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MCQs

  1. An achromatic doublet is constructed by lenses of crown glass and flint glass of dispersive powers 0.05 and 0.03. If the focal length of the crown glass is 20 cm, the focal length of the doublet becomes,
    a. – 60 cm                      
    b. – 30 cm                  
    c. 12 cm                      
    d. – 12 cm
  2. The power produced by a prism of small angle A
    a.   depends on materials of prism only
    b.   depends on A only
    c.   depends both on material and A
    d.   depends neither on material nor on A
  3. An impure spectrum of Green, Blue, Orange and Yellow is observed on a screen when light passes through a transparent prism. The refractive index of the prism with respect to the colours will be maximum for
    a. Blue                
    b. Green                     
    c. Orange                    
    d. Yellow
  4. To make an achromatic combination a convex lens of focal length 42 cm having dispersive power 0.14 is placed in contact with a concave lens of dispersive power 0.21. The length of the concave lens should be
    a. 14 cm             
    b. 21 cm                     
    c. 63 cm                      
    d. 42 cm
  5. A beam of light composed of red and green ray is incident obliquely at a point on the face of rectangular glass slab. When coming out on the opposite parallel face, the red and green ray emerge from
    a.   Two points propagating in two different non parallel directions
    b.   Two points propagating in two parallel directions
    c.   One-point propagating in two different directions
    d.   One-point propagating in the same directions
  6. The focal length of a converging lens is measured for violet, green and red colours. It is respectively fv, fg and fr. We will get
    a. fv < fr                          
    b. fg > fr                      
    c. f­v­ = fg                     
    d. fg = fr
  7. The blue colour of the sky is due to the phenomenon of
    a. Scattering                   
    b. Dispersion              
    c. Reflection               
    d. Refraction
  8. Rainbow is formed due to
    a.   Scattering and refraction
    b.   Total internal reflection and dispersion
    c.   Reflection only
    d.   Refraction and dispersion
  9. An achromatic lens having a focal length of + 20 cm is to be constructed by combining a crown lens with a flint glass lens. What must be the focal length of the component lenses if the dispersive power of the crown and flint glasses are 0.0158 and 0.0324 respectively?
    a. – 21 cm and 10.2 cm                          
    b. – 42 cm and 52.62 cm
    c. – 7 cm and 41.25 cm                          
    d. – 21 cm and 7.58 cm
  10. Deviation by a prism is lowest for
    a. Violet ray                   
    b. Green ray               
    c. Red ray                   
    d. Yellow ray
Q1
CONCEPTUAL PROBLEMS
  1. When white light dispersed by a prism, red light appears at the top of the spectrum whereas violet at the bottom. Why?
  2. Why does the sky appear blue in a day?
  3. How do the colours of light related with the refractive index of a lens? Explain.
  4. Define dispersive power of a lens and write expression for it in terms of the refractive indices of lens with respect to different colours used.
  5. How does the refractive index relate with the wavelength of light?
  6. A spherical mirror cannot give rise to chromatic aberration. Why?
  7. The sun looks red at sun – rise and sun – set. Why?
  8. If a plane glass slab is placed on letter of different colours, then violet-coloured letters appear more and more raised up, why?
Show answer
Correct answer: (B)
Q2
THEORY BASED PROBLEMS
  1. What is chromatic aberration in lens? Deduce the condition for achromatism in two thin lenses in contact.
  2. What is chromatic aberration? Show that for a lens, the chromatic aberration is the product of dispersive power and focal length of mean light.
Show answer
Correct answer: (A)

Numerical problems

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