To obtain a sharp image of the building instead of the window pane, Shoba will have to move the screen slightly towards the lens. The focal length of this lens will be 15 cm approximately.

Yes, a convex lens of focal length 20 cm can produce a magnified virtual as well as real image. The statement is correct. For magnified virtual image, the object must be held at a distance less than 20 cm from the lens. For magnified real image, the object must be held at a distance between 20 cm and 40 cm from the lens.

\(^{ g }{ n }_{ d }=1.6.{ n }_{ g }=1.5,{ n }_{ d }=?\\ ^{ g }{ n }_{ d }=\frac { { n }_{ d } }{ { n }_{ g } } \Rightarrow 1.6-\frac { { n }_{ d } }{ 1.5 } \\ or\quad { n }_{ d }=1.6\times 1.5=2.40\)

We know that \(n=\frac { c }{ v } \), where 'c' is velocity of light in vaccum and 'v' is the velocity of the light in the medium. If v₁ is velocity of light in a medium of refractive index n₁ and v₂ is velocity of light in another medium of refractive index n₂, then
\({ n }_{ 1 }=\frac { c }{ { v }_{ 1 } } \quad and\quad { n }_{ 2 }=\frac { c }{ { v }_{ 2 } } \\ ^{ 1 }{ n }_{ 2 }=\frac { { n }_{ 2 } }{ { n }_{ 1 } } =\frac { { c/v }_{ 2 } }{ { c/v }_{ 1 } } =\frac { { v }_{ 1 } }{ { v }_{ 2 } } \)

When a rectangular glass slab is immersed in any medium, the say incident on the slab suffers two refractions and emerges parallel to it but displaced / shifted laterally as shown in the figure. This is because deviation suffered by the ray in first refraction at one face of slab is equal to deviation suffered by the way in 2nd refraction at the other face of the slab.