A convex lens of power +6D and a concave lens of power -4D are combined together (in contact). Find the power of the combination of two lenses.

When an object is placed at 25 cm from a concave lens, a virtual image is produced at a distance of 10 cm. Calculate the magnification produced by the lens.

An object placed at 50 cm from a lens produces a virtual image at a distance of 10 cm in front of the lens. What is the focal length of the lens? Is it converging or diverging?

An object is placed at a distance of 12cm in front of a concave mirror. It forms a real image four times larger than the object. Calculate the distance of the image from the mirror.

The refractive index of water is 1.33 and the speed of light in air is 3 x 10⁸ms¹. Calculate the speed of light in water?