Optics 3 Question 4
4. Formation of real image using a biconvex lens is shown below. If the whole set up is immersed in water without disturbing the object and the screen positions, what will one observe on the screen?
(2019 Main, 12 Jan II)
(a) No Change
(c) Image disappears
(b) Magnified image
(d) Erect real image
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Answer:
Correct Answer: 4. (d)
Solution:
- When given set up is immersed in water, focal length of lens increases,
Note $\quad$ Now, $n _{g a}=\frac{3}{2}$ and $n _{l a}=\frac{4}{3}$
$$ \therefore f _{\text {liquid }}=f _{\text {air }} \frac{\frac{1}{2}}{\frac{1}{8}}=4 f _{\text {air }} $$
As focal length increases there is no focussing of image on screen
$\therefore$ image will disappear
Actually a virtual image is formed on same side of object.
