Physics Prac - Mesuring Wavelengths of Light



To determine the approximate value of l for red light using a diffraction kit.


Diffraction of light is one explanation of the wave theory of light. Francesco Grimaldi first observed this type of behvaiour from light. However, Thomas Young was the scientist that was able to derive a mathematical formula where l can be calculated. The original experiment involved sending white light into a screen with two small holes to serve as point sources. The light from these sources was then projected onto a screen and light and dark bands were observed. The mathematical formula could then be used after taking certain measurements.


Light source (Ray box), Red cellophane, Double-slit slide, and Measuring rod (can be already attached to Ray box.


1. Set up the ray box and place red cellophane in clip so that it is over the bulb.

2. Switch on ray box and stand approximately two meters away from the ray box, hold the slide up to your eye and look through one of the double slit patterns into the light. Observe disturbance pattern.

3. While looking through the slide, have someone move the white marker along the slide rule until its edge is approximately halfway inside a dark or light band.

4. Take measurement down and then repeat step 3 and gain measurement for other dark or light bands.


The results gained from the experiment were conclusive and accurate when compared to the original value for l for red light. Below is a table of the measurements recorded and calculations to gain l.

Calculation (con't):

The value d was calculated by placing the slide in a projector and then on the whiteboard (acting as a screen), marks were made. The slide had a measurement of 1cm and from this the ratio 35:1 was established. Measurements were then made and the slides actual measurements were determined. For this particular experiment, the slide marking D was used. L was measured also using simple trigonometry. The person viewing through the slide sat at a table and then looked down at the ray box on an angle. From measurements taken, L was able to be calculated quite accuratly.

Conclusion and Discussion

1. The l for red light is approximately 662nm (averaging out three results gained). This is within the acceptable range being 650nm ? 700nm.

2. Difficulties were experianced when trying to take measurements for x. It was hard to define the edge of the white marker and also count the number of fringes. There is no editing of the experiment that would make a significant difference unless different equipment is used.