A point source of light S is placed on the axis of a lens of focal length 20 cm as shown. A screen is placed normal to the axis of lens at a distance x from it. Then INCORRECT statement.

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As x is increased from zero, intensity continuously decreases.

As x is increased from zero, intensity first increases then decreases.

Intensity variation at the centre of the screen from (x = 90 cm to x = 100 cm) and (x = 100 cm and x = 110 cm) is same.

Radius of the bright circle obtained on the screen is equal to 4/5 cm for x = 20 cm.

answer is A.

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Detailed Solution

1v−1u=1f⇒1v⇒1v−1(−25)=1201v=120−125=1100⇒V=100 cmFrom O to I intensity increases and then decreases at x = 90 cm and 110 cm intensity is same (d) Radius at x = 20 cm is equal to radius of lens.

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