We then go on to describe the construction and working of some important optical instruments, including the human eye. Using the basic laws of reflection and refraction, we shall study the image formation by plane and spherical reflecting and refracting surfaces. In this chapter, we consider the phenomena of reflection, refraction and dispersion of light, using the ray picture of light. The path is called a ray of light, and a bundle of such rays constitutes a beam of light. In this situation, a light wave can be considered to travel from one point to another, along a straight line joining them. How to reconcile the two facts? The answer is that the wavelength of light is very small compared to the size of ordinary objects that we encounter commonly (generally of the order of a few cm or larger). The intuitive notion that light travels in a straight line seems to contradict that light is an electromagnetic wave of wavelength belonging to the visible part of the spectrum. The speed of light in vacuum is the highest speed attainable in nature. For many purposes, it suffices to take c = 3 × 10 8 ms –1. Its presently accepted value in vacuum is c = 2.99792458 × 10 8 m s –1. It took some time for people to realise that the speed of light is finite and measurable. First, that it travels with enormous speed and second, that it travels in a straight line. There are two things that we can intuitively mention about light from common experience. It is mainly through light and the sense of vision that we know and interpret the world around us. ![]() Electromagnetic radiation belonging to this region of the spectrum (wavelength of about 400 nm to 750 nm) is called light. ![]() Nature has endowed the human eye (retina) with the sensitivity to detect electromagnetic waves within a small range of the electromagnetic spectrum.
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