lens is used in optical instruments. In other mentioned options, a concave lens is not used.
  2. A concave lens diverges a straight light beam from the source to produce a diminished, upright virtual image.
  3. It always generates virtual images, i.e., it is used in optical instruments.
  4. It is used in the manufacture of binoculars and telescopes.
  5. Convex lenses are used in binoculars and telescopes to enlarge objects and make them look closer, but they distort and blur images instead of precisely transmitting light. Concave lenses are therefore included by telescope and binocular makers in or before the eyepieces to aid in sharpening pictures for the viewer.

Hence, option (B) is correct.

The explanation for incorrect options:

  1. The middle of the magnifying glass is thicker than the edges. Convex lenses, like magnifying lenses, produce magnified images. Convex lenses, like magnifying lenses, allow you to magnify objects.
  2. Spotlight makes use of a plano-convex lens, which is convex on one side and flat on the other. This lens is housed in a basic housing that also houses a bulb and a reflector.
  3. A concave lens always generates a virtual image, i.e., it is usually used in optical instruments.
  4. Convex lenses are used in binoculars and telescopes to enlarge objects and make them look closer, but they distort and blur images instead of precisely transmitting light. Concave lenses are therefore included by telescope and binocular makers in or before the eyepieces to aid in sharpening pictures for the viewer.
  5. A camera's primary lens is convex, and when used by itself, it can distort images in ways known as chromatic aberrations. It also refracts the light of different colors at different angles, creating a fringe effect around bright objects in the image. Combining convex and concave lenses eliminates both undesirable effects.

Hence, Options (A),(C), and (D) are incorrect.

Hence, option (B) is correct.


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