BCIT Physics Department
- To show that an E-field is unable to pass through a conductor, but is able to pass through a dielectric. It is also shown that a conductor with holes can block E-fields.
Place the small radio on the aluminium plate, and hit the scan button to tune to a local station (preferably not JR FM). Ensure that the antenna on the radio is down such that it is unable to make contact with the object that will cover it. Get the students to speculate what will happen when the radio is covered with the plastic pail and with the tin can.
After demonstrating that the tin can cuts out the signal and the plastic bucket allows the signal to pass, ask the students what will happen when the strainer is used (the air gaps are dielectrics). Use the strainer to cut out the signal. From this, you can get the students to conclude that the strainer does stop EM radiation. Once they have concluded this, look at the students through the strainer. As they can see you, light is passing through the strainer, and light is an EM wave, and so the previous conclusion must be wrong!
At this point, explain that what is important is the wavelength of the EM radiation. Using C=(frequency x wavelength), calculate the wavelength of the radio wave. At around 3 m, the wavelength is much larger than the mesh spacing. Visible light is able to pass through as its wavelength (around 555nm) is much smaller than the mesh spacing.
Finally, mention that the door on a microwave oven uses this same principle to allow you to see what's cooking (visible light) while not allowing the microwaves inside the oven to escape.