What Is Light?

Most of us think of light as just the opposite of darkness. It’s what illuminates our world and allows us to see. If there’s too much light it hurts our eyes. If there isn’t enough light we can’t see.

Seems simple enough, right? And it is, for the purposes of this course anyway.

I will not be delving into the fascinating nature of light here. However, if you would like to learn more exactly what light is, here are few videos that cover most of the basics.

Helpful Videos

Drawing Light

Being able to represent light in diagrams is extremely useful in learning how different types of lenses work.

Luckily, you don’t have to be an artist to be able to draw light. All you have to do is draw a straight light with an arrow tip that shows the direction the light is taking.

Here’s an example:

a single light ray

This simple diagram represents a single light ray travelling from left to right. When drawing light using lines like this it is called a ray diagram.

Here is another ray diagram showing several light rays reflecting off a mirror.

light rays reflecting off a mirror

Important Take-Home Message #1: 

Unless it encounters a different medium, light always travels in a straight line.

Let’s explore this important take-home message in a little more depth.

What is a medium?

A medium refers to any substance through which light can travel. For example, light can travel through outer space, air, water, glass, etc.

Can light change direction?

Yes it can and that will be crucial to our understand of lenses, but for now it’s equally important to understand that so long as light doesn’t encounter a new medium, it does not change direction, i.e., it travels in a straight line.

light rays travelling straight

Light rays do this

Light rays do not do this

Where Does Light Come From?

Primary Light Sources


Light originally comes from primary light sources. A primary light source is anything that generates lights. Here are some common primary light sources:

Image of The Sun Primary light source - a fire Primary light source - Light bulbs

The Sun


Light bulbs


Now let’s incorporate a light source into a ray diagram.

ray diagram of light coming from a light bulb
Example of a primary light source.

In this ray diagram, you can see the light source (light bulb) and the light rays being emitted from it in all directions.

Its worth noting that even though I’ve only drawn 5 light rays here, I could have drawn as many or as little as I wanted. Generally speaking, to keep ray diagrams neat and tidy, only a few light rays are included.

Secondary Light Source

By contrast a secondary light source is anything that reflects light. That reflected light acts like the light emitted from a primary light source.

It may seem arbitrary to define since almost everything can be a secondary light source. Essentially, anything that you can see but does not generate light acts as secondary light source.

ray diagram of light coming from a cat
Example of a secondary light source.

The difference between primary and secondary light sources is largely pedantic as they both behave in the exact same way as far as the optics of vision is concerned.

Since virtually anything can be a secondary light source, we tend to refer them as objects ray diagrams.

Important Take-Home Message #2: 

Anything that we can see acts like a light source (whether it’s a primary or secondary light sources is largely irrelevant). 

Introduction to Vergence

If you’ve just been skimming through this lesson so far looking for the right place to start paying attention, this is it!

The concept of light vergence will apply to literally everything else we cover from now on.

Now, if you research ‘light vergence’ on the internet, you’re going to find a lot of technically explanations involving wavefront curvatures and some weird looking equations. Forget all of that!

Here all you (remember that this lesson is intended for non-licensed optometric and optical staff members) need to know.

There are 3 vergence states:

Negative Vergence

Zero Vergence

Positive Vergence

light rays with negative vergence light rays with zero vergence light rays with positive vergence
Light rays move away from each other. Light rays travel parallel to each other. Light rays move towards each other.

Now let’s examine the key points of each types of vergence.

Negative Vergence

  1. When the light rays are travelling away from each other, that light has negative vergence.
  2. When light has negative vergence, each individual light ray gets further and further apart from each other. As such, they will never cross.
  3. Light emitted from any* light source has negative vergence.
    • Vergence is the most negative at the light source
    • As light gets further away from the light source is tends towards zero vergence
    • Light reaches zero vergence at ~20 feet from the light source.
  4. Another name for light with negative vergence is diverging or divergent light.

*Except lasers and other crazy contraptions.

Point number 3 was difficult to explain in words. This diagram will make things more clear.

ray diagram showing divergent light from object becoming parallel over a distance of 20ft
Click to enlarge

In the above diagram you can see that the object is emitting divergent light. The further the travels away from the object, the less divergent it becomes. At approximately 20 ft (6 m), light rays become parallel.

Take-home about negative vergence:

Light emitted from objects is initially divergent but becomes more parallel as it travels further away. 

Zero Vergence

  1. Light rays of light with zero vergence travel parallel to each other.
  2. They never get closer to each other.
  3. They never get further apart.
  4. Another name for light with zero vergence is parallel light or collimated light.

Positive Vergence

  1. Light rays of light with positive vergence travel toward each other.
  2. Light rays of converging light eventually all meet at a single point called a focal point.
  3. Light beyond a focal point becomes divergent.
  4. Another name for light with positive vergence is converging or convergent light.

ray diagram of converging light, focal point, and diverging light

So far you’ve learned that light is divergent when it is emitted from a light source (whether primary or secondary) and that it becomes parallel when it reaches a certain distance from the source.

You may then be asking… how does light become convergent?

That is a great question! We will be tackling that very topic in the next lesson!

==> The Optics of Vision – Lesson 2: Introduction to Lenses <==

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