PHYSICS

The angle of incidence of a ray in a denser medium for which the angle of refraction in rare medium is 90°, is called Critical angle for those pair of media.  The critical angle depends on  i) the density of medium. ii) the colour of the light.

1. The light ray should travel from denser to rarer medium. 2. The angle of incidence in the denser medium should be greater than the critical angle for the given pair of media. 

When a ray of light travels from denser to rarer medium at the angle of incidence greater than critical angle for the pair of media, the ray gets reflected back to the denser medium without any loss of energy. This phenomenon is called total INTERNAL reflection.

The properties of non contact forces( gravitational,  electrostatic and magnetic forces) are as follows, 1. They are central forces. It means the non contact force acts along the line joining the centres of two bodies. 2. They follow inverse square law. It means a non contact force is inversely proportional to the square of the distance. 3. They can both be attractive and repulsive.

We don't feel the effect of enormous atmospheric pressure. Why? We don't feel the effect because the pressure due to the blood and other fluids in our body balance it.

The efficiency is less than 100% because of the weight of the pulley and the friction in its axle.

Mechanical advantage of the block and tackle system of pulleys increases because the number of strands balancing the load increases.