10. LIGHT – REFLECTION AND REFRACTION
Optics is the study of behaviour and properties of light.
An object reflects light that falls on it. Eyes receive the reflected light and thus we see that object.
We see through a transparent medium as light is transmitted through it.
Light seems to travel in straight lines. A small source of light casts a sharp shadow of an opaque object. It is usually indicated as a ray of light.
If there is very small opaque object on the path of light, it shows a tendency to bend around it. This effect is called diffraction. Here, the straight-line concept fails.
Such phenomena can be explained by considering light as a wave. But Wave theory often becomes inadequate for treatment of the interaction of light with matter, and light often behaves like a stream of particles.
Quantum theory says that light is neither a ‘wave’ nor a ‘particle’. It reconciles the particle properties with the wave nature.
Laws of reflection of light:
(i) Angle of incidence is equal to angle of reflection.
(ii) The incident ray, the normal to the mirror at the point of incidence and reflected ray, all lie in same plane.
These laws are applicable to all types of reflecting surfaces including spherical surfaces.
A highly polished surface, such as a mirror, reflects most of the light falling on it.
Image formed by a plane mirror is virtual and erect. The size of the image is equal to that of the object. The image formed is as far behind the mirror as the object is in front of it. Further, the image is laterally inverted.
Image formation by curved mirror:
Consider the curved surface of a large shining spoon is a curved mirror. The image at its concave side is real, inverted and diminished. If the spoon is moved away, image becomes smaller.
Image at the convex side is virtual, erect & diminished. If the spoon is moved away, image becomes smaller.
The most commonly used type of curved mirror is the spherical mirror.