An object placed at a distance of 24 cm from a concave mirror forms an image at a distance of 12 cm from the mirror. If the object is moved with a speed of $12 \mathrm{~ms}^{-1}$, then the speed of the image is

When the object and the screen are 90 cm apart, it is observed that a clear image is formed on the screen when a convex lens is placed at two positions separated by 30 cm between the object and the screen. The focal length of the lens is

When a monochromatic light is incident on a surface separating two media, both the reflected and refracted lights have the same

The power of a thin convex lens placed in air is +4 D . The refractive index of the material of the convex lens is $\frac{3}{2}$. If this convex lens is immersed in a liquid of refractive index $\frac{5}{3}$, then