Some physics background related to photography

This is the 1st part of a six-part tutorial about photography basics.

Visual properties of objects

Every material, has some visual properties like color, brightness and opacity. Visual properties are directly related to what happen to incident rays of light.

Say you are on a room with white walls, illuminated by an light bulb. We see the walls because the light that is emitted by the bulb is made of light rays that, as they hit the walls, are reflected. The ability of reflecting the rays is a property of these walls.

But the wall is a rough surface, even a wall that is smooth to the touch has enough ups and downs to be rough to light dimensions. So, any point of it will reflect light like if there was a mirror, tangent to the wall in the very point of the incident ray.

The light ray will be reflected with a reflection angle that is equal to the incident angle, but because the wall is rough, every light ray will arrive in a different angle and each one will have a different reflection angle, as a result, any ray of light will go to a different direction, in a very unordered way.

That is why, when we look at a wall we don’t see an image reflected on it, like we do when we look at a mirror. Nevertheless, what we see on the wall is an image of everything on the room that is emitting or reflecting light.

At the same time, any point of the wall receives rays of light that come from many (we could say infinite) other points of the room. So, any point of the wall is reflecting light from infinite other points that come in many colors. The red one from a shirt, the blue from the flower vase, the green from the leaves, the yellow from the flower, and so on.

A red object is red, because, it absorbs all rays of light except the red ones that are reflected. The light that reflects in a red object and the reflects to our eyes is darker than the original, because two thirds of the incident light will be absorbed by it. It will also be red, because those the third reflected is made of only one frequency range: the red range.

Still on the subject, blacks are black because they absorb every single ray of light and none is reflected, that is why we say that black is the absence of color.

What makes a wall white? Well, is just that it has the ability (or property) to reflect all kinds of colors and absorb none. When all these rays of light hit your eyes, the light is “added up” and you don’t see all the individual rays, but the sum of them.

The structure of the human eye

Human eyes have photoreceptor structures called cones that can detect red, green and blue rays of light. No other color is perceived by the human eye, what happens is that the brain mixes amounts of these three seamlessly and we feel like seeing other colors.

People use to say that red, green and blue are primary colors because they are the primary components of white light. This is not untrue, but we must realize that this is only true when associated with human perception of light, not with light itself.

Anyway, ultimately, all light we see can be divided into three parts, and we can rely on understanding only these three to study all the light we see.

Subtractive colors

As you probably know since kindergarten adding paints does not bring white into the picture. The more colored paint you add, the darker it gets.

That is because the paint absorbs all the incident rays except the ones on the paint color, that are reflected. So, the more paint you add, the more rays are absorbed, eventually fading to black.

The Newton’s Color Wheel

Just because adding colored paint turns into black, it doesn’t mean that “white is the sum of all colors” is not true, fiction or just a theoretical model, but you have to make sure you are adding light, and not only paint.

We can do an experiment that shows that the sum of colors is white. Take a disc and divide it in three circular sectors, then paint each one with one of these colors: red, green and blue.

If you nail the circle by its center and spin it fast enough the circle will seem white. Looking at any point of the circle will make you eye receive a rapid succession of intermit red, green and blue, so fast that the eye will blend them together and see white.

This is called the Newton’s Color Wheel.

Conclusion

A rough object, like the wall on the example, reflects light on every possible direction and builds infinite images that are projected all around the place and mix with images of other objects.

Our eyes only see reds, greens and blues and they are sent to the brain that mixes them, based on amount of light, incapable of separate the different rays or images formed.

To take a picture we need to isolate one of these images from the others and keep it.