Geometrical optics  The Sun is our natural source of light.  Objects such as the sun, a light bulb, a fire, which give off their own light are called luminous.  Objects which can only be seen when light falls on them are called non-luminous objects.

 Geometrical optics is the study of how light rays interact with matter.  In order to understand how light behaves it is necessary to understand some of the properties of light. Light travels in straight lines.

Light rays, beams.  The fact that light travels in straight lines explains the formation of shadows as well as the fact that we are unable to see around corners.  In our study of optics we represent a ray of light as an arrow.  Several arrows drawn together represent a beam of light. Parallel Beam Converging Beam Diverging Beam

Reflection of Light  When light strikes an object some of the light is absorbed and some bounces back.  The bouncing of light off an object is known as reflection.  The reflection of light can either be regular reflection or diffuse reflection.

 Most objects reflect light in a diffuse manner which means that the light is reflected in all directions.  Such an object can be seen when viewed from any direction. Light rays Object Diffuse reflection!

 Objects which are perfectly smooth or polished will reflect light in a regular manner.  Light which is reflected in this way will bounce back from an object at one particular angle or in one particular direction.  Objects which reflect light in a regular manner are general called mirrors.  Mirrors can either be flat (plane) or curved (spherical). Light rays Object Regular reflection!

Ray diagrams.  We can use simple diagrams to explain how light is reflected, called ray diagrams.  Ray diagrams help us to describe how images are formed by both plane and spherical mirrors.

 The ray of light striking the mirror is called the incident ray  The ray of light reflected is called the reflected ray.  The point where the incident ray strikes the mirror is called the point of incidence.  A line drawn at right angles to the mirror at the point of incidence is called the normal, ‘N’.  The angle between the incident ray and the normal is called the angle of incidence, ‘i’..  The angle between the reflected ray and the normal is called the angle of reflection, ‘r’.

The Laws of Reflection When light is reflected from the surface f a plane mirror;  The incident ray, the normal and he reflected ray all lie in the same plane.  The angle of incidence ‘i’ is equal to the angle of reflection ‘r’. N i r i = r Incident ray Reflected ray

Images formed by plane mirrors.  The image formed by a plane mirror will be 1. A virtual image 2. The same distance behind the mirror as the object is in front of the mirror 3. Laterally inverted, (reversed left to right). A virtual image is one formed by the apparent intersection of light rays. Object Virtual Image Page 10

Uses of plane mirrors.  Because of ‘lateral inversion’, the practical uses of plane are limited.  Emergency vehicles will print names right to left so that when viewed in a rear view mirror they can be read properly, i.e. left to right. Plane mirrors are used to view one own reflection.

The periscope.  The periscope is an instrument used to view around obstacles.  Two plane mirrors are placed at 45º and used to reflect light in a regular manner.  Submarines, sports events.

Locating images produced by plane mirrors.  The apparent movement of one object relative to another due to the motion of the observer is called parallax.  The object which is farthest away moves with the observer.  No parallax occurs if the two objects are at the same point.  The method of no parallax is used to locate the image formed by a plane mirror.