Physics 10. Lecture 29A. "There are two ways of spreading light: to be the candle or the mirror that reflects it." --Edith Wharton

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Physics 10

Lecture 29A

"There are two ways of spreading light: to be the candle or the mirror that reflects it."

--Edith Wharton

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Converging Lenses

What if we wanted to use refraction to

converge parallel light rays to a single focal point? What type of shape should we use?

Recall our prism example:

What would happen when if flipped the prism?

A parallel light ray was pushed downward.

A parallel light ray was

pushed upward.

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Converging Lens

This is the basis behind constructing different types of lenses for distorting light.

We can construct the following lens:

This is known as a converging lens.

It is a lens consisting of plastic or glass that

refracts light.

It focuses parallel light rays at a single point

known as the focal point.

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Converging Lens

Converging lenses are thick in the middle and thin near the edges.

They have positive focal lengths.

A thin lens has two focal points, corresponding to

parallel light rays from the left or from the right.

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Diverging Lens

We can also construct the following lens:

This is known as a diverging lens.

It is also a lens

consisting of plastic or glass that refracts light.

It defocuses parallel light rays to make it

appear that it came from

a single point known as

the focal point.

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Diverging Lens

Diverging lenses are thick on the edges and thin in the middle.

They have negative focal lengths.

A thin lens has two focal points, corresponding to

parallel light rays from the left or from the right.

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Interference

Treating light as a particle helped us to

understand how images are formed by lenses and mirrors by constructing ray diagrams.

But we can also understand different phenomena by treating light as a wave.

For example, light waves will interfere with

each other just like the sound waves and waves on a string that we dealt with earlier.

This means we can get areas of constructive

interference and destructive interference just

like we had when we produced standing waves.

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Double Slit Experiment

Two narrow slits, S

1

and S

2

, can act as sources of waves.

The waves emerging from the slits originate from the same wavefront and

therefore are always in phase (coherence).

The light from the two slits form a visible pattern on a screen.

The pattern consists of a

series of bright and dark

parallel bands called fringes.

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Double Slit Experiment

Constructive Interference occurs at the center point.

There is no path length difference between these two waves.

Therefore, they arrive in phase with each other.

This will result in a bright area on the screen.

This bright spot is called the central maximum

(zeroth order maximum).

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Double Slit Experiment

If we moved to a spot, Q, below the center spot, P, we find that the upper wave, S

1

, to travel farther than the lower wave, S

2

.

If the upper wave

travels one wavelength farther than the lower wave; the waves will still arrive in phase.

A bright spot will occur.

This bright spot is called

the first order maximum.

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Double Slit Experiment

The fringe pattern formed by a Young’s Double Slit

Experiment would look like the picture to the right.

Alternating bright and dark fringes are created.

Constructive interference

occurs where a bright fringe appears.

Destructive interference

results in a dark fringe.

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Single Slit Experiment

A single slit placed between a distant light source and a

screen produces a diffraction pattern (similar to a double slit experiment).

It will have a broad, intense central band.

The central band will be flanked by a series of narrower, less intense

secondary bands (secondary

maxima).

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Interference in Thin Films

Have you ever looked a

soap bubble and observed patterns of different colors?

Light wave interference can be observed in thin

films (such as an oil film on water or soap bubbles).

The interference in thin films is caused by not

only a path length difference but also by a phase shift as the light ray reflects

With this phenomenon some colors are being enhanced.

This comes about due to the wave nature of light.

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Interference in Thin Films

When light hits a semi-transparent region it has two choices: 1) to be reflected or 2) to go

through.

If two separate light rays travel the different paths, then they are no longer equivalent.

One light ray had to travel

further than the other light ray.

The interference in thin films is caused by not

only a path length difference but also by a phase

shift as the light ray reflects

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Interference in Thin Films

This path length difference can lead to light rays now having crests meet up with troughs.

This can lead to destructive interference (if the path length difference is just right).

So, some colors will not be seen, making the other colors appear more vibrant.

The interference in thin films is caused by not

only a path length difference but also by a phase

shift as the light ray reflects

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Thin Film Interference

Thin film interference is used by Morpho butterflies to intensify the colors reflected.

The Morpho butterfly has “ridge stacks” on its

wings that causes blue to be emphasized and

all other colors to be de-emphasized.

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Polarization of Light

Light from the sun is produced by the vibrations of multitude of atoms located there.

Each atom produces a wave with its own orientation of the electric field.

All directions of the

electric field vector are equally possible and are in a plane perpendicular to the direction of

propagation.

This type of wave is

known as an unpolarized

wave.

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Polarization of Light

A wave is said to be linearly polarized if the resultant electric field vibrates in the same direction at all times at a particular point.

It is possible to polarize an unpolarized beam.

The most common technique for

polarizing light is called

polarization by selective

absorption.

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Polarization of Light

In this technique, you use a material that

transmits waves whose electric field vectors in that plane are parallel to a certain direction.

This material also absorbs waves whose

electric field vectors are perpendicular to that direction.

This device is known as a polarizer.

The material

is known as

a Polaroid.

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Polarization of Light

Light can also be polarized by scattering it off of

particles.

The electrons in the medium can absorb and reradiate

part of the light.

Sunlight that reaches an

observer on earth becomes

polarized as it scatters off

of air molecules.

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Clicker Question 29A-1

Suppose the viewing screen from our in-class double-slit demonstration is moved closer to the double slit apparatus. What happens to the