Snell’s Window

underwater mark tipple

When you are under water and you look upwards, you see everything above the surface of the water through a round window of light. I think we are so used to this phenomenon, if not from experience then at least from films, that we don’t think about it. At least I didn’t until I saw Neil Gower’s beautiful drawing for the cover of the Watermarks anthology.


The round window of light is known as Snell’s Window and is to do with refraction of the light as it travels from the air to the water. The area outside of the cone of light that forms the window will be completely dark or will reflect objects within the water. It sounds complicated, and in terms of physics it is, but a photograph taken from under the water reminds us how familiar the effect is even if, like me, you don’t fully understand it.

Art work for Watermarks by Neil Gower, underwater photograph by Mark Tipple



Face in the water


Many people tell me before they come to their fist swimming lesson that they can’t put their face in the water. So far every single adult I have ever taught has been able to do it within about five minutes of arriving at the lesson. Some people have had a fear of this for years but for some reason, and with a little bit of guidance they are always able to do it. I am not sure exactly how many people I have taught but it is certainly in the hundreds.

Children are quite a different matter. They will not put their faces in the water until they are ready for it. This may take five minutes or it could take several months or longer. You absolutely cannot persuade a reluctant child to do it if they don’t want to. This is one of the main ways in which teaching children differs from teaching adults.

To learn to swim you do really need to be able to put your face in the water. This is a simple matter of physics. Our bodies are less dense than water, but only slightly. That means that most people float (there are some very rare exceptions). But any part of the body that is held out of the water is heavy. This means that you cannot float without doing something. If your head is out of the water you have to move your arms or legs to keep you afloat. If your face is in the water you don’t have to do anything.

It takes time to learn what the resistance of the water feels like, how to ‘catch’ the water, and how much effort you need to use to propel yourself forward, or to keep your head out of the water.  Once you have learned to swim it is easy to swim with your face out of the water, but to learn to swim this way is not impossible, but quite difficult.

As I say I have never had any problem at all persuading any adult, however scared they are, to put their face in the water. This is never a stumbling block. But there is a point where many people get stuck and that is in allowing themselves to float. Many people find it very difficult to let go and allow the water to hold them.

Swimming monkeys–apes-prefer-breaststroke.html#.Ur1nc8uYapr

Interesting to see that the chimpanzees here swim with their faces both in and out of the water. I find with many people I teach that it is the fear or reluctance to put the face in the water that stops them from swimming efficiently and effectively.


“To be weightless for the better part of a year, to never even have to lift my head
and then to have to be subjected again to my own inevitable weight having to
pick up my arm […..] It was an extremely unfair feeling”

Canadian astronaut Col chris-hadfieldChris Hadfield speaking on the Today programme, (BBC Radio 4 29th October 2013),  about how it felt to return to Earth and experience the heaviness of his own body .

Most of us will never go into space and will never experience the weightlessness Chris Hadfield is talking about here. Floating and swimming in the water is probably the closest we can get. That is why they train astronauts underwater in special zero gravity tanks.

The secret to learning anything

einstein-laughingSometimes parents say to me that they are worried that all the children are doing in my lessons is playing. I am not sure that these parents always appreciate that, especially in the water, it is through playing that the children learn. When I teach non-swimming adults the main difference between them and the children is that they don’t play in the water. It is true that sometimes the children can enjoy the lessons so much that things get very lively. Then I have to remind them of two basic rules

  • don’t jump in without asking me
  • don’t hold onto each other in the water.

These have to do with safety and are not too difficult to remember, although we do have to over them quite often. I believe that to learn any new skill properly, at any age, you have to enjoy the process. Here is a letter from Albert Einstein to his eleven year old son. It was written in 1915.

“I am very pleased that you find joy with the piano. This and carpentry are in my opinion for your age the best pursuits, better even than school. Because those are things which fit a young person such as you very well. Mainly play the things on the piano which please you, even if the teacher does not assign those. That is the way to learn the most, that when you are doing something with such enjoyment that you don’t notice that the time passes. I am sometimes so wrapped up in my work that I forget about the noon meal. . . .”

He wrote this letter just after having completed his General Theory of Relativity.

My thanks to Maria Popova for drawing my attention to this quote in her blog

Fear of flying

?????????????????????????????????????????????????????????????????????????????I have worked with many non swimmers who tell me they are afraid of putting their faces in the water.  Although it is a common fear, putting your face in the water is easy and almost everyone can do it without a problem more or less straight away. I have never, ever had anyone finish a lesson without being able, quite happily, to put their face in the water. I tell them to hum a nice song. That way their mouth is closed and the air comes out of the nose, and they are also a bit distracted. They may be afraid of seeming foolish but in the water no one can hear you anyway.

A much more persistent fear, and one that is much more difficult to let go of, is the fear of floating. It is not usually an inability to float that is the problem, that is only very, very rarely the case; rather it is the fear of being ‘untethered’ and free. We are so used to holding on in life, that to let go and float is very difficult for some people.

One woman said that it felt like the moment when you are falling asleep and you suddenly catch yourself and wake yourself up with a start. This made a lot of sense to me. Similarly in the water, you are letting go and allowing yourself to float freely and for a non swimmer this can be an unfamiliar and frightening experience, and it can feel like falling if you don’t realise that the water is there to catch you. Not everyone can overcome this fear quickly. For some people it can take a long time to let go and trust the water. This is true for children as well as adults. Sometimes people are OK as long as they have something to hold on to. One pupil could swim as long as she was holding my hand, another one was fine as long as she could touch me with one finger. But as soon as I took my hand away, even though I was not supporting her at all, she got that falling feeling and was snatched by a panicky fear .

A woman I was teaching last week  who had been rather quiet until then told me she had just come back from a Kite festival in France. It sounded so beautiful watching all the colourful kites flying high up in the sky. I told her that swimming is the closest you can come to flying in scientific terms, and it is.