# Measurement of the wavelength of monochromatic light

When monochromatic light is passed through a diffraction grating a number of bright lines (fringes) are formed as a result of diffraction and interference. A spectrometer, which must first be adjusted, is used to allow a parallel beam of light to fall on a diffraction grating and to view the fringes. The angles (θ) at which the fringes are produced can then be measured. In this simulation a grating with 350 lines per mm scatters yellow light. When the angles have been measured for each fringe (n = 1, 2 ..) the wavelength (λ) of the light can be calculated using the formula:
λ = d.sinθ / n.
Note: Angle measurements are not sufficiently accurate in this simulation; a vernier scale is used in a laboratory spectrometer.
Also, it is sometimes impossible to get the interference fringe exactly on the vertical cross-wire.

Procedure:
1. Using the slider drag the telescope up or down until the yellow line (1st. order image) is lined up with the vertical cross-wire.

2. Press the "Measure Angle" button and record the angle.

3. Move the telescope to the second order image and again record the angle.

4. Repeat for the third and fourth (if present) order images.

5. Move the telescope in the other direction and repeat steps 1 to 4. Calculate the average angle for each order number.

6. Click on "450 Grating" (450 lines per mm) to repeat the entire experiment with a different grating.

Precautions:
• Ensure that the slit, collimator, table, telescope and eyepiece are properly adjusted before using the spectrometer, use a narrow slit.
• In the lab. ensure that the bright fringes are exactly centered on the vertical cross-wire when readings are taken.
• Measure angles on both sides of the n = 0 position and take the average of the two readings.
• Have your eye directly above the scale when taking angle readings to avoid parallax errors

Light Experiments           All Physics Experiments