Spherical Surfaces Refraction

A source of light may be considered to give out energy either as a stream of particles called photons or as a continuous stream of energy along a ray or as wave motion in the medium. The concept of light particles such as photons is of importance in the study of the interaction of light with matter as in the case of photoelectric effect.

However, as far as propagation of light through a medium is concerned it is more convenient to suppose that light energy is propagated as wave motion. From the point of view of wave motion, a ray can be defined as an imaginary line drawn in the direction in which the wave is travelling. In the case of reflection and refraction, the light energy is propagated along the wave normals. Defining rays as wave normals, it is easy to conceive that behaves as a stream of energy propagated along the direction of the rays. The rays are directed outwards from the source and they obey the laws of reflection and refraction. Therefore, the ray concept is of great importance in the study of geometrical optics.

The processes involved in geometrical optics are only reflection and refraction. The section on geometrical optics refers only to the study of light rays undergoing reflection and refraction in optical instruments. Here, one is mainly concerned with the dominant direction of the ray and other considerations (e.g. diffraction, interference) are not taken into account. In a pin hole camera, the geometrical optics deals with the formation of a well defined image only and does not take into account the interference and diffraction phenomena.

This is the reason why an optimum size of the pin hole (0.035 cm) is necessary in a pin hole camera to obtain a well defined image. Well defined image is not formed when the size of the pin hole is too large because for each point of the object, there will be a corresponding patch in the image due to large number of rays. The overlapping of the patches results in a blurred image when the size of the hole is comparable to the wavelength of light, the diffraction effects become prominent and the ray concept does not hold good.

Thus, the geometrical treatment is sufficient as long as the surfaces and other discontinuities encountered by the light wave during its propagation are very large as compared to the wavelength of light. As long as this condition is satisfied, the geometrical treatment concerning reflection and refraction is equally applicable to light waves, ultrasonic waves, earthquake waves etc.

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