Friday, January 9, 2015

Ray Optics Introduction and Reflection of Light

Light is a form of energy and it satisfies law of conservation of energy. Light travels like a electromagnetic wave. Human eye has a sensitivity to detect the electromagnetic waves of certain wavelength ranging from 4000 Å to 8000 Å.

Light travels with a very high velocity that is equal to 3 into 10 power 8 m/s in vacuum. The velocity of the light is maximum in vacuum and in any other medium it is less than that value. When the light falls on the objects whose size is much larger than that of the wavelength of the light, it appears like a straight line. Further properties of this and its applications are called ray optics.

Optics is a known subject for us for many years and it is a part of classical physics. We are able to see any objects because sunlight falls on the body and the body absorbs some particular colors, and reflects the other colors. What is the color that the body reflects is the color we see as the color of the body.

Light is a form of energy and it exhibits a wide variety of properties. To understand all these properties of the light we shall follow different theories of light that are evolved over the time.

Initially we have Newton’s corpuscular theory according to which light consists of  tiny particles called carpuscules. The size of the particle decides the color of the light. They travel in straight lines with high velocities.

Further we have different theories like Huygen’s wave theory, Maxwell’s electromagnetic theory and Plank’s quantum theory. Each theory is successful in explaining some properties of light and failed to explain to some other properties.

Finally we have a concept of dual nature. Here we assume that light travels like a wave and interacts with objects like a particle.

A ray of light gives the direction of propagation of light. In the absence of the obstacle, light advancing straight line without changing its direction.

 Reflection of light

The phenomena of the light coming back to the same medium after striking an obstacle is called as reflection. A light ray is reflected by the smooth surface in accordance to the rules of reflection. There are two laws of reflection.

The first law is that the angle of incidence is equal to angle of reflection. Here the angle of incidence is the angle between the incident ray and the normal. The point at which the light ray strikes the surface is called point of incidence. A line drawn through the point of incidence perpendicular to the surface is called normal.

The angle between the reflected and the normal is called angle of reflection. As per the first law angle of incidence is equal to angle of reflection.

As per the second law the incident ray, the reflected ray and the normal lies in the same plane.

The angle of deviation is the angle between the original path of the light and the path taken by of the light. We can derive the equations for the as shown below.

From the diagram it is clear that the reflected ray moves away from a plain surface. It is very clear from the diagram that the reflected ray is diverging from the obstacle and it is not going to form a real image. By extending these two light rays we can identify the location of the image and this kind of the images is called virtual image. From the obstacle the distance of the object in this case is equal to the distance of the virtual image.

Spherical mirror

A spherical mirror is a part that is cut from a hollow sphere and in general made up of the glass. One surface of the glass is silvered therefore it can behave like a mirror. The reflection of the light takes place on the other surface. If the reflection takes place at the convex surface then the mirror is called a convex mirror. If the reflection of the tile light takes place at the concave surface then the mirror is called as concave mirror. There were as shown in the diagram below.


The Centre of the sphere from which the mirror is drawn is called Centre of curvature of the mirror.

The radius of the sphere is called radius of curvature of the mirror.

The point of the mirror at the middle of the surface is called as pole.

The line joining the pole and the Centre of the curvature is called principal axis.

The point where all the reflected light rays converge or from where the reflected light rays appears like diverging is called principal focus.

The distance between the principal focus and the pole of the mirror is called focal length.

The light rays that are close to the principal axis are called paraxial light rays.

The area of the spherical surface which is available for the reflection of the light is called Aperture.

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