Why can’t you put aluminum foil in the microwave?

If you are like me, you often forget to set your ingredients out ahead of time to allow them to reach room temperature.  So when you realize that the recipe you are baking calls for softened butter, you reach into the fridge, pull out a stick of butter, and pop it into the microwave, wrapper and all.  I know, I know, it’s not the proper way to soften butter, but it gets the job done.  Admit it, we’ve all done it.

The other day, I was baking and realized that I had forgotten to take the cream cheese out of the refrigerator.  Not wanting to dirty yet another bowl, I left the cream cheese in its aluminum wrapper and put it in the microwave.  I punched in thirty seconds and pressed the on button, but immediately hit stop once I saw sparks coming from the microwave.  Not my best idea.

So this got me thinking.  How exactly do microwaves work and why can’t we microwave certain materials such as aluminum?

Microwave ovens use microwaves to heat food.  Microwaves are part of the electromagnetic spectrum – waves which are created by the vibration of electric charges and have both electric and magnetic components.  All electromagnetic waves transport energy through a vacuum at a speed of 3.00 x 108 m/s, known as the speed of light. When you turn on the microwave, the electrical energy from the power outlet is converted and emitted as microwaves.  When absorbed by the water, fats, and sugar in food, these microwaves convert into atomic motion – heat.  As a result, microwaves can heat food very quickly.


However, sometimes when you heat food in the microwave, the outside is hot, yet the inside remains cold. This is because microwaves can only penetrate about an inch into food.  In comparison to other electromagnetic waves, microwaves are relatively low in energy. As a result, when microwaves enter the food, they rapidly lose their energy and cannot travel very far.


Let’s talk a little bit more about the properties of waves. Wavelength is the distance between two peaks of a wave and is usually measured in meters (m). Frequency is the number of cycles of a wave to pass some point in a second. The units of frequency are thus cycles per second, or Hertz (Hz).  Multiply wavelength by frequency and you get the speed of the wave. Since all electromagnetic waves move at the constant speed of light, wavelength and frequency are inversely related – as one goes up the other goes down.

c = λυ  

where c = speed of light (3.00 × 10 8 m/s)

λ = wavelength (m)

υ = frequency (Hz)

Furthermore, the energy of a wave is directly related to its frequency.  Waves carry energy in small packets called photons.  The photon energy is the energy carried by a single photon with a certain electromagnetic wavelength and frequency. The higher the photon’s frequency, the higher its energy.  This explains why microwaves are relatively low in energy and cannot travel very far throughout food.

E = hυ

where E = energy (J)

h = Planck’s constant (6.63 × 10 -34 J·s)

Now back to the real question- why can’t you put aluminum foil in the microwave?

Firstly, it is important to note that the walls of microwaves are made of thick metal.  The metal acts almost like a shield or a mirror, reflecting the waves and preventing them from escaping from the microwave.  However, small, thin pieces of metal do not react the same way.  When microwaves hit the metal, a current of electrons flow through the metal.  Thick metal can tolerate the current, but thinner, jagged pieces cannot.  As a result, small pieces of metal heat up very quickly and can become dangerous.

Pointed metal objects, such as forks, crumpled aluminum foil, and metal wires, can create an electric arc or sparks when microwaved.  High concentrations of charge reside on the tips of metal objects, which can exceed the dielectric breakdown of air (3.00 x 106 V/m).  Even though air is an insulator (does not conduct electricity very well), if high voltage applied across it exceeds the breakdown voltage, the insulator can become electrically conductive. When this happens, the air forms a conductive plasma, which is visible as a spark. The plasma can jump across the pointed edges, forming a conductive loop and a longer lived spark.



For this reason, its best to avoid putting any metal in the microwave.  Play it safe. . . even when you are feeling lazy like me.

Inspired by my cream cheese incident, I chose to make cheesecake brownies for Valentine’s Day.  Enjoy!


The batter starts with loads of melted chocolate and butter.


Swirl in the cheesecake mixture using a fork.


Bake and. . . Voila! Cheesecake swirl!



Allow to cool and then cut with a plastic knife to avoid any sticking.


Plate and enjoy!  Perfect for Valentine’s Day. . . or any other day of the year.


Cheesecake Brownies


1 1/4 cups butter

16 oz semisweet chocolate chips

1 cup brown sugar

5 eggs

1 Tbsp vanilla

1 tsp salt

2 1/4 cups flour

1/4 cup heavy cream or milk


8 ounce cream cheese

1/4 cup sugar

1 egg

1 tsp vanilla

Splash of heavy cream or milk

Preheat the oven to 350º.  In a large bowl, melt together the butter and chocolate and stir until smooth.  Mix in the sugar, eggs, vanilla and salt.  Slowly add the flour and then the heavy cream.  Set aside.

In another bowl, mix together all the cheesecake ingredients.  Pour the brownie mix in a greased baking pan and spoon the cheesecake mixture on top.  Swirl together using a fork.

Bake for 35-40 minutes or until a toothpick comes out clean.  Let cool and then cut into squares.

Makes 24 brownies.

Explain That Stuff

Nasa Science


How Stuff Works





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