Inside the center of the sun a huge mass squeezes down from all sides. A pressure and temperature so intense that four hydrogen nuclei give their lives to form one helium atom. The mass of the helium ‘ash’ is slightly less than that of the original nuclei – the mass is not lost; it is transferred into energy. Some of that energy is released in the form of light.
If the light produced is lucky enough it will travel the 150 million kilometers in a near perfect vacuum towards the Earth. After the 8 minutes and 20-second journey, it will arrive and hit our atmosphere – an atmosphere that loves to reflect and suck up tasty photons.
When the photons hit, 29% of them are reflected back into space by clouds, particles or bight surfaces like snow. A further 23% of the light is absorbed in the atmosphere by water vapour, dust, and ozone, only 48% reaches the surface.
After a laborious journey, the photons that make it to the surface are free to do whatever they want – some of them may even end up entering your eye.
It is only possible for humans to see things because of the light the sun provides us with. Light hits an object and it is reflected or absorbed, the properties of the object influence the light that is reflected – a green circle will absorb all light that is not green and reflect light with a green wavelength.
The light will then travel towards your eye, eventually passing through your cornea that will focus the light towards the back of your eye (towards the retina) – focusing is aided by the lens.
Your retina is made up of cells called cones and rods. Rods are extremely sensitive to light and are responsible for your vision in dim conditions (black and white).
Cones come in three varieties – red, green and blue. They are active at high light levels and provide us with our colour vision. Red light stimulates red cells, blue light blue cells ect. A combination of RGB cells allows for different colours to be made.
The rods and cones in the retina absorb and convert light into electrical signals which are then passed to the brain via the optic nerve where the signals are processed into an image.
Why is the sky blue?
Sunlight in Earth’s atmosphere is scattered in all directions by particles in the air. In comparison to most other colours of light, blue light has a short wavelength (~495 nanometers). The amount of scattering is inversely proportional to the fourth power of the wavelength, meaning blue light scatters more than other light because it has a short wavelength. Because it scatters more than other light – the sky is blue.
You might also notice that the appearance of the sun seems to change as the day goes on – this is because the amount of atmosphere the light travels through changes. In the mornings/evenings, the light has to travel through a lot of atmosphere which causes a reddish colour.