Thursday, Feb 26, 2009

Reviewed reflection and refraction.

Asked what value students got for the index of refraction of light in water. Used this value to calculate the speed of light in water.

Showed again that when going from fast to slow the wave bends towards the normal. When going from slow to fast, the wave bends away from the normal.

In going from slow to fast, as you increase the angle of incidence, you reach a point where the angle of diffraction is 90 deg. If you increase the angle of incidence any more, you do not get a refracted ray, instead you get total internal reflection. Demonstrated total internal refraction with laser and optic fiber.

Defined and calculated the critical angle.

Other examples of refraction are mirages and the fact that the Sun is already really well below the horizon when you see it setting.

Talked about diffraction - bending or spreading of a wave as it passes through an opening or around a barrier. Sent student out into hallway. They can hear you but not see you. Calculated the wavelength of the sound wave and compared it to the wavelength of light. Longer wavelengths are diffracted more. That is why you can hear around corners but not see around corners.

Used circular pattern overheads to demonstrated diffraction and interference.

Young's double slit diffraction experiment was used to measure the wavelength of light. Described the set-up and math for the experiment.
We used a double slit to measure the wavelength of laser light.

I then showed how to measure the wavelength using a ruler. We got a value of 590 nm.

Handed out worksheet on reflection and refraction.