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  • Lenses are objects made of transparent materials such as glass or clear plastic that have curved surfaces. Diverging lenses are thicker at their edges than at their centres and make light rays pass

Lenses are objects made of transparent materials such as glass or clear plastic that have curved surfaces. Diverging lenses are thicker at their edges than at their centres and make light rays passing through them spread out. Converging lenses are thicker in their middle than at these edges and make light rays passing through them focus at a point. These are used in spectacles to help people with poor vision see better. The converging lenses magnify by bending the rays of light that pass through them to meet at a point called focus. The thicker the converging lens is at its centre, the more it magnifies and the closer the focus is to the lens. (i) Ravi uses two lenses A and B of the same size and same material as shown. P1 and P2 are the powers of A and B. An object is kept at the same distance from the lens between F and 2F of each lens on the principal axis in turn. Let I1 and I2 be the image formed by two lenses respectively. What is the relation of image distances of both lenses? (ii) Write down the relation between the power of the lens of both lenses. (iii) Meenakshi uses the above two lenses A and B along with another two lenses C and D, as shown

She can see the subject matter on the black board while sitting in the front row of the classroom but is unable to see the same matter while sitting in the last row. Which of the above four lenses will she require to correct the defect in her vision? Why?

OR

(iv) Natasha places an object on the principal axis of the above-given lens A. One end of this object coincides with the focus F and the other end with 2F. What will be the nature of the image formed by the lens on the other side?

Answers (1)

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(i) relation between image distances of both lenses

  • Given: Two lenses A and B have the same size and same material, so they have same focal lengths.

  • The object is placed between F and 2F (i.e., focal length and twice the focal length).

  • This is a converging lens situation.

Answer:
Since both lenses are identical and the object is placed at the same distance, I1=I2

(ii) Write the relation between the power of both lenses

  • Power PPP of a lens depends on the focal length f:

P= 100 /f (in cm)

Answer:
Since both lenses A and B have the same focal length (same material and size),

P1=P2

(iii) Which lens will Meenakshi require and why?

  • Problem: Meenakshi can see clearly from front row but not from the last row → She has myopia (nearsightedness).

  • For myopia, a diverging lens (concave) is used to spread the light rays before they enter the eye.

 Answer:

Meenakshi will require a diverging lens (lens B or D)

→ Because diverging lenses help people with myopia to see distant objects clearly, like the blackboard from the last row.

(iv) What is the nature of the image if object is between F and 2F?

  • The object lies from F to 2F in a converging lens.

  • In this case, the image formed is:

    • Real

    • Inverted

    • Magnified

    • Formed beyond 2F on the other side

Answer:

The image is real, inverted, and magnified. It forms beyond 2F.

 

Posted by

Saumya Singh

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