Question

Answer the both questions with explanation​

Answer 1

Answer:

Let the focal length of concave lens be f so focal length of convex will be -3/2 f { Ratio is given

【 Note that in books we take focal length of concave lens positive 】

Now use lens equivalence formula

$\frac{1}{30} = \frac{1}{f} - \frac{1}{ \frac{3}{2} f} = \frac{1}{3f} \\ \\ f = \frac{30}{3} = 10 \: cm \\ \\ \frac{ - 3}{2} f = - 15 \: cm$

Option 4

Now other question

Use lens formula for first image

$\frac{1}{v} = \frac{1}{f} + \frac{1}{u} = - \frac{1}{15} + \frac{1}{30} = \frac{ - 1}{30} \\ \\ v = - 3 0\: cm$

So first image forms at 20 cm inside of plane mirror so u can guess second image will form at same distance in opposite side of mirror 【think like when you see in normal mirror 】or outside mirror means it will be real image .

If you can't guess u can also use mirror formula

For plane mirror taking it's focal length infinite

$\frac{1}{v} = \frac{1}{ \infty } - \frac{1}{u} = - \frac{1}{20} \\ \\ v = - 20 \: cm$