The journey of a photon

In the example of our perception of the countryside, the story begins about 8 minutes earlier, when light is born on the surface of the sun. It does not matter, for the moment, what we mean by saying 'light is born'. We will just picture light as a tiny particle (a photon), which suddenly materialises at the surface of the sun.

Countless photons leave the sun every second and go out into the universe in all directions. They travel through space with a speed of 3 x 108 ms-1 for billions of years, unchanged and unhindered. A very small fraction 'just happen' to go in the direction of the earth, and reach us, having traversed about 100 million miles

Lough Conn, Co. Mayo, Ireland.

of empty space. In turn, a small fraction of these might hit a leaf on a tree and be absorbed, giving energy to help the tree to grow. Others get reflected, and some of these, an even tinier fraction, reach our eyes and are focused on the retina. There are still enough photons remaining to activate the 'photosensitive' cells in the retina. (These photosensitive cells are closely packed in three layers and interconnected by tiny fibres.) Having arrived safely after their long journey, the photons have done their job. They give up their energy to electrons, which flow through nerve fibres to the brain, as electrical currents.

The sun not only emits light which enables us to see, it is also our main source of energy. Solar flare. Courtesy of NASA/ESA.

About 600 million tonnes of hydrogen are 'burned' on the sun every day (equivalent to about 1025 J of energy). The earth receives just about 5 x 10-8 % of this, which amounts to 2 x 1016 J a day, more than ample for our needs. The sun is 'captured' in this image from the Solar and Heliocentric Observatory, as its surface erupts in a large 'prominence'. An image of the earth is shown here to illustrate the scale of the eruption.

1.4.3 The eye is like a digital camera

The front of the eye forms a complex optical system which focuses the light on the retina. To ensure that the image is of the highest quality this system is capable of rapid adjust-



The eye.

nerve muscles

The eye.

retina nerve ments to control the viewing direction, focusing distance, and the intensity of the light admitted. The retina consists of millions of photosensitive cells which send out an electrical signal when struck by light. (This process is called the photoelectric effect, about which we shall have much more to say in Chapter 13.) These signals are then transmitted to the brain via the optic nerve. In this way the eye works like a television camera, rather than a conventional camera which uses photographic film.

The shape of the lens is controlled by the ciliary muscles. When the ciliary muscles contract, the lens becomes more rounded and therefore more strongly focusing. It is interesting to note that the eye contains a range of different muscles, to control the lens, to adjust the iris, and to rotate the eyeball up, down and sideways!

The function of the retina is not merely to generate electrical signals at the spots where the light lands. It does much more, and acts as a kind of microcomputer, pre-analysing the information before it is transmitted to the brain. In particular it is responsible for the sensation of colour. The human eye is sensitive only to a small range of the electromagnetic spectrum. Different colours are characterised by different wavelengths of the electromagnetic wave. Information from the two eyes is compared and coordinated and tiny differences between the two images are used to measure perspective and distance and even to estimate the speed of approaching objects.

Digital Camera and Digital Photography

Digital Camera and Digital Photography

Compared to film cameras, digital cameras are easy to use, fun and extremely versatile. Every day there’s more features being designed. Whether you have the cheapest model or a high end model, digital cameras can do an endless number of things. Let’s look at how to get the most out of your digital camera.

Get My Free Ebook

Post a comment