We will now be looking at light via the wave theory. Let’s review Constructive & Destructive Interference: When a source emits sound/light, it radiates outwards with the __________ having an ___ shape. (e.g. ripples in a pond) When you get far away enough, the __________ are close enough to being _________ lines.

When these parallel wavefronts reach a solid barrier that has a very small (single) slit in it, then they go through that point and __________ outwards again. We now have these waves head for another barrier, this time containing 2 small slits at very small distance apart. (aka ________ ________ ________ ______)

The same wavefront arrives at these double slits and they now produce 2 new radial sources of waves. These waves are now __________ (in phase). The 2 sets of wave patterns now __________ with each other. If the original source of waves is a single colour (wavelength) of light (aka _____________), and we put a screen on the other side of the double slit barrier, then we can see the following results.

When the wavefronts ______________ interfere, the light is ______. _____________ interference leads to no light. Diagram.(a) Both waves travel the same distance and interfere at point P.

Diagram.(b) wave from s 1 travels 1 extra than s 2 at point Q.  interference. (in phase) Diagram.(c) wave from s 1 travels ½ extra than s 2 at point R.  interference. ( out of phase)

Geometric View of Young’s Double Slit

Note: 1.) r 2 travels further than r 1 by a length of ________ 2.) If ______ =0, or some _______ _______ of the, then the 2 waves will hit the screen in phase and a _______ fringe will result. (Constructive interference.) 3.) If ______ is an ____ multiple of /2, then the 2 waves will hit the screen out of phase and a _______ fringe will result. (Destructive Interference)

Formulae: Constructive Interference / Destructive Interference. Where m represents the ________ in the number of wavelengths between the 2 waves, and m = _____________ and is known as the ___ order.

The intensity at each order is as follows: Note that the intensity of the brightness fringes __________ as the m th order increases.

Now, let’s look at the distance (y) between the fringes.

Assumptions/Approximations:  L>>d so,  is very small,  Tan   Sin  = and L  h  And  i.e.  So now,, which leads to: Bright fringe Dark fringe

Diffraction Gratings: A piece of glass/plastic that has multiple straight line scratches etched very close to each other, is known as a Diffraction Grating. It is simply a _______ of the Young’s Double Slit and acts as such.