31. A convex lens of focal length 80 cm and a concave lens of focal length 50 cm are combined together. What will be their resulting power ?

30. If $f_V$ and $f_R$ are the focal lengths of a convex lens for violet and red light respectively and $F_V$ and $F_R$ are the focal lengths of concave lens for violet and red light respectively, then we have ?

13. Light travels through a glass plate of thickness t and refractive index μ. If c is the speed of light in vacuum, the time taken by light to travel this thickness of glass is ?

12. One face of a rectangular glass plate 6 cm thick is silvered. An object held 8 cm in front of the first face forms an image 12 cm behind the silvered face. The refractive index of the glass is ?

Intensity of an electric field (E) depends on distance r, due to a dipole, is related as ?

A point charge +q is placed at mid point of a cube of side ’L’. The electric flux emerging from the cube is ?