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  • a) Define the focal length of spherical lens. b) A divergent lens has a focal length of 30 cm. At what distance should an object of height 5cm from the optical center of the lens be placed so that its image is formed 15 cm away from the lens?

a) Define the focal length of spherical lens.

b) A divergent lens has a focal length of 30 cm. At what distance should an object of height 5cm from the optical center of the lens be placed so that its image is formed 15 cm away from the lens? eFind the size of the image.

c) Draw a ray diagram to show the formation of image in the above situation.

 

 

 

 
 
 
 
 

Answers (1)

a) Focal length od spherical lens is defined as between optical center and peripheral focus of lens 

b) V= -15 cm

   f = -30 cm 

   h = 5 cm 

  U = ? 

height of image (h_1)= ? 

According to the formula : - 

\frac{1}{f} = \frac{1}{v} - \frac{1}{u} \\\\ \frac{1}{-30} = \frac{1}{-15}- \frac{1}{u}\\\\ \frac{1}{u} = \frac{1}{-15}+ \frac{1}{30}\\\\ \frac{1}{u} = \frac{1}{-30}\\\\ u = -30 cm

Hence The object placed at 30 cm away from the lens magnification is 

m = \frac{v}{u} = \frac{h_1}{h_2}\\\\ 0.5 = \frac{h_1}{5}\\\\ h_ 1 = 2.5

so the height of image formed is 2.5 cm

c)

it is desired to obtain an erect image of an object using concave ...

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Priyanka Kumari

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