Introduction

Light and the Ray Model

Light is the part of the electromagnetic spectrum that our eyes can detect. It lies in a narrow wavelength range: from about $400\ \text{nm}$ to $750\ \text{nm}$. This is what allows us to see and understand the world.

From everyday experience, we know two things about light:

• It travels at an enormous speed.
• It travels in straight lines.

Speed of Light

The speed of light in vacuum is finite and measurable. Its accepted value is:

For most calculations, we use the approximate value:

This is the highest speed attainable in nature.

Reconciling Waves with Straight-Line Travel

In Chapter 8, you learned that light is an electromagnetic wave. But how can a wave travel in a straight line? The answer lies in the wavelength of light.

The wavelength of visible light is very small (about $400\ \text{nm}$ to $750\ \text{nm}$) compared to the size of everyday objects (a few cm or larger). Because of this, the wave nature of light does not cause noticeable bending (diffraction) around ordinary objects. Therefore, we can treat light as traveling along a straight line from one point to another.

Ray and Beam of Light

• Ray of light: The straight-line path along which light travels.
• Beam of light: A bundle of such rays.

What We Study in This Chapter

Using the ray picture of light, we will study:

• Reflection (bouncing off surfaces)
• Refraction (bending when entering a medium)
• Dispersion (splitting into colors)

We will apply the basic laws of reflection and refraction to understand image formation by:

• Plane and spherical reflecting surfaces (mirrors)
• Plane and spherical refracting surfaces (lenses)

Finally, we will describe the construction and working of important optical instruments, including the human eye.

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