From my previous post you already know some principles about the light and human perception of the light. In this part I will focus more on sight defect releted with color vision.

Do we all see the world in the same way?

So previously I said that for color detection we use cones in our eyes. For most of us is obvious that lemon or banana is yellow, strawberry is red and orange is orange... But what if our cones would not work properly? Actually quite big percent of human population have issues with colors perception. Almost 8% of mans and 0.5% women suffers from color vision deficiency (CVD) - color blindness in other words. Why males are more affected than females? Answer is hidden in chromosome X, which is responsible for this genetic defect. As females have two X chromosomes, a defect in one is typically compensated for by the other, while males only have one X chromosome.
The most common form of colorblindness is red-green color blindness, but there are some people who can see only monochromatic. Color vision problems can be group in 3 main types:

• monochromacy with only one type of working cones
• dichromacy where only 2 types of cones work properly
• trichromacy where all 3 types of cones work, but one of them has some shifted in spectral sensivity

Below you can find how colorblind people see depends on type of CVD (dichromacy type):

As you can see we can divide dichromatic blindness on three main types:

• deuteranopia (green cones are reduced or missing)
• protanopia (red cones are reduced or missing)
• tritanopia (blue cones are reduced or missing)

this is how colorblind people see the rainbow...

If you want to check if everything is OK with your sight, you can do Ishihara test:

Colorblindness can`t be cured, but there are some glasses which makes colors visible:

Coming to the end, I have one more thing about color perception. Above I described what is happening when cones are not working properly, Below you can find what is happening when cones are "tired"

"The trick works due to an effect known as 'afterimage'. This is where
overexposure to a given colour causes the retina to become tired of
that colour and desensitises the cone cells to it for a short period.
When the colour stimulus is removed and the eye is exposed to a black
and white image under white light, then the complementary colour is
perceived for a brief period of time."

Here you can find sources where you can find more details:
http://www.dailymail.co.uk/sciencetech/article-3320480/The-image-HACK-brain-Optical-illusion-transforms-black-white-picture-colour-simply-staring-it.html#ixzz4k6bd3Yzr
http://mkweb.bcgsc.ca/colorblind/
http://blog.usabilla.com/how-to-design-for-color-blindness/
https://en.wiipedia.org/wiki/Color_blindness

I next post I will come back to light again and describe one more property of electromagnetic waves - polarization.

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