Difraction on microobjects


  1. Acknowledge yourselves with the experiment „Diffraction of electromagnetic radiation on microobjects“ (in our case, the microobject is a slit) found on the following link http://proj-x.karlov.mff.cuni.cz/sterbina_en.html.
    • Make sure, that you can imagine the experiment and describe terms, which are tematically connected with it. Can you draw the scheme of this experiment and describe functions of individual devices?
    • Which parameters can you change?
    • Think how you can effectively use measured data.
  2. Learn theory.
  3. Make detailed measurements of the diffraction pattern intensity distribution , distance a = (2152 ± 1) mm from the slit. Make measurements for both sources of light (red laser of the wavelength λ1 = (632 ± 10) nm and a green one with the wavelength λ2 = (532 ± 10) nm) and for both sizes of slits.
  4. Determine the magnitudes of both slits d . It is possible to use either positions of minima (equation (3)) or maxima (equation (4)). For determining of the corresponding angles of minima (maxima), it is necessary to measure the locations of maxima (minima) ymax,min ,taken by the photodiode as accurately as possible and then calculate the corresponding angles of minima (maxima) according to the following relation
    equation 1
    For more accurate determination of magnitudes of the slits it is possible to use an optimalisation programme fitting the model dependence (1) by the measured points (for example using the software SigmaPlot, Origin, eventually program GNUPLOT - http://www.gnuplot.info).
  5. Evaluate the errors of magnitudes of the slits.

Error analysis

Determine an error of the width of the slit, d, for example from a location of the first minimum value, for which from equation (3) is

equation 2
as well as
equation 3

Assuming that for small angles we are working with it is ( angles in arc units! )

equation 4
it is possible to write the following expression for the slit width
equation 5
and then for its error
equation 6

Discuss, which of the quantities has the largest impact on a value of the error of the microobject.

  1. At the end of the experiment, convice yourselves, that you can answer these questions (about experiment, measurement, terms, theory):
    • What phenomenon is observed in this experiment?
    • Which parameters are fixed in this experiment?
    • Which parameters are changeable (by you) in this experiment?
    • Is parameter changed discretly or continuously?
    • What source of light can be used?
    • Can we use arbitrary source of light, if we want to observe diffraction? What is the limiting factor?
    • Are the source of light here monochromatic?
    • What is the wavelength of the green laser?
    • What is the wavelength of the red laser?
    • It is possible to simultaneously use both lasers in this experiment?
    • Compare wavelenghts of green and red laser. Which is higher / lower?
    • Which parameters change, when the width of the slit is changed? Was your estimation correct? Repeat the measurement and verify.
    • What is the dependence of intensity on screen coordinate behind the slit?
    • How will change the interference patter, if we use another slit? E.g. 2D optical grid? (see Motivation)
    • Which laser has higher intensity in main maximum - red or green?
    • What change do you observe on interference patter, if we change the width of the slit (light source is unchanged)?
    • Have you figured out, that you can choose the measurement time and download it to PC?
    • Have you tried to choose the measurement data?
    • Have you created table for measurement data?
    • Explain the term electromagnetic waves.
    • Explain the term light.
    • The word "beam of light" is often used. Why do you think people use it to describe light?
    • What is interference and diffraction?
    • Light has wave-like and corpuscular behaviour. Which one does apply in diffraction?
    • When do we talk about diffraction of waves?
    • How does the distribution of the intensity change when the source is very far?
    • How many maxima are observable in this experiment? How are they called?
    • Estimate and then calculate the angle for second minor maximum.
    • How many angles can be determined from the experiment? What is their use?
    • Do we observe interference and diffraction of light in the way as mechanical and acustic waves?
    • Which interference and diffraction is easier to observe? Acustical or optical?

Were you able to answer above questions? If not, find theory and read it, go through the experiment once more.