In geometrical optics we analyze the formation of images by assuming the light to travel in a straight line path or rays. For this reason geometrical optics is also known as ray optics.
Ray of Light:
A ray of light gives the direction of propagation of light. When light meets a surface, which separates the two media, reflection and refraction take place. An image or an array of images may be formed due to this.
Beam of light:
A beam of light is a bunch of light rays. It may be convergent, divergent or parallel.
- A convergent beam is that in which the rays are directed towards a point.
- A divergent beam is that in which the light rays are directed away from a point.
- In a parallel beam of light all the ray are parallel to each other.
Object and image
The point of intersection of the incident rays is called object and the point of intersection of the corresponding reflected / refracted rays is called image.
Real Object:
If the incident rays diverge from a point, it is called a real object.
Virtual Object:
If the incident rays converge and appear to intersect at point behind the optical device (mirror, lens etc.), the point is virtual object.
Real Image:
If the reflected or refracted rays actually intersect at point, the point is called real image.
Virtual image:
If the reflected or refracted rays do not actually intersect at point, but appear to come from a point, the point is called a virtual image.
Note:
we cannot distinguish between real and virtual image just by observing it. Only difference is that, a real image can be taken on screen, where as a virtual cannot be formed on a screen.
Note:
In most of the problems in this section our primary goal is to locate the final image formed by certain optical systems for a given object. The optical system may be just a mirror, or a lens or a combination of several reflecting and refracting surfaces. In such problems, first of all, we identify the sequence in which the reflection and refraction are taking place.
All cases of reflections and refractions are governed by ‘laws of reflection’ and ‘laws of refraction’ respectively.
BASIC LAWS of Geometrical Optics:
- Law of Rectilinear Propagation of Light states that “light propagates in straight line in a homogeneous medium”.
- Law of Independence of Light Rays states that “ray do not disturb each other upon intersection”.
- Law of Reversibility of Light Rays states that “when a ray of light is moved in opposite direction after going through a number of reflections and refractions, it retraces the path through which it was incident”.
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