Explanation

An insulator has a completely filled valence band. 

Explanation: In an insulator, all the valence electrons are tightly bound to their atoms and cannot move freely, resulting in a filled valence band.

Since there are no electrons available in the conduction band, they cannot move to it and conduct electricity. 

Explanation

 A semiconductor has electrical conductivity between that of a conductor and an insulator, and silicon falls within this category

Explanation

The photoelectric effect best explains the particle nature of light. 

Explanation

 The direction of a vector in space is typically specified by three angles.

These angles, often referred to as azimuth, elevation, and tilt (or polar/inclination), describe the vector's orientation in 3D space relative to a chosen reference frame. 

Explanation

The least distance of distinct vision for a normal human eye is 25 cm.

This means that when an object is placed at a distance of 25 cm or more from the eye, a clear and distinct image is formed on the retina.