Lecture 14: Chapters 34 (Young’s Double Slit) and Section 35-11 (Polarization)

Gone fishing, 1

To shoot a fish with a gun, how should you aim?

1. aim directly at the image
2. aim slightly above
3. aim slightly below

Ans: C. Due to Snell's law, the fish looks higher than it really is.

Gone fishing, 2

To shoot a fish with a laser gun, how should you aim?

1. aim directly at the image
2. aim slightly above
3. aim slightly below

Ans: A.

Superposition

Phase difference

The two waves shown are

1. out of phase by $\pi$.

2. out of phase by $\pi / 2$.

3. out of phase by $\pi / 4$.

4. in phase.

Ans: B. One wavelength corresponds to $2\pi$. Quarter wave length difference here.

Young’s double slit experiment

An interference pattern is seen from two slits. Now, cover one slit with glass, introducing a phase difference of $\pi$ (180$^\circ$; half wavelength) at the slits. How is the pattern altered?

1. pattern vanishes.
2. pattern expands.
3. bright and dark spots are interchanged.
4. no change at all.

Ans: C. Oveall phase difference is what matters.

Young’s double slit experiment

In a double slit experiment, if the wavelength of the light is increased, the interference pattern will

1. spread out.
2. shrinks together.
3. stays the same.
4. disappears.

Ans: A. $d\sin \theta = m \lambda$. With $d,m$ fixed, bigger $\lambda$ means bigger $\theta$.