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# Lecture 40: Final Review Lecture

## Young's double slit experiment

If we treat each of the slits as a point source of circular wavefronts. The condition for constructive interference (bright fringes) is

$d\sin\theta=m\lambda$       (m=0,1,2,..)

and for destructive interference (dark fringes)

$d\sin\theta=(m+\frac{1}{2})\lambda$       (m=0,1,2,..)

## Reflection from a transparent medium

As in the case of a wave on a rope that is incident on a heavier rope and is reflected with a 180o phase change when a light wave is reflected from a more optically dense media a 180o phase change occurs. This effect is important when we want to consider interference effects in thin films.

## Air wedge and Other Effects

Interference effects can also be observed when light is reflected from the gap between two glass surfaces, which leads to the phenomena known as Newton's Rings.

A similar problem is that of the air wedge, such as shown here.

For a single wavelength dark stripes will occur whenever

$2t=m\lambda$      (m=0,1,2,..)

and bright stripes will occur whenever

$2t=(m+\frac{1}{2})\lambda$      (m=0,1,2,..)

For white light different colors will experience constructive interference at different thicknesses, leading to the colorful lines we see when an air gap is under normal light.

This can be dramatically demonstrated with a soap bubble.

## Anti reflective coating

For most lenses we want as much of the incident light to be transmitted as possible.

Suppose we take a glass lens with refractive index $n=1.52$. We can see from the reflectance equation

$R=(\frac{n_{0}-n_{1}}{n_{0}+n_{1}})^2=(\frac{1-1.52}{1+1.52})^2=0.043$

that about 4% of the incident light is reflected.

This percentage can be reduced by the use of an anti reflective coating.

Ideally we would use a coating that produced an equal amount of reflection at both interfaces, but there is no suitable material with the required refractive index, $n=1.26$, so we use magnesium flouride MgF2.

As the two reflections both occur from more optically dense media they both experience a phase change of $\pi$ on reflection which corresponds to advancing the wave by $\frac{\lambda}{2}$ .

To have the light be out of phase we need to light that goes through the coating to have advanced by $\frac{\lambda}{2}$ for destructive interference to occur.

Critically, when destructive interference occurs the light is not lost, but is instead transmitted.

As the wavelength of light in a medium is given by $\lambda=\frac{\lambda_{0}}{n}$ where $n$ is the refractive index of the medium and $\lambda_{0}$ is the wavelength of the light in free space, the thickness of the coating should be $\frac{\lambda}{4n_{2}}$.

In practice the light incident will not all be the same wavelength, so the thickness of the coating is typically chosen to work optimally in the center of the visible band (~550nm).

## Intensity pattern for a single slit

Rather than writing our equation in terms of k

$I_{\theta}=I_{0}\mathrm{sinc^{2}}(\frac{Dk}{2}\sin\theta)$

it is convenient to write it in terms of the wavelength, using $k=\frac{2\pi}{\lambda}$

$I_{\theta}=I_{0}\mathrm{sinc^{2}}(\frac{D\pi}{\lambda}\sin\theta)$

This function has a central maxima and then minima at

$D\sin\theta=m\lambda$       $m=\pm 1,\pm 2,\pm 3,..$

## Diffraction in the double slit

Now we have an expression for the intensity from a slit of width $D$

$I_{\theta}=I_{0}\mathrm{sinc^{2}}(\frac{D\pi}{\lambda}\sin\theta)$

we can use consider slits as sources instead of the point sources we considered earlier which gave

$I_{\theta}=I_{0}\cos^{2}(\frac{d \pi}{\lambda}\sin \theta)$

The combination of these gives

$I_{theta}=I_{0}\mathrm{sinc^{2}}(\frac{D\pi}{\lambda}\sin\theta)\cos^{2}(\frac{d\pi}{\lambda}\sin \theta)$

## Diffraction grating

Irrespective of the number of slits $n$ the condition for maxima is the same as for the double slit

$d\sin\theta=m\lambda$       (m=0,1,2,..)

but that the larger the number of slits from which diffraction occurs the sharper the maxima will be.

## Polarizers

The polarization of the light after it has passed through the polarizer is the direction defined by the polarizer, so the way to approach the calculation of the magnitude of the intensity that passes through the polarizer is by finding the component of the electric field which is in that direction

$E=E_{0}\cos\theta$

the intensity is then

$I=E^{2}=E_{0}^{2}\cos^{2}\theta=I_{0}\cos^{2}\theta$

This formula is known as Malus' Law.