DISPERSION OF LIGHT AND OPTICAL INSTRUMENTS - BRIEF ANSWERS

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ANSWER IN BRIEF

1. WHY DOES DISPERSION OF LIGHT OCCUR ? EXPLAIN.
   

   ANS:
   
   White light consists of different colours having different wavelength.When white light passes through vacuum the velocity of all colours is same.When light passes through a refractive medium( like water) the velocities of all components differ because waves of different colours are refracted in different proportions.This results in dispersion of white light.
   



2. WHAT ARE PRIMARY AND COMPOSITE COLOURS ? EXPLAIN GIVING EXAMPLES.
   

   ANS:
   
   

   
   
   • Red, blue and green are three primary colours of light.
   • White light can be obtained with proper combination of these three colours.
   • Additive mixture of these primary colours in different proportions produces a wide range of different colours.
   • Colours obtained by such mixing are called composite colours.
   • Mixing of BLUE and RED gives MAGENTA. Mixing of BLUE and GREEN gives CYAN and mixing of RED and GREEN gives YELLOW. Thus, magenta, cyan and yellow are composite colours of light.
     

   
   
   
3. WHY DO DEFECTS OF VISION ARISE ? EXPLAIN IN BRIEF.
   

   ANS:
   
   

   
   
   • We can see an object clearly when its image is formed exactly on retina.
   • The thickness of the eye lens changes according to the distance of the object.
   • Ciliary muscles help the lens to change its thickness.
   • When the lens cannot change its thickness properly, it cannot focus the object exactly on retina and the defects of vision arise.
   • If the lens of the eye does not become sufficiently thin to focus a distant object, the defect is called nearsightedness or myopia.
   • If the lens cannot become sufficiently thick to focus a nearby object, the defect is called farsightedness or hypermetropia.
     

   
   
   
4. MENTION DEFECTS OF VISION AND THEIR REMEDY.
   

   ANS:
   
   There are two types of defects of vision:

   • Nearsightedness or myopia
   • Farsightedness or hypermetropia.
     

   
   
   A concave lens of appropriate focal length corrects the defect of nearsightedness whereas a convex lens of appropriate focal length corrects the defect of farsightedness.
   

   
   
5. EXPLAIN : COMPLIMENTARY COLOURS.
   

   ANS:
   
   Any two colours which on mixing produce white light are called complimentary colours. For example :

   • Blue and yellow are complimentary colours of each other.
   • Green and magenta are complimentary colours of each other.
   • Red and cyan are complimentary colours of each other.
     

   
   

   
   
6. MENTION TWO POINTS OF DIFFERENCE BETWEEN COMPOUND MICROSCOPE AND ASTRONOMICAL TELESCOPE.
   

   ANS: