DISPERSION OF LIGHT AND OPTICAL INSTRUMENTS - III :LONG ANSWERS

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1. EXPLAIN THE DISPERSION OF WHITE LIGHT BY PRISM.
   

   ANS:
   
   When a narrow beam of white light is incident on the transparent side of a prism, the prism decomposes it into its constituent colours and we obtain a group of bands of different colours called SPECTRUM OF WHITE LIGHT. This separation of white light into its constituent colours is called DISPERSION OF WHITE LIGHT.
   
   
   White light consists of different colours having different wavelength. When white light passes through vacuum the velocity of all colours is same. When the light passes through a refractive medium (like water) the velocities of all components differ because the waves of different colours are refracted in different proportions. This results in dispersion of white light.
   
   The colours obtained on the screen starting from the bottom are : violet, indigo, blue, green, yellow, orange, red.
   
   This shows that in a transparent medium, the velocity of violet light is the least and that of the red light is the highest. Hence the violet colour at the bottom and the red at the top of the spectrum.
   

   
   
2. DESCRIBE IN SHORT HOW THE SEVEN COLOURS OF LIGHT CAN BE COMBINED TO GET WHITE LIGHT.
   

   ANS:
   
   
   Take two prisms P₁ and P₂ with same prism angles and arrange them as shown in the figure. When you allow a narrow beam of white light to be incident on prism P₁, it decomposes into its seven constituent colours (which can be obtained on a screen).
   
   When these waves are allowed to be incident on the second prism P₂, they recombine and emerge from P₂ as a beam of white light. Thus, recombination of colours of white light is a reverse process of dispersion.
   

   
   
3. EXPLAIN : PRIMARY COLOURS OF LIGHT AND THEIR SUPERIMPOSITION.
   

   ANS:
   

   
   
   • Red, blue and green are three primary colours of light.
     
   • White light can be obtained with proper combination of these three colours of light and we need not combine all seven constituent colours.
     
   • Additive mixture of these primary colours in different proportions produce a wide range of different colours(shades). This method is called ADDITIVE MIXTURE METHOD.
     
   • Colours obtained by such method are called COMPOSITE COLOURS.Magenta, cyan and yellow are examples of composite colours.
     
     
   • SUPERIMPOSITION:
     
     
     
     • Take three torches fitted with transparent glass plates of red, blue and green colour.
       
       
       
     • If the torches are arranged as shown in the figure, and their lights after passing through the coloured plates are projected on a screen (or wall), we find different regions with different colours.
       
       
     • The portion where all three colours superimpose appears white.
       
       
     • The portion where red and blue colours superimpose appears magenta.
       
       
     • The portion where red and green colours superimpose appears yellow.
       
       
     • The portion where blue and green colours superimpose appears cyan.
       
     
     

   
   

   
   
4. EXPLAIN : SUBTRACTIVE METHOD OF MIXING OF PIGMENTS.
   

   ANS:
   

   
   
   • The mixing of primary colours of light(i.e/ red, blue, green) by additive method gives white light.
     
   • The mixing of red, blue and green pigments, however, does not give white pigment as this mixing is done by subtractive method.
     
     
   • As shown in the figure when white light is incident on blue pigment; violet, green and blue colours of light are reflected and remaining colours are absorbed.
     
   • When white light is incident on yellow pigment; only yellow, orange and green colours are reflected and remaining colours are absorbed.
     
   • Thus, both yellow and blue pigments do not absorb green colour. If blue and yellow pigments are mixed, then that mixture will reflect only green colour of light.
     
   • Cyan, magenta and yellow are primary pigments and pigments of any colour can be produced by appropriately mixing these pigments.
     

   
   

   
   
5. EXPLAIN : DEFECTS OF VISION
   

   ANS:
   

   
   
   • We can see an object clearly when its image is formed exactly on retina.
     
   • The thickness of the lens of the eye changes according to the distance of the object.
     
   • Ciliary muscles help the lens to change its thickness.
     
   • When the lens can focus the object properly its image is formed exactly on retina and we can see it clearly.
     
   • When the lens cannot change its thickness properly, the defects of vision arise.
     
   • There are two types of defects of vision :
            (1)Near-sightedness (Myopia)
            (2)Far-sightedness
     

   
   

   NEAR-SIGHTEDNESS

   
   

   
   
   • If the lens of the eye does not become sufficiently thin to focus a distant object, the rays coming from the object after being refracted are focussed before reaching the retina.
     
   • As the lens remains thick, it can focus nearby objects perfectly on the retina. Therefore the defect is called nearsightedness.
     
   • This defect arises due to excessive convergence of the light rays.
     
   • A concave lens of appropriate focal length corrects this defect.
     

   
   

   FAR-SIGHTEDNESS

   
   

   
   
   • If the lens of the eye does not become sufficiently thick to focus a nearby object, the rays coming from the object, after being refracted are focussed behind the retina.
     
   • As the lens remains thin, it can focus distant objects perfectly on the retina. Therefore the defect is called far-sightedness.
     
     
   • This defect arises due to less convergence of the light rays.
   
   
   
   • A convex lens of appropriate focal length corrects this defect.
   
   

   
   
   

   
   
6. EXPLAIN : COMPOUND MICROSCOPE
   

   ANS:
   

   
   
   • An instrument using two convex lenses to obtain the magnified image of a small object is called COMPOUND MICROSCOPE.
     
   • The lens towards the object is called objective lens whereas the lens towards the eye is called eyepiece.
     
   • The focal length of the objective is smaller than that of the eyepiece.
     

   
   
   

   WORKING

   
   

   
   
   • The object to be observed(AB) is placed at a distance slightly greater than f_o (focal length of objective lens).
     
   • A magnified, real and inverted image(A'B') is formed on the other side of the objective lens.
     
   • A'B'(image of AB) works as an object for eyepiece.
     
   • The position of the eyepiece is so adjusted that A'B' lies within its focal length(f_e).
     
   • The eyepiece now acts as simple microscope and forms a highly magnified, virtual and erect image A"B".
     

   
   

   
   
7. EXPLAIN : ASTRONOMICAL TELESCOPE
   

   ANS:
   

   
   
   • Astronomical telescope is used to observe far away heavenly objects like planets and stars which appear very small and closer to one another because of their great distances.
     
   • An astronomical telescope consists of two convex lenses arranged coaxially.
     
   • The lens towards the far away object is called objective lens and that near the eye is called eyepiece.
     
   • The focal length of the objective lens is larger than that of the eyepiece.
     

   
   
   

   WORKING

   
   

   
   
   • When the telescope is focussed on a distant object, parallel rays coming from the object form a real, inverted and small image A₁B₁.
     
   • This image (A₁B₁) acts as an object for the eyepiece.
     
   • The eyepiece can be moved to and fro in such a way that virtual and magnified image A₂B₂ of A₁B₁ is formed.
     
   • The final image A₂B₂ formed by the telescope is inverted (compared to object).
     
   • Thus, far away objects can be seen as magnified image.
     

   
   

   
   
8. DISTINGUISH BETWEEN NEAR-SIGHTEDNESS AND FAR-SIGHTEDNESS.
   

   ANS: