Question

29.(i) Define focal length of a spherical lens.

(ii) A divergent lens has a focal length of 30 cm. At what distance should an

object of height 5 cm from the optical centre of the lens be placed so that its image

is formed 15 cm away from the lens? Find the size of the image also.

(ii) Draw a ray diagram to show the formation of image in the above situation.​

Answer 1

I) Define focal length of a spherical lens.

Focal length of a spherical lens is the distance between optical centre and focus of the lens. The Focal length of a lens is half of its radius of curvature.

ii) A divergent lens has a focal length of 30 cm. At what distance should an

object of height 5 cm from the optical centre of the lens be placed so that its image

is formed 15 cm away from the lens? Find the size of the image also.

Substitute, v=−15cm, u=−30cm and h1=5cm to find the value of h2. Therefore, the object should be placed 30 cm from the optical centre and the size of the image formed is 2.5 cm. Additional information: The distance between focal point and lens is called focal length.

Answer 2

Answer:

i)Focal length of a spherical lens is the distance between optical centre and focus of the lens. The Focal length of a lens is half of its radius of curvature

ii)We know that for a divergent lens both u and f are -ive , from lens equation

v= uf/u+f , as both u and f are -ive therefore v will be -ive ,

given f=−30cm,v=−15cm ,

so u= fv/f−v

= 30×−15/−30−(−15) =−30cm ,

now linear magnification m=I/O=v/u ,

given height of object O=5cm,I=? ,

therefore I/5=15/30 ,

or I=5×15/30=2.5cm

hope it helps you mate

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