In what seems like a counter-intuitive demonstration, we can place a polarizing filter in between two other filters which do not transmit light in order to cause light to pass through again. This is because each filter will permit the components of electric field vectors of the electromagnetic waves that are parallel to its axis of polarization according to the equation $$A = A_o \cos{\theta}$$ where $A_o$ is the original amplitude of the unpolarized wave incident on the filter and $\theta$ is the angle between the electric field vector and the axis of polarization. Each time the wave passes through a filter, it undergoes a reduction in amplitude according to the equation so that by the third filter, its resultant amplitude is $$A = A_o \cos{\theta_1} \cos{\theta_2}$$ where $\theta_i$ is the angle between the axis of polarization of the i^th filter and the electric field vector direction of the incident light on the i^th filter. According to Malus' law, the intensity of the light that passes through these two filters is given by $$I=I_o\cos^2\theta$$ where I₀ is the initial intensity and θ is the angle between the light's initial polarization direction and the axis of the polarizer. The resulting intensity for light that passes through 3 filters is given by $$I=I_o \cos^2{\theta_1}\cos^2{\theta_2}$$ where $\theta_1$ is the angle between the axes of the first and second filters and $theta_2$ is the angle between the axes of the second and third filters.

Using a pair of polarizing sunglasses, you can demonstrate the effects of polarization together with a computer screen which is also polarizing. When the axes of polarization of the two polarizing screens are rotated, the brightness alternates between bright and dark. Light coming from a computer screen is usually polarized. In the video below, when polarized light passes through another polarizer, the intensity of the light is given by Malus' law: $$I = I_o cos ^2{\theta}$$ where $$\theta$$ is the angle between the two axes of polarization and $$I_o$$ is the original intensity of the unpolarized light. Only the components of electric field vectors in electromagnetic radiation that are parallel to the axis of polarization of a polarizing filter will be permitted through. Those electric field components that are perpendicular to the polarization axis are blocked by the filter. Hence, the amplitude of a vector A that passes through is given by $$A = A_o \cos{\theta}$$. Since intensity is proportional to the square of amplitude ($$ I \propto {A^2}$$), we have Malus' law. The purpose of having polarizing filters in sunglasses and computer screens is to cut out glare due to light from other sources.

Audacity is a free audio recording and editing software downloadable from http://audacity.sourceforge.net. It is cross-platform meaning you can run it on PC or Mac.

Once you have the software installed, you can try out some of its simpler functions, such as recording and playing back. You can also generate tones of known frequencies, which will be useful for experiments such as using stationary waves to determine the speed of sound in air.

Frequency and pitch The first activity serves as an introduction to the software and can be easily carried out. All you need is a computer with Audacity installed and a microphone connected to it. Tuning forks of different frequencies will be best for this activity because of the simple and pure waveforms generated. A regular wave pattern can also be recorded with the help of a musical instrument such as a guitar or by singing a note.

To make a sound with the tuning fork, strike it against something hard such as the heel of your hand. The two prongs of the fork, known as "tines,"  will then vibrate with a fixed frequency, thus generating a waveform with a displacement that is almost sinusoidal. Place this tuning fork next to the microphone and you should see a densely packed waveform like the following:

Zoom into the peaks recorded by clicking on the peaks of interest and pressing Ctrl 1 for Windows or Command 1 for Mac. You should observe a repeated pattern in the waveform. By reading off the time difference on the horizontal axis between two peaks, you can measure the period of the wave. Using $$f=\frac{1}{T}$$ where f is the frequency of the wave, and T is the period, you can verify the frequency measured with the known value of the tuning fork's frequency. In the above graph, the period is 2.8190-2.8155 = 0.0035s. The frequency of the tuning fork used is given by $$f=\frac{1}{T}=\frac{1}{0.0035}= 286 \text{ Hz}$$ which is roughly that of a D note.

image taken from http://courseweb.stthomas.edu/apthomas/SquishyCircuits/howTo.htm I came across this Ted video on Squishy Circuits, presented by AnnMarie Thomas from the University of St Thomas and found it to be a suitable activity for kids. I shall attempt to make some when I am free with instructions from the following site: http://courseweb.stthomas.edu/apthomas/SquishyCircuits/index.htm Be sure to watch this page for photos and videos! As I was contemplating the potential of combining conductive and insulating dough to make fun toys with the help of electric motors and the learning that can come from it. Apart from the obvious learning related to electrical resistance and current, we can even learn about flotation and fluid dynamics by building floating boats of different hull shapes.

With the help of a simple manual vacuum pump that is used to keep food fresh, we can demonstrate the effect of a reduced pressure on the boiling point of water. This leads students to a discussion on what it takes to boil a liquid and a deeper understanding of the kinetic model of matter. Materials

1. Vacuum food storage jar with hand-held vacuum pump
2. Hot water

Procedure

1. Boil some water and pour them into the jar such that it is half filled. This is necessary as hand-held vacuum pumps are not able to lower pressure enough for boiling point to drop to room temperature.
2. Cover the jar with the lid and draw out some air with the vacuum pump.

Explanation When water boils, latent heat is needed to overcome the intermolecular forces of attraction as well as to overcome atmospheric pressure. Atmospheric air molecules would prevent a significant portion of the energetic water molecules from escaping as they will collide with one another, and cause them to return beneath the liquid surface. Removal of part of the air molecules within the jar lowers the boiling point of water because less energy is needed for molecules to escape the liquid surface.