Ray Optics and Optical InstrumentsIntroduction
It is a sleepy summer afternoon at Riya's grandmother's house. The wooden shutters are closed against the heat, but sunlight leaks in through the gaps, and the dust in the air turns every leak into a sharp, straight beam that runs all the way to the floor. Light, it seems, travels in straight lines.
Our eyes, or rather the retina at the back of them, can sense electromagnetic waves only in a narrow band of the spectrum, with wavelengths from about 400 nm to 750 nm. Radiation in this band is what we call light, and most of what we know about the world around us reaches us through it.
Two things about light are familiar from everyday life: it is enormously fast, and it travels in straight lines. Its speed in vacuum is now known to be c = 2.99792458 × 10⁸ m/s; for most purposes c = 3 × 10⁸ m/s is good enough. Nothing in nature travels faster.
The straight lines are more puzzling. In Chapter 8 you met light as an electromagnetic wave, and waves are known to spread. The way out is size: the wavelength of light is tiny compared with ordinary objects. Here green light (λ = 0.5 μm) passes through a gap of width a.
Change the width of the gap and watch how much the light spreads.
gap width a =
The blue curve falls steeply: once the gap is a few dozen wavelengths wide, the light spreads by less than a degree. A window, a coin or a lens is a few centimetres across or more, tens of thousands of wavelengths, so for them a light wave can be taken to travel from one point to another along the straight line joining them (Chapter 10 explains why). That path is called a ray of light, and a bundle of rays is a beam.
Fast, and in Straight Lines
Using the ray picture we can follow light as it is reflected, refracted and split into colours, work out where mirrors and lenses form images, and see how microscopes and telescopes are built. First, a feel for the speed.
The Sun is about 1.5 × 10¹¹ m away. How long does its light take to reach us?
Time = distance ÷ speed = 1.5 × 10¹¹ m ÷ 3 × 10⁸ m/s = 500 s, a little over 8 minutes. The sunlight you see left the Sun 8 minutes ago.
In this chapter you will
- use the laws of reflection and refraction to find images formed by spherical mirrors, spherical surfaces and thin lenses,
- see why light can be trapped inside glass by total internal reflection, and how optical fibres use it,
- follow a ray through a prism and find its minimum deviation,
- build a microscope and a telescope and work out how much they magnify.