16. Diffraction.

16.1. Look at the difference between the simulation of green light passing through a small slit and through a slightly bigger slit using the following two buttons. What difference does the slit size make in the amount of spreading in the two cases?

16.2. Light, with it's very small wavelength, passes through a doorway without bending because the door is much larger than the wavelength. Sound, however, is a wave with wavelengths close to the size of the opening of a door. Explain why we can hear noise through a doorway to another room even though the source (a person,  radio, TV. etc.) is not in our direct line of sight.

16.3. Why would it be an advantage to have the largest possible opening for an optical device such as a teloscope?

16.4. Now look at the case of a slit of the same size with two different colors with the following two buttons. Which wavelenght bends the least, short or long?

16.5. All optical instruments (teloscopes, microscopes, even radio teloscopes which look at radio waves instead of light waves) have openings which act like a single slit. This means diffraction will be a problem for that instrument for some sizes of waves. If you want to reduce the effects of diffraction for a particular instrument, would you want to try to use longer or shorter wavelengths? (This is how electron microscopes can provide much higher magnification, electron waves can be much smaller than light.)

Diffraction can also be explained as a type of interference resulting from a path difference, as was the case for the double slit patterns. Recall in the ripple tank simulation of two sources (Simulation 10.4) waves from the source on the left must travel a longer path to get to a point at the top right of the simulation than waves from the source on the right. This path difference changes depending on how far to the right we look resulting in spots of destructive and constructive interference along the top. For a single opening instead of two separate sources we can imagine a row of sources filling up the single opening. Again there will be a path difference from different sources but the pattern will look different because of the sources in between.

16.6. Compare the patterns from the slits in the following two simulations with a much smaller wavelength (and greater resolution) which alows you to see the diffraction pattern (similar to a double slit interference pattern). Describe how the location of the spots of destructive interference change between the two opening sizes.