An object is placed 30 cm in front of a convex lens of focal length 20 cm — JEE Physics
An object is placed 30 cm in front of a convex lens of focal length 20 cm. Find the image distance, magnification, and nature of the image.
1 Answer
VAVidaara Admin
✓ Vidaara Team
✓ Accepted
· 2mo ago
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Using $\dfrac{1}{v} - \dfrac{1}{u} = \dfrac{1}{f}$, with $u = -30$ cm:
$$\frac{1}{v} = \frac{1}{20} + \frac{1}{-30} = \frac{1}{20} - \frac{1}{30} = \frac{3-2}{60} = \frac{1}{60}$$
$$v = +60 cm \quad \Rightarrow Real, inverted image$$
$$m = \frac{v}{u} = \frac{60}{-30} = -2 (inverted, magnified)$$
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Discussion (4)
KK
Is there a faster shortcut for this in the actual exam? Time is tight.
AR
Quick doubt: would this method still work if the numbers were not so clean?
E
Why do we take the positive value only in the last step?
MT
Plotting it roughly also helps you sanity-check the sign.