Geometrical Optics and Reflection of Light

Geometrical optics is also known as ray optics. Using the term rays geometrical optics approximate the paths light travels. Rays are used to define the movement of light depending on the environments and mediums around it. Light rays travel in a straight line until they hit a medium. Depending on the medium, the light rays may be absorbed or reflected, bend or divide or even curve if the refractive index of a medium varies (such as water).

Reflection of Light

Reflection is the process by which a light ray hits a medium and then bounces or rebounds off. The amount of reflected light depends on:

• a. The nature of the medium the light ray propagates through
• b. The color of the light ray
• c. The angle at which the light ray strikes the surface of the boundary.

Some facts about the reflection of light:

• The light ray that hits the medium is called incident ray and the ray that bounces off is termed reflected ray.
• When light falls on a plane mirror (flat mirror), about 80% of the incident light is reflected.
• A black or opaque body is a poor reflector while a white body is a good reflector.
• From a highly polished surface, nearly all the light is reflected.

Regular Reflection: (Specular reflection) is defined as reflection of light from a smooth surface.

Irregular Reflection: (Diffuse Reflection) Light rays reflecting from a rough or uneven surface is called irregular reflection or diffuse reflection.

(Plane Mirror)

1. 1
   Ray of light

   :

   A ray of light is the straight line along which light travels.

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2. 2
   Incident Ray

   :

   is the ray of light which falls on the mirror surface. In the figure, mm' is the mirror and AO is the incident ray.

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3. 3
   Reflected Ray

   :

   The ray of light that bounces back from the mirror is called reflected ray. In the figure, OB is the reflected ray.

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4. 4
   Normal

   :

   The normal is the line that is perpendicular to the mirror at the point of incidence. ON is normal.

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5. 5
   Angle of Incidence

   :

   The angle between the incident ray and the normal is called incident angle. In the figure, AON is the angle of incidence, denoted by 'Li'.

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6. 6
   Angle of Reflection

   :

   The angle between the reflected ray and the normal is called reflected angle. NOB is the angle of reflection, denoted by 'Lr'.

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Laws of the Reflection of Light

There are two laws that explain the reflection of light first discovered by Ibn-Ul Haitham and written in his book Kitab al-Manazir (Book of Optics) in the early 1000's ^[1]. These laws are as follows:

• (1) The incident ray, the reflected ray and the normal (at the point of incidence) all lie in the same plane.
• (2) The angle of incidence (Li) is always equal to the angle of reflection. (Lr) mLi = mLr

Given the laws of reflection, different types of reflective mediums are used effectively for different purposes.

Mirror: is defined as a surface from which light undergoes regular reflection. It is also defined as the smooth and the polished surface of a medium from where most of light reflects. Spherical Mirror: is a mirror where either its inner or outer surface is polished to reflect light. A spherical mirror, as its name implies, has the shape of a segment of a sphere. Spherical mirrors are classified as concave and convex mirrors.

1. 1
   Concave Mirror

   :

   is a spherical mirror where the inner side of the shell is polished to reflect light. Example, the inner shining surface of a spoon.

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2. 2
   Convex Mirror

   :

   is a spherical mirror with a polished outer side. It diverges a parallel beam of light, therefore it is also called a diverging mirror. The back side of a shining spoon is an example of a convex mirror.

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Uses of Spherical Mirrors

Spherical mirrors have different uses:

1. 1
   Shaving Mirrors

   :

   Concave mirrors are used as a shaving mirror. When a person looks through this mirror, the enlarged, virtual and erect image of his face will be seen in the mirror, allowing for a clean and neat shave.

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2. 2
   Medical Use

   :

   Concave mirrors are used by doctors for the medical examination of ear, nose, and throat (ENT Doctors). They are also used by dentists to examine backside of the tooth.

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3. 3
   Microscopes

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   Concave mirrors are used in a microscope to concentrate light on a microscopic slide.

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4. 4
   Automobiles

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   Concave mirrors are used in car head lights, search lights, and spot lights.

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5. 5
   Vehicles

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   Convex mirrors are used in vehicles for rear view mirrors. It makes small, erect and virtual images of the object behind the vehicle. Convex mirrors are also posted on dangerous roads to avoid accidents especially in sharp turns on hillsides.

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