How to measure the speed of light at your university with a dinner budget

Abstract

Although the idea of using a modulated laser signal to measure the speed of light is not new, most methods found in international literature are still expensive, as a result of either the instruments or the circuits used. In the present approach, we provide an alternative that requires equipment that most universities own for their undergraduate programs, plus some inexpensive circuitry, allowing the students of any undergraduate physics laboratory course to obtain an eight-point graph using relatively small distances (such as corridors), combining a low budget with an accurate result that is equally as good as those achieved by kits available on the market. Keywords: speed of light, low budget equipment. Embora a ideia de utilizar um sinal modulado de laser para medir a velocidade da luz não seja nova, a maioria dos métodos encontrados na literatura internacional ainda são caros, devido aos instrumentos ou circuitos utilizados. Na presente abordagem, oferecemos uma alternativa que requer equipamentos que a maioria das universidades possuem para os seus cursos de graduação, além de alguns circuitos de baixo custo, permitindo que os alunos de qualquer curso de laboratório de física de graduação obtenham um gráfico de oito pontos com distâncias relativamente pequenas, combinando um baixo orçamento com um resultado preciso, queé tão bom quanto os resultados obtidos com kits disponíveis no mercado. Palavras-chave: velocidade da luz, equipamento de baixo orçamento.

Speed of light demonstration using Doppler beat

“…On the other hand, when using a modulated beam at a constant frequency, it is necessary to have a stabilised modulation. Phase shift measurements include amplitude-modulated light beams produced by means of Kerr cell [9], modulated LEDs [10,11], laser pointers [12,13], or modulated laser [14,15]. Recently, Pegna [16] has published an apparatus that modulates a collimated red laser at a frequency of 433.92 MHz, making possible a speed of light demonstration within a sub-metre measuring base.…”