Light reflection and refraction Worksheet-9

Light reflection and refraction Worksheet-9

 

  1. An object is placed at a distance or 50 cm from a concave lens of focal length 30 cm. Find the nature and position of the image.

 

  1. An object of height 2 cm is placed at a distance of 15 cm in front of a concave lens of power –10 dioptre. Find the size of the image.

 

  1. A thin lens has a focal length of –25 cm. What is the power of the lens ? Is it convex or concave ?

 

  1. The power of lens is 2-5 D. What is its focal length?

 

  1. A convergent lens of power 8 D is combined with a divergent lens of power –10 D. Calculate focal length of the combination.

 

  1. A concave lens is kept in contact with a convex lens of focal length 20 cm. The combination works as a converging lens of focal length 100 cm. Calculate power of concave lens.

 

  1. Find the focal length and nature of lens which should be placed contact with a lens of focal length 10 cm  so that the power of the combination becomes 5 dioptre.    

 

  1. The radius of curvature of a spherical mirror is 20 cm. What is its focal length ?

 

  1. Name a mirror that can give an erect and enlarged image of an object.

 

  1. Find the focal length of a convex mirror whose radius of curvature is 32 cm.

 

Answer:

  1. Here,  = –50 cm,   f = –30 cm,  v = ?

As  

         

          –18.7 cm

As v is negative, the image must be virtual and erect.

 

  1. Here,  h1 = 2cm, u = –15 cm

          P = –10D, h2 = ?

Now,

As    

∴    

         

            As v is negative, image is virtual

As    

∴    

          h2 = 0.8 cm. As h2 is positive, image is erect.

 

  1. Here, 

From  

Negative sign shows that lens is concave.

 

  1. Here,   

∴    

          = 40 cm.

 

  1. Here,  P1 = 8 D, P2 = –10 D, f = ?

As     P = P1 + P2 = 8 – 10 = –2D

∴     –0.5 m

 

  1. Here, f1 = 20 cm,

         

          F = 100 cm,  

As     P1 + P2 = P,  ∴ P2 = P – P1 = 1 – 5 = –4 D

 

  1. Here, f1 = ?, f2 = 10 cm,

         

          P = 5D.

As     P1 + P2 = P

∴     P1 + 10 = 5  or  P1 = 5 – 10 = – 5D.

∴     –20 cm.

Lens must be concave.

 

  1. Here, R = 20 cm,  f = ?

As               ∴

 

  1. A concave mirror gives an erect and enlarged image of an object held between pole and principal focus of the mirror.

 

  1. Here, focal length, f = ?

radius of curvature,

As