LAB: Determining the Wavelength of a Red Laser Pointer.
Purpose: to learn the difference between continuous and line spectra, and to
determine the wavelength measurement of a red laser light.
Materials: laser pointer, 500 lines/nm diffraction grating, string, ring stand, ring,
clay triangle, ruler, clamp, white paper, flashlight
Procedure:
Part 1: Measuring the Wavelength of a Laser Pointer
1. Place a piece of white paper at the base of the ring stand. Adjust a small ring to
the stand so that it is approximately 200 mm from the paper. Place the clay triangle
on top of the ring, and then place the 500 line/mm diffraction grating on top of the
clay triangle.
2. Clamp the laser pointer to the stand so it is just above the diffraction grating. Its
red beam will point down through the diffraction grating onto the paper.
CAUTION: DO NOT POINT THE LASER POINTER INTO ANYONE’S EYES!
3. Turn the laser pointer on so that the red dots are on your white paper. You
should see a central bright red dot and two somewhat dimmed red dots to both sides
of this dot (some distance away). These dots are the first-order diffraction image of
the laser beam. (you may even see two even dimmer red dots further away which
are the second-order diffraction image of the laser beam). Record your
observations.
4. While shining the pointer through the diffraction grating, mark the location of
the central dot and the first order dot with a small “X” on the white paper. Measure
the distance between the central dot and the first order dot. This is distance X.
Record this measurement in mm in the data section.
5. Measure L by stretching a string from the first-order dot to where the laser light
passes through the diffraction grating. Use a ruler to determine the length of the
string. Record this measurement in mm in the data section.
Part 2: Observing the Wavelengths of a Flashlight
1. Replace the laser light with a flashlight.
2. Record your observations and turn off the flashlight.
Observations.
Data.
Calculations.
Pre-Lab Questions
1. What is a diffraction grating?
2. You see a girl with a beautiful diamond ring that has the sun shining
brightly on her finger. If purple and green are reflecting onto a piece
of paper near her hand, what wavelengths of light do you suspect are
reflecting off of her diamond?
3. A laser light was shone through a diffraction grating whose lines
were 1/1000 mm apart. The distance was measured between the
center spot and the first side spot and found to be 99 mm. The
distance from the diffraction grating to the first side spot was 154
mm. Calculate the wavelength of light in nm that the laser pointer
was emitting.
nλ = d • X/L
n = 1 d = ____________ X = _____________ L = ______________
( HINT: convert d into a decimal number)
4. What color was the light in the laser for question 3?
Post Lab Questions
1. Is the value you obtained for the wavelength of the laser light
consistent with the range of wavelengths for red light? Is this what
you expected?
2. Why was the pattern seen when shining the flashlight through the
diffraction grating different when shining a laser pointer light through
it?