Microondas, regaliz y la velocidad de la luz

Benjamín Montesinos

¿Tienes una barra de regaliz en casa? Si no, baja a la tienda de chuches más cercana, cómprate unas cuantas y disponte a medir la velocidad de la luz usando tu microondas.

Este es un sencillo -y sorprendente experimento- para medir la velocidad de la luz con una barra de regaliz y un microondas:

Paso 1: quítale al microondas la pieza de plástico que está en el eje de giro, para evirtar que el plato de vueltas. Con dos trozos de cello pega la barra de regaliz en el plato y ponla de modo que la barra quede paralela a la puerta.

Paso 2: cierra el microondas, y a baja potencia, por ejemplo a 200 watios, enciéndelo. Quizás tengas que hacer algunas prueba, pero en eso consiste la experimentación. Verás que el regaliz se curva en forma de onda: algunas zonas quedan al nivel del plato y otras se elevan algo así como 3 cm. Apaga el microondas y saca el plato.

Paso 3:  Cuando apagas el microondas la barra de regaliz se enfría y tiene el aspecto que se muestra en la fotografía de arriba: verás que las zonas que se han elevado están quemadas pero hay otras zonas intactas (los dos pequeños «valles» que se aprecian en la foto). Con una regla mide la distancia aproximada entre los centros de esos dos valles. En nuestro caso, es más o menos 6 cm.

Paso 4: dale la vuelta al microondas y mira la frecuencia de las ondas. Viene indicada en la placa con las características técnicas. En el microondas que nosotros usamos es 2450 MHz. «MHz» significa «Megaherzio» (millones de herzios) y los «herzios» son simplemente número de oscilaciones por segundo, de modo que las unidades son la inversa de un tiempo, que se indica como «s-1»

Multiplica por 2 la distancia que mediste antes, y multiplicala por 2.450.000.000 s-1.
El sorprendente resultado es 12 × 2.450.000.000 = 29.400.000.000 cm/s =294.000 km/s… ¡la velocidad de la luz!

¿Es causalidad? ¿Qué está pasando? La explicación es sencilla: el microondas es una cavidad que produce ondas estacionarias. Hay puntos del microondas donde la amplitud de las ondas es cero y otros donde esa amplitud es máxima. En nuestro caso, el hecho de que cada seis centímetros el regaliz no se haya quemado, significa que en esos puntos el calentamiento es nulo, y por tanto la amplitud de las microondas es cero.

La longitud de onda de la radiación es dos veces la distacia entre dos de estos puntos estacionarios (llamados técnicamente «nodos»). El regaliz por tanto nos dice que la longitud de onda en el microondas es de 12 cm.

Una fórmula muy conocida en física dice que la velocidad de la luz es igual al producto de la longitud de onda por la frecuencia. Haciendo la operación que hemos indicado, se ve que la exactitud del experimento es grande. El valor «oficial» de la velocidad de la luz es 299.792,458  km/s.

¡Ánimo y enviadnos vuestros resultados!

ADDENDUM (DByN):
Midiendo la velocidad de la luz: el contexto histórico

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