Fishing…

I am not a great fan of New Year resolutions (why wait for New Year?) but I would like to resolve to do some fishing on the Cherwell in 2012. We have an excellent stretch of the river adjoining our fields and the North Oxford Angling Society, which runs our stretch of water, is very happy for us to fish there. It would not be game fishing (as there are relatively few trout on our stretch) but the earthier brand of angling known as coarse fishing. The distinction is really in the species caught. In game fishing the angler is usually pursuing trout and salmon that may then be taken home and eaten. The coarse fisherman is after a wider range of species such as carp, tench, perch, chub, pike, bream, barbel and many others. These are not taken home but carefully returned to the water unharmed. The tactics used tend to be different too. The game fisherman uses artificial lures of one sort or another almost exclusively, whilst the coarse angler uses bait as well as lures in a range of different tactics to suit many species and water conditions.

Our stretch of the Cherwell is noted for its bream, chub, perch, roach and tench. I have spoken to some local anglers who say that the bream are very large along the banks of what is a very clean stretch of water. By clean I mean unpolluted; the river carries a great deal of silt so the water appears very dark. Some of you may have joined Mr Sparrow on one of his crayfish excursions. There are a huge number of these invaders, but plenty of more interesting species too. My favourite is the chub. These are thick-set and powerful animals that are very shy and lurk in ambush under any bit of cover they can find. Catching them is not easy; stealth is required if they are not to be frightened off as you approach the water’s edge.

They have the laws of physics on their side. Although it is hard to look into the murky waters to see any fish clearly, it is relatively easy for them to detect movement on the bank from the little light that does penetrate the gloom. They have Snell’s Law to help them spot you before you see them. Snell’s Law describes the bending of light as it travels between materials of different densities – e.g. air and water. As light enters the water it refracts, so fish can see movement further back on the bank than a direct line of sight might indicate. The diagram below attempts to show this.

The dotted line shows where you "ought" to stand in order to be seen by the fish, were it not for the refraction of light.

In a vacuum, light travels at 299,792,458m/s (we often round this off to 300,000,000m/s) but through different media it travels more slowly. The value by which light slows in a given material is called the index of refraction (refractive index). The table below shows these values for a number of common materials. You can see that the indices of refraction for a vacuum and for air are so close that we can ignore the difference on most occasions.

The greater the difference in refractive index, the greater will be the degree of bend as light travels between two materials.

On entering the denser perspex, the light beam bends towards the normal but when emerging into the less dense air, it bends away from the normal to finish parallel to the incident ray.

You can use the index of refraction to calculate the speed of light through different materials – just divide the speed of light through a vacuum by the index of refraction for the material in which you are interested.
For example, to calculate the speed of light through water…

For further reading about light and waves in general, you might try reading about colour, light or vision.

Questions…

1. To which taxonomic group do crayfish belong?
2. What is the speed of sound in air (in m/s)?
3. What would be the speed of light through diamond?