Ray Optics and Optical Instruments: How is Image Formed by a Lens, Uses of the Lens (For CBSE, ICSE, IAS, NET, NRA 2022)

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  • Laws of refraction and reflection are valid for all the surfaces and pairs of media at the point of incidences
  • Reflection:-
Ray Optics and Optical Instruments
  • Light: it is an agent which produces in us the sensation of sight. It is a form of energy.
  • Transparent medium: It is a medium through which light can be propagated easily. (e. g. , sun, candle, electric arc)
  • Translucent medium: It is a medium through which light is propagated partially. (e. g. , paper, ground, glass)
  • Opaque: It is a medium through which light can be propagated. (e. g. , wood, iron)
  • Reflection: It is the property of light by virtue of which, light is sent back into the same medium from which it is coming after being obstructed by a surface.

How is Image Formed by a Lens?

  • The real image of a body which is being positioned between the 2f and f from a convex lens can be captured on a screen which has been placed at the location of the image even if the screen is removed.
  • This screen simply diffuses the rays from a given point on the object are converging to an image in space and again diverging away.
  • Laser show projection is an appropriate example of this phenomena.
  • The formation of an image needs regular refraction/reflection.
  • The rays generated from a single point are expected to reach the same image point.
  • So, certain types of objects such as the page of a book doesn՚t produce an image.

What Are the Constituents of White Light?

  • Variety of colors of objects is witnessed due to the presence of constituents colors of the light falling upon them.
  • White light may produce a different perception about the colors on a substance when compared with monochromatic light. Thick lenses also produce colored images due to dispersion.
  • In case of a simple microscope, the angular size of the object is similar to the angular size of the image.
  • It produces magnified results because the position of the object is closer to the eye and hence the subtended angle will be larger.
  • The human eye is a convex lens having a focal length of 2.5 cm. The image is always formed in the retina and this ability of the eye is termed as accommodation.

Uses of the Lens

  • For myopia, the image is focussed before the retina which can be fixed with the help of a diverging lens.
  • In hypermetropia, the image is focussed beyond the retina which can be fixed with the help of a converging lens. Cylindrical type of lenses is used to fix astigmatism.

Magnifying Power of a Simple Microscope

  • The formula for finding the magnifying power of a single microscope is

m = 1 + (D/f)

  • The distinct vision has the least distance as 25 cm. If the image is at infinity and f is referred to the focal length, then m = D/f.

Magnifying Power of a Compound Microscope

  • Magnifying power of a compound- microscope is given by
  • where, l is length of tube D is least distance of clear vision is focal length of objective is focal length of eyepiece
  • So, clearly, it can be seen that focal length of objective and eyepiece needs to be decreased so that magnifying power increases.