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MCQs

  1. If in a plano – convex lens, radius of curvature of convex surface is 10 cm and the focal length is 30 cm, the refractive index of the material of the lens will be
    a.   1.5                              
    b.  1.66                       
    c.  1.33                       
    d.  3
  2. The distance between an object and a diverging lens is ‘p’ times the focal length of the lens. The lateral magnification ‘m’ produced by the lens will be
    a.   p                                 
    b.   [latex]p^{-1}[/latex]                      
    c.  1 + p                      
    d. [latex](1+p)^{-1}[/latex]
  3. The focal length of lens in air is 30 cm. Find the focal length of the lens in water (given, [latex]\mu_w = 1.33[/latex] and [latex]\mu_g = 1.5[/latex])
    a.   120 cm                       
    b.  20 cm                    
    c.  90 cm                     
    d.  60 cm
  4. If we put the object at the focus of convex lens, the refracted rays are
    a.   Converging                                        
    b.  Diverging
    c.   Both converging and diverging         
    d.  Parallel
  5. When a convex lens having focal length 12 cm is immerged in water, its focal length becomes,
    a.   12 cm                         
    b.  24 cm                    
    c.  36 cm                     
    d.  48 cm
  6. An object is placed 1 cm from lens having magnification 5. Then, its focal length is
    a.   0.8 cm                        
    b.  1.2 cm                   
    c.  – 1.2 cm                 
    d.  – 0.8 cm
  7. An object is placed 10 cm in front of a diverging lens of focal length 20 cm. The image will be
    a.   Real inverted, large                            
    b.  Real, erect, diminished
    c.   Virtual, inverted, large                       
    d.  Virtual, erect, diminished
  8. A plano-convex lens is made of refractive index 1.5. If radius of the curvature of the curved surface is 20 cm, then, focal length of lens is
    a.   10 cm                         
    b.  20 cm                    
    c.  30 cm                     
    d.  40 cm
  9. The focal length of a convex lens is f. An object is placed at a distance x from its focal points the ratio of size of real image to that of object is
    a. [latex]\frac{f}{x^2}[/latex]                                  
    b. [latex]\frac{x^2}{f}[/latex]                            
    c.  [latex]\frac{f}{x}[/latex]                              
    d.  [latex]\frac{x}{f}[/latex]
  10. A plano convex lens is made of a material of R.I. 1.6, the radius of the curvature of the curved surface is 60 cm the focal length of lens is
    a.   50 cm                         
    b.  100 cm                  
    c.  200 cm                   
    d.  400 cm
Q1
CONCEPTUAL PROBLEMS
  1. A lens is immersed in water. Is there any change in its focal length? Explain.
  2. Sun glasses have curved surfaces but their power is zero, why?
  3. Can a concave and a convex lens be combined so that the combination behaves like a diverging lens? Justify your answer.
  4. A converging lens is dipped in a liquid having refractive index greater than the lens. Does its power change? Explain.
  5. If a converging lens and a diverging lens having the same focal length be in contact, how will the combination of lenses behave?
  6. Draw the ray diagram showing the formation of real image by a concave lens.
  7. Can a concave lens form a real image? Give the condition.
  8. Draw the ray diagram showing the formation of a virtual image by a convex lens.
  9. At what distance from a convex lens should an object be placed on the axis, so as to form a real image of the same size? Illustrate your answer with a ray diagram.
  10. What is the deviation produced by a thin lens of a ray passing through its optical centre?
  11. The central portion of a lens is covered with a black paper. Will the lens form full image of an object?
  12. A convex lens of refractive index  is immersed in medium of refractive index [latex]\mu_m[/latex]. How will it behave if i) [latex]\mu_g > \mu_m[/latex]  ii)  [latex]\mu_g < \mu_m[/latex]   iii)  [latex]\mu_g = \mu_m[/latex]
Show answer
Correct answer: (C)
Q2
THEORY BASED PROBLEMS
  1. Describe a method with a suitable diagram to determine the focal length of a convex lens.
  2. Define lens maker formula. Prove that [latex]\frac{1}{f} = (\mu – 1)(\frac{1}{R_1} + \frac{1}{R_2})[/latex] where the symbols have usual meaning.
Show answer
Correct answer: (D)

Numerical problems

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