Evans SPH 4U1

Physics Grade 12

Unit 5: Waves

Note 5: Light and Young's Double Slit


Reference: Chapter 9.5


Attempts at Showing Interference in Light:

Many scientists attempted to see if light would demonstrate an interference pattern like that shown by water waves in a ripple tank.  However, they used two incandescent light sources as their 2 point sources.  However, the two incandescent light sources emit light randomly and out of phase so no single interference pattern was seen (since the phase relationship was changing rapidly and randomly).  Finally Young used one incandescent source and directed the light through two pinholes. Since the light emanated from one source, the two beams of light were always in phase and an interference pattern could be seen.

A monochromatic (one colour and one wavelength) light source was required.


For Young's experiment, L is much greater than xn and the following can be derived:

where d is the separation of the slits, L is the perpendicular distance from the slits to the screen and is the distance between adjacent nodal lines.



                was for destructive interference (Nodal Lines)


Often when we are dealing with light we are usually finding the constructive interference (Maximum) and so the formula becomes

Wave theory could still not explain the movement of light through a vacuum, since waves needed a material medium for their transmission. However, scientists NOW believed so much in the wave theory that they made up a fluid filling space and called it "ether".  No experiment has ever detected ether.


Example 1:

A student stands 4.0 m from a monochromatic light source and observes the light through a pair of narrow parallel slits 2.0 x 10-2 cm apart. He determines that the distance from the first node to the eighth node in the interference pattern is 8.0 cm. Calculate the wavelength of the light.


Text Questions:

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