The distance from point P to point C as measured perpendicular to the central antinodal line will be referred to as y. For water waves in a ripple tank, the resulting pattern would include locations along the water's surface where water was vibrating up and down with unusually large amplitudes (antinodes). That is, the y value will be two times its original value. The use of trigonometric functions allows one to relate the angle theta to the reliably measured distances d, y and L. The above logic has consistently assumed that the screen upon which the interference pattern is projected is very far away; that is, L >>> y. Inverse Square Law: Using The Equation I = W+4nd’, Determine The Intensity In W/m? | This can be proven by returning to the assumption that the screen is very far away (L >>> y). In 1801, this experiment was performed for the first time by Thomas Young. Complicating the task of observing the interference of visible light waves is the fact that light from the two sources must be coherent. Since there are one billion nanometers in a meter, and one thousand millimeters in a meter, the wavelength of red light is less than one-thousandth of a millimeter. Thus, the effects of interference for visible light waves are difficult to observe. As mentioned earlier in Lesson 3, each antinodal and nodal line is assigned a number or order value (m). That is, the y value will be three-halves its original value. The light diffracts through the slits and interferes in the space beyond the slits. Frequency and wavelength of reflected light are unchanged, Intensity of reflected is not equal to the intensity of the incident light. Even if the sources of light do not stay in step with each other, as long as the amount by which they are out of step remains the same over time, the light sources are said to be coherent. Terms The sin of the angle of refracted ray according to Snell's Law is, But it must be less than unity. In the previous section of Lesson 3, it was shown that the path difference (PD) for any point on the pattern is equal to m • λ, where m is the order number of that point and λ is the wavelength. Reflected from two mirrors at right angle, the light goes directly back for any incident angle Refraction . Since this ratio is a constant ratio, one could conclude that the distance across the screen between any two bright spots - whether adjacent or non-adjacent and whether to the central bright spot or any bright spot - can be measured and used in Young's equation as long as it corresponds to an m value which represents the number of spacings. The yellow triangle in the diagram on the left above is enlarged and redrawn in the middle of the graphic. Thus, a pattern of bright red and dark fringes or bands is observed on a screen as shown in the diagram below. The extra distance traveled by waves from S2 can be determined if a line is drawn from S1 perpendicular to the line segment S2P. In this section, the logic and mathematics associated with Young's equation was presented. That is, the y value will be three times its original value. And being a short wave, the distance between positions of constructive interference and positions of destructive interference is very small. The discussion of the interference patterns was introduced by referring to the interference of water waves in a ripple tank.