Headteacher Graham Able expects to have 20 applicants for the physics teacher’s post he recently advertised. He thinks this is a “small number” but he is confident that he will be able to shortlist five and find several that he wants to appoint.

But then Mr Able is head of Dulwich College, in south London, one of our most famous public schools. He can guarantee whoever he appoints will never have to teach anything other than physics, in dedicated physics lessons, and will be able to teach large numbers of A-level students. Not every head can make the same promises.

Elsewhere, physics teaching is in trouble. A recent report by the Institute of Physics said: “There is a crisis in the teaching of physics in our schools. The shortage of new physics teachers and the loss of physics teachers from the profession has concerned (us) more than any other factor facing the future of physics in the UK.”

Many pupils are now taught physics by teachers who are really unqualified.

Two-thirds of those teaching physics at key stage 4 in England do not have a related degree. Three out of 10 do not even have A-level physics (see table below).

This is despite the fact that schools try to use their specialists mainly for older pupils. The Royal Society of Chemistry says: “In order to preserve teaching by specialists at KS4, schools use those staff whose specialism is in excess - normally biologists - to teach at KS3.” However, since many regard KS3 as a crucial point in fostering enthusiasm for science, this policy may have unfortunate consequences.

It is difficult to put a figure on the shortfall of physics teachers, but it certainly runs into thousands. The problem exists in other science subjects, but is not as severe, as the table shows - numbers of physicists accepted to teacher-training courses are about a quarter the number of biologists. The table illustrates shows how entries have changed over the past decade.

Many experts believe the fact that many lessons are taught by teachers outside their specialism shows that science is in a mess. Dr Ian Gibson, who chairs the House of Commons select committee on science and technology, described GCSE science teaching as “a disaster”. “School science can be so boring it puts young people off for life.”

A report for the Treasury by Sir Gareth Roberts, president of the Science Council, last summer spelled out the consequences: “Pupils are turning away from science, technology, engineering and mathematics. This is a significant factor in explaining the difficulties experienced by employers in recruiting people with high-level science and engineering skills.” It is not an exaggeration, experts stress, to say that the UK’s industrial base depends on the quality of school science.

At primary level things are going well. A 2002 Ofsted report painted a positive picture, with standards rising steadily and good-quality teaching.

There remain concerns about levels of teachers’ subject knowledge, and a feeling that investigative work could be stronger, but the consensus is positive.

So the focus is on secondary schools where teacher shortages are being tackled while the curriculum is updated.

Alan Smithers of the centre for education and employment research at Liverpool University says staff shortages mean children are being put off science by badly-qualified teachers.

They may be taught physics by an unenthusiastic biologist. By contrast, they may well be taught humanities by keen, well-qualified teachers, and this is skewing their A-level choices. That means fewer physicists at university and a smaller pool to recruit from.

Professor Smithers believes the way out is to return to teaching separate sciences to some pupils: most take dual science in a programme which combines the three. “We have to ensure that there are appropriate steps through the education system for people with a talent for the physical sciences,” he says. He notes that numbers taking sciences in the sixth form have fallen less in independent schools and grammars, which have more freedom to offer separate sciences up to age 16.

He wants science for the talented, as well as science for all, and says that we should not boost numbers taking science at the expense of the most able.

The Institute of Physics is also trying to break the cycle, this time by a scheme to support physics teaching by non-physicists at KS3. Teacher support manager Chris Shepherd says: “We have to enthuse chemistry and biology teachers to tackle physics better.” The institute hopes it will lead to a rise in A-level numbers.

Another debate has been generated by plans for a new GCSE exam to allow pupils to skip much traditional theory and make science more relevant to their lives. Critics have called the changes “popular science”, although proponents such as Derek Bell, chief executive of the Association for Science Education, reject the charge. “Just because you are dealing with a science topic through an issue doesn’t mean you can’t grab the brightest pupils,” he says. “You can go that much further with the science that is behind it.”

Chris Shepherd backs the exam but says that extension modules for bright pupils could have been made “more adventurous” and adds that curriculum change is no panacea.

The Government has accepted that school science is in urgent need of a boost and is looking into reform. Measures could include better pay for science teachers, a pound;50 million investment in professional development centres, extra money for school labs and employing science students to help teachers.

But whether this will be enough to entice the brightest children back remains to be seen - and it is left to Professor Smithers to spell out the consequences of failure. “The prosperity and quality of understanding in this country depends on the talents of people who can take physics to a high level,” he says. “For the sake of our quality of life, we need to develop those particular talents.”