LASER FUN

Materials:
~ 660 nm pocket laser pointer, hair, CD pieces (clear & mirrored), scissors, tape
paper, protractor, meter stick, vernier caliper

Day 1/2

Objective:
TLW use a laser to measure very small distances using diffraction

Terms:
vernier caliper, reflection, diffraction, refraction,
Pocket LASERs can be used for more than just exciting your dog or cat. In the right arrangement, they can be used to measure very small objects or detect small vibrations.

Aside from reflection, light rays can also be bent when they pass near an object as well. This is called diffraction. If a hair with diameter do is placed in the path of a laser beam of wavelength λ it will demonstrate wavelike properties by making an unexpected interference pattern on a screen a distance L from the hair. The distance the interference spots are apart d form a geometric relationship that can be exploited to find the thickness of the hair causing the diffraction pattern. Use a low power laser pen and try this at home using a hair or fish line!

Example problem:
A _ m diameter fiber cuts a 660 nm (.00000066 m) wavelength laser beam making a pattern 3.8 m away with the bright spots .0025 m apart
We use the simplified diffraction equation do=λL/d
So, do = .00000066 x 3.8 / .0025 = .001 m ( or 1 mm)

Here are 2 diffraction patterns. The upper one is air and the lower one was refracted through water. The ratio of the bright bar spacing of air over water = 1.33 (the index of refraction for water!). n = x1 / x2


  1. Learn how to use the vernier caliper, taking multiple measurements of various known objects to calibrate
  2. Record in your JOURNAL
  3. Measure the diameter of 3 different hair colors using the equation do=λL/d and a laser (λ=660 nm or .00000066 m )
  4. Record in your JOURNAL

Day 3/4/5

Objective:
TLW use trigonometry (d = x * tan(θ) ) to calculate larger distances and then make a laser rangefinder that will measure medium distances

  1. Go to a large area (hallway, commons, outdoors) and stand in front of your target (more than 30 feet away). Step 10 feet to the right or left and aim your modified protractor at the target and measure the angle (θ).
    Plug it into this equation: d = 10 * tan(θ).
    Measure the actual distance with a yardstick and compare.
    Repeat with 2 more targets with different distances
    Record in your JOURNAL

  2. Over the next 2 days, using a laser, a clear CD piece (splitter), a mirrored CD piece and protractor, create and test a rangefinder
  3. Take 3 known measurements to compare for accuracy
  4. Record in your JOURNAL
  5. What weaknesses are inherent in this method?