MPTC
Lasers
Introduction
Laser light is distinguished from other common light sources because it is amplified,
monochromatic, collimated, and coherent. These properties can easily be investigated with a low
power laser, such as that used for a pointer. Light is monochromatic if it consists of a single
color, or equivalently, a single wavelength or frequency. Collimated means that the light
produced is composed of parallel light wavefronts (recall the diffraction lab). Coherent light is
composed of light waves that have a constant relative phase; at any given place, all the light
passing through from the source has a maximum at the same time. Coherence implies that the
light is already monochromatic, otherwise different colors would soon get out of phase because
of their different frequency.
Non-laser light sources may achieve some of these properties but not all, at least not to
the same extent as a laser source and not with the same intensity (power/area). Quantum
coherence allows the production of photons of the same energy (monochromatic), travelling in
the same direction (collimated), and with wavefunctions that are in-step (coherent). At the
atomic level, the laser achieves this by Einstein’s idea of stimulated emission of coherent light
from excited atoms, light that goes on to stimulate further emission and hence the high intensity
of the laser beam. The geometry of the laser cavity that holds the atoms ensures that only
photons travelling in the same direction escape to form the external laser beam.
Equipment
• flashlight containing white LED (light emitting diode), red LED, laser pointer
• diffraction grating (in slide mount)
• 1 sheet of white paper (8.5” x 11”)
Caution!
Never look directly into the laser beam or point the beam at anyone else! Never
look at laser light reflected off of a highly reflective surface like a mirror or a ring.
Instead, view a reflection of the beam from a piece of paper or a wall. Viewing
laser light directly can burn your eye, causing permanent damage.
For all sources of light, so your eyes are not strained observe in short bursts rather
than viewing continuously for long periods.
Procedure
Monochromaticity
Hold the diffraction grating near your eye and look at the lights in your classroom.
Draw and label a diagram of what you see.
Hold the white LED at about arm’s-length and point it sideways, so the beam is not directed
directly at your eye. Hold the diffraction grating near your eye and look at the white LED.
Draw a diagram showing what you observe, and label the colors that you see.
Repeat the previous step for the red LED.
In the next step, for safety, point the laser down at the table, so the beam hits a piece of paper.
Never look directly into laser light or point the light at anyone else. It can burn your eye. Place a
piece of paper on the table, below eye level. Shine the laser light down onto the paper to form a
bright spot. View the laser light spot through the diffraction grating at a distance of an arm’s
length away.
Draw a diagram showing what you observe and label the colors that you see.
Collimation
Place a piece of paper on the table and hold the red LED about 10 cm above the paper, pointing
straight down at the paper. Slowly move the light source a further away from the paper. Notice
what happens to the illuminated area on the paper produced by the light source as the distance
between the light source and the paper increases. Repeat for the laser.
To gather some quantitative evidence for the laser, you will take measurements of an illuminated
spot’s diameter for three different distances, two between 0 and 100 cm and one very large
distance. Place the laser pointer on a steady surface. No more than a meter away, tape a piece of
paper vertically to a wall or a book. Shine the laser directly at the paper.
Safety check - Make sure you never look directly into the light coming from the laser. When you
are doing this experiment, make sure the eye level of anyone in the class is above the level of the
laser beam.
Measure the distance between the front of the laser and the paper. Outline the central illuminated
spot on the paper and label this spot with the distance between the laser and the paper. Repeat
this for a different distance between the laser and the paper. Then repeat for a very long distance
– for example, across the lab room, or along the corridor – taking care to keep it below eye level
and reflecting surfaces. Measure the diameter of each spot and record your data with error
estimates.
Turn off the laser and measure the size of the aperture through which the laser light emerges.
Calculate the angular size of the central bright maximum expected from diffraction through a
circular aperture and compare with your data (you will need to look up a suitable diffraction
formula).
Coherence
Hold the laser pointing away from you close to a sheet of paper so that it makes a shallow angle
about 30o with the paper.
Draw a diagram and describe the detail you see in the spot on the paper.
Repeat for red LED.
Draw a diagram and describe what you see.
Discussion

For each light source used today:
Do you think the light source produces a single wavelength or multiple wavelengths?
What evidence do you have?

Is the laser diffraction angle you calculated consistent with the one you observed?
What does this tell you about collimation of the laser light?

Paper fibers make the surface of the paper appear rough at a microscopic level. When
light strikes the paper at a shallow angle and reflects off of the paper fibers, the light
waves travel various distances to get to your eye and these waves interfere.
How would this explain what you observed for the laser and red LED source?
Further Reading
http://www.laserfest.org/resources/lesson-teacher.pdf