Introduction

Ray tracing is an important skill for optics engineering. It’s something you may have learned within Ontario’s grade 10 science curriculum.

Ray tracing allows you to do things like determine the position behind a lens that an image will appear based on its position in front of the lens. This allows you to do things like determine object size in image plane, figure out the appropriate focal length to use for your camera, and more.

For more complex optical applications, there’s softwares like Zemax and TracePro that do ray tracing simulations for you. But understanding the basics of ray tracing and optics is a first step to being able to work with applications like that.

Rules & Concepts

There are a few main rules and conventions that you need to understand about ray tracing before you can make a ray diagram.

• Rays can only go straight, unless they encounter a lens or a mirror.
• All objects are assumed to transmit light in all directions, even if the object is not a light source. For non-source objects, the assumption is that there’s enough ambient light such that it bounces off the object in all directions uniformly.

Terminology

• The plane on which the lens lies is called the principal plane
• The axis running through the centre of the lens, perpendicular to the principal plane, is called the principal axis
• Often times, “objects” are just represented as singular points or arrows to a point. These really represent a single point on the object that we’re looking at.
• The plane on which the object lies is called the object plane
• The plane on which the image lies is called the image plane
• The focal point is a point at which all light rays parallel to the principal axis either converge to or diverge from (depending on whether the lens is concave or convex)

• Lenses have a focal point on either side - if light comes in from the left, it gets focused onto the right focal point. If light comes in from the right, it gets focused onto the left focal point

Mirrors

• Rays must always reflect off mirrors such that the angle between an incident ray and the normal to the surface of the mirror is equal to the angle between the reflected ray and the normal.