Some Preliminaries What is modern physics? How does it differ from, and in what ways is it similar to, classical physics?" What central ideas of classical physics are carried over into twentieth-century physics, in which one encounters the very small and the very fast? Which of the classical ideas remain unchanged, and which must be modified or replaced? These questions and other important ones are dealt with in this introductory chapter.

THE PROGRAM OF PHYSICS

THE CONSERVATION LAWS OF PHYSICS The conservation laws of classical physics : 1.conservation of mass 2. conservation of energy 3. conservation of linear momentum 4.conservation of angular momentum, 5.conservation of electric charge.

The Law of Mass Conservation: The total mass of an isolated, or leakproof, system is constant. Mass cannot be created or destroyed

The Law of Energy Conservation: If no work is done on or by a system, and if no thermal energy enters or leaves the system as heat, the total energy of the system is constant.

The Law of Linear-momentum Conservation: When a system is subject to no net external force, the total linear momentum of the system remains constant both in magnitude and direction.

The law of Angular-momentum Conservation: When a system is subject to no net external torque, the total angular momentum of the system remains constant, both in magnitude and direction.

The Law of Electric-charge Conservation: The total charge of an isolated electrical system is constant.

THE CLASSICAL INTERACTIONS There are only two basic origins of force in classical physics: Gravitational mass  The universal gravitational force Electric charge; these give rise  electromagnetic forces

The forces between electric charges may be separated into two basic types: Electric force Magnetic force: The magnetic force arises when two electric point charges are in motion with respect to the observer.

ELECTROMAGNETIC FFELDS AND WAVES The electric-energy density The magnetic energy density

A changing electric field creates a magnetic field (Ampere‘s law), and a changing magnetic field creates an electric field (Faraday's law). This is the origin of electromagnetic waves; an oscillating, and therefore accelerating, electric charge produces in space electric and magnetic fields, whose frequencies are the same as that of the electric charge. Electromagnetic field travels through space at the peed of light c:

The instantaneous intensity I of an electromagnetic wave, the energy flow per unit time, through a unit area oriented at right angles to the direction of wave propagation Electromagnetic waves were first observed in the laboratory by Heinrich Hertz in 1883.

When an electromagnetic wave is absorbed by the electrically charged particles in a material, the electric field does work on the charged particles and produces a magnetic force on them in the direction of the propagation of the wave. The radiation force F, is given by where P is the power of the absorbed wave.

THE CORRESPONDENCE PRINCIPLE Any theory or law in physics is, to a greater or lesser degree, tentative and approximate. We know in advance that any new theory in physics, whatever its character or details, must reduce to the well- established classical theory to which it corresponds when it is applied to the circumstances for which the less general theory is known to hold.

Projectile motion (1) The weight of the projectile is constant in magnitude (2) the earth may be represented by a plane surface (3) the weight of the projectile is constant in direction, vertically downward Satellite motion (1) the weight of the body is not constant in magnitude but varies inversely with the square of its distance from the earth's center, (2) the earth's surface is round, not flat, (3) the direction of the weight is not constant but always points toward the earth's center.

Now, if we apply the second, more general, theory to the motion of a body traveling a distance small compared to the earth's radius at the surface of the earth, notice what happens: The weight appears to be constant both in magnitude and direction, the earth appears flat, and the elliptical path becomes parabolic. This is precisely what the correspondence principle requires

RAY OPTICS AND WAVE OPTICS Ray (geometrical) optics: rectilinear propagation, reflection, and refraction of light Wave(physical) optics: interference and diffraction