Double Slit Experiments II by Robert Nemiroff Michigan Technological University

Physics X: About This Course Pronounced "Fiziks Ecks" Reviews the coolest concepts in physics Being taught for credit at Michigan Tech o Michigan Tech course PH4999 o Title: Extraordinary Concepts in Physics o Aimed at upper level physics majors o Light on math, heavy on concepts o Anyone anywhere is welcome No textbook required o Wikipedia, web links, and lectures only o Find all the lectures with Google at:  "Starship Asterisk" then click on "Physics X" or 

Double Slit Experiment: Gamma Rays Substituted The usual double-slit experiment is done except now the photons used are gamma rays. This means that the wavelength of the photons is now much smaller than the width of the slits and the distance between slits. What happens? 1.An interference pattern appears. 2.No interference pattern appears. 3.The image screen remains blank. 4.A safety inspector insists you need a permit.

Double Slit ExperimentDouble Slit Experiment: Gamma Rays Substituted 2. No interference pattern appears. Comments: Gamma rays act like bowling balls and just plow through the slits without significant interference. The image screen will show a bright band behind each slit, the lit projection of the slits onto the image screen. Powerful enough gamma rays will just plow through the slit screen itself making the slits irrelevant.

Double Slit Experiment: Gamma Rays Substituted Slider: λ. As λ increases: light reddens from gamma ray to optical diffraction increases, dispersing the light directly behind each slit interference peaks move further apart, eventually becoming discernible

Double Slit Experiment: Radio Waves Substituted The usual double-slit experiment is done except now the photons used are radio waves. This means that the wavelength of the photons is now much larger than the width of the slits and the distance between slits. What happens? 1.An interference pattern appears. 2.No normal interference pattern appears. 3.The image screen remains blank. 4.A song plays that is too noisy to be a hit.

Double Slit Experiment: Radio Waves Substituted 3. The image screen remains blank. Comments: Radio waves will not be able to fit through the thin slits. Slider: Lambda. As lambda increases light reddens from optical to radio waves diffraction increases, concentrating less light behind each slit interference peaks move further apart, eventually disappearing.

Double Slit Experiment: Ordinary light bulb The usual double-slit experiment is done except now an ordinary light bulb is used emitting many colors all at the same time. What happens? 1.An interference pattern appears. 2.No interference pattern appears. 3.Interference for different colors appears with different spacings. 4.Psychedelic music begins to play.

Double Slit Experiment: Incoherent source 3. Interference for different colors appears with different spacings. Different colors of light have different wavelengths and hence the slit spacing and slit width will be different multiples of each wavelength. Each wavelength will still interfere, but now it will be harder to see because all of these interference patterns will appear on top of each other.

Double Slit Experiment: Color Filter Placed Over One Slit The usual double-slit experiment is done with an ordinary light bulb, except now a color filter is placed over one of the slits. Say this filter only passes red light from the white light source. What happens? 1.An interference pattern appears. 2.No interference pattern appears. 3.Only the red light interferes. 4.Only the red light avoids interference.

Double Slit Experiment: Color Filter Placed Over One Slit 3. Only the red light interferes. Comment: The red filter blocks all but the red light, so that only the red photons can go through both slits and interfere. Other colors only go through the slit with no filter. The image screen is therefore a superposition of red light interfering through both slits and non-red light shining through the no-filter slit without interference.

Double Slit Experiment: Wall Placed from Source to Slits The usual double-slit experiment is done except now an opaque wall is placed that extends from the source to the slit screen that divides the two slits. (A red-only laser is used.) What happens? 1.An interference pattern appears. 2.No interference pattern appears. 3.The image screen remains blank. 4.The wall becomes radioactive.

Double Slit Experiment: Wall Placed from Source to Slits 1. A normal interference pattern appears. Comments: Placing the wall does nothing to stop the coherence of the two slits, which act as two sources interfering. Placing the wall also does not give any observer "which way" information, meaning that the observer will know which photon went through which slit.

Double Slit Experiment: Wall from Slits to Image Screen The usual double-slit experiment is done except now an opaque wall is placed that extends from the slit screen to the image screen. The wall separates the two slits. What happens? 1.An interference pattern appears. 2.No interference pattern appears. 3.The image screen remains blank. 4.The wall makes strange barking noises.

Double Slit Experiment: Wall from Slits to Image Screen 2. No interference pattern appears. Comments: This wall separates the slits in a way that allows an observer to track which photons go through which slit. This "which way" information identifies specific slits which means that a photon cannot go through both slits at the same time and interfere with itself.

Double Slit Experiment: Identical sources replace slits The usual double-slit experiment is done except now two identical point sources replace the two slits. What happens? 1.An interference pattern appears. 2.No interference pattern appears. 3.Sometimes an interference pattern is visible, sometimes not. 4.Only identical twins can know the answer.

Double Slit Experiment: Identical sources replace slits 3. Sometimes an interference pattern is visible, sometimes not. Comment: The two slits are really placeholders for two identical sources that are coherent. Typically, two independent sources even chosen to be identical are only coherent for short periods of time, so that interference patterns may appear, disappear, and drift rapidly over even a short time. coherent Is this in conflict with Dirac's "Particles interfere only with themselves." quote? Possibly, but this itself is a matter of some debate!