CRITICAL ANGLE

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.

CONDITIONS FOR TOTAL INTERNAL REFLECTION. CLASS 10

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. 

Total internal reflection (ICSE CLASS 10)

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.

Characteristics of non contact forces. (ICSE CLASS 9)

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.

Atmospheric pressure and blood pressure

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.

Efficiency of single movable pulley is always less than 100%. Give reason.

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

Mechanical advantage of a block and tackle system increases on increasing the number of pulleys. Give reason

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