If the far point of a short sighted person is 400 cm , then the power of the lens required to enable him to see very distant objects clearly is

In Young's double slit experiment, the wavelengths of red and blue lights used are $7.5 \times 10^{-5} \mathrm{~cm}$ and $5 \times 10^{-5} \mathrm{~cm}$ respectively. If $n$th bright fringe of red color coincides with $(n+1)$ th bright fringe of blue colour, then the value of ' $n$ ' is

The force between two point charges kept with a separation of 9 cm in air is 98 N . If a dielectric slab of constant 4, thickness 6 cm and another dielectric slab of constant 9 , thickness 3 cm are introduced between the two charges, then the new force becomes

Three point charges shown in the figure lie along a straight line. The energy required to exchange the position of central charge with one of the negative charges is