Which one of the following is the best shape of a solid metal rod to form the top end of a lightning conductor?

Which of the following forces is/are fundamental in nature?

1. Gravitational force

2. Electromagnetic forces

3. Strong and weak nuclear forces

Select the correct answer using the code given below :

A spherical shell of outer radius R and inner radius R/2 contains a solid sphere of radius R/2 (see figure). The density of the material of the solid sphere is ρ and that of the shell is ρ/2. What is the average mass density of the larger sphere thus formed?

The human eye is like a camera that has a lens with :