20_a wave diffraction interference refraction_pre

8/3/2019 20_A Wave Diffraction Interference Refraction_pre

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PHYSICS IN LIFE SCIENCE

Wave diffraction, interference and refraction


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Diffraction and its dependence on obstaclesize.

Constructive and destructive interference.

Light speed in vacuum and refractive index.

Snells law of refraction.

Myopia and hyperopia and how to correctthem.

Key concepts


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Reminder: Wave phenomena

A mechanical wave is the spreading ofvibration in a medium.


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The wave equation

Wave speed = wavelength / period

= wavelength

frequency All forms of wave obey this relationship.

v = l/T = lf


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Difference between particle motion

and wave motion

Visithttp://micro.magnet.fsu.edu/primer/java/particleor

wave/diffraction/ for theparticle-wave simulation.

Classical particles like bullets leave straightshadows.

Wave can bend into the shadow region and reach

places where the wave source is not directly visible.
http://micro.magnet.fsu.edu/primer/java/particleorwave/diffraction/http://micro.magnet.fsu.edu/primer/java/particleorwave/diffraction/http://micro.magnet.fsu.edu/primer/java/particleorwave/diffraction/http://micro.magnet.fsu.edu/primer/java/particleorwave/diffraction/http://micro.magnet.fsu.edu/primer/java/particleorwave/diffraction/http://micro.magnet.fsu.edu/primer/java/particleorwave/diffraction/


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Diffraction: when wave meets anobstacle

Part of the wave willbe reflected by theobstacle echo.

Part of the wave willmove around theobstacle, spreading to

points behind theobstacle diffraction.

Waves rejoining after moving

around


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If the obstacle is much smaller than the wavelength,

diffraction is significant, i.e., small obstacles cannotblock a wave.

If the obstacle is much larger than the wavelength,

diffraction is much less obvious, i.e., large obstaclescan leave an obvious shadow region behind it.

Wavelength and size of obstacle


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Diffraction through an aperture

When the obstacle is big but has a small holeon it, a wave can move around it through the

hole still it is the phenomenon of diffraction.


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Wavelength and aperture size

If the aperture is muchlarger than thewavelength, diffraction

is not obvious.

If the aperture is muchsmaller than thewavelength, diffraction is

significant.

wavelength distance between wavefronts


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Examples of diffraction

You can still hearsound from the other

side of the wall.

Water wave diffractsthrough a narrow

opening.


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Two wave pulses are travelling on a string.They pass through as if the other one doesnot exist.

Principle of superposition: the totalwaveform at any moment is simply the

summation of two waveforms.

What happens when waves meet?


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When two waves add up, they can cancel each

other or enhance each other: (a) destructiveinterference (b) constructive interference.

Interference

Watch video wave interference on slinky.


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Interference of 1-D waves

Peak meets peak;Valley meets valley.

Constructiveinterference

Peak meets valley;Valley meets peak.

Destructiveinterference

Peaks and valleysdont meet.

Partialinterference

For stable interference, two waves must have the

same l and f.


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Interference of wave in 2D or 3D medium

Some points have constructive interference;

Some points have destructive interference.

Interference pattern of two spherical waves


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Double-slit Interference

The two slits are two wave sources with the same f.

The interference pattern shows alternating regions

of constructive and destructive interference.

Peak meetspeak

(constructive)

Peak meetsvalley

(destructive)


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Double-slit Interference of light

Constructive or destructive interferencehappens at certain angles.

Light seems to be split into multiple beams.

what you see on ascreen after the slits


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Double slit experiment

Lasers are very good monochromatic (singlefrequency) sources and are commonly used for

double slit experiments


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When a wave goes from one medium intoanother, the direction of the wave often changesat the interface. This is refraction.

Refraction of wave


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Reflection and Transmission of Waves

Refraction

Diffraction

Water waves travel faster on the surface of deep waterthan they do on shallow water.

Faster v longer wavelength or larger distance betweenwave fronts. However, the wave front at the interfaceneeds to be continuous. This leads to a tilted wave frontin shallow water.

Refraction happens because wave speed is changed.

Refraction of water wave


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Light speed in a medium and

refractive index Speed of light:

In vacuum c=3108 m/s.

In a medium v=c/n, n>1 is the refractive index ofmaterial.

Wavelength:

In vacuum l0 = c/f.

In a medium l= v/f = l0/n.

Larger n slower speed and shorter wavelength. Refraction will happen when light goes from one

medium into another medium of different n.


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n2sin

2= n

1sin

1

Snells Law of Refraction

n1

n2

q1

q2

q1: angle of incidence (or reflection)q

2: angle of refraction

q1

Snells law:

Note: all angles are between the light beam and

the normal direction!


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Example

q1

q2

n1=1.00

n2=1.33

q1

q2

n1=1.51

n2=1.33

From air to water From oil to water

q1>q2 q1


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Refraction of light

Light speed is slower in a medium than in air:c/v = n (refractive index)>1.Refraction changes the direction of light,

making things in water look shallower.
http://www.qedoc.org/en/index.php?title=Image:Pencil_in_a_bowl_of_water.png


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The problem of a fisherman

Fishes in water are merely images ofrefraction. Where should a fisherman aim?


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Refraction of curved surfaces lens

With curved surfaces, lenses can converge ordiverge light rays.


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Convex and concave lens

Concave lens is

thinner in the middle.

It defocuses light.

Convex lens isthicker in the middle.

It focuses light.


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Image projection by a convex lens

Light from differentpoints of a far object isfocused onto differentpoints on the other side

an image is formed. It projects an object

into an inverted image.

The image can be

magnified or reduced,depending on thedistance between theobject and the lens.


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Lens in human eye

The focus of lens is adjusted by the surroundingmuscles.

To see a closer object clearly, the lens need to have

a shorter focus.


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Image projection in our eyes

An inverted reduced image of external objects is

formed on the retina.


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Focusing problems of an eye

Refraction by the eye in (A) emmetropia; (B)hyperopia (farsightedness); and (C) myopia

(nearsightedness).


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Correcting myopia and hyperopia

Nearsightedness: eyesfocusing too tight use

concave lens to relax focusing.

Farsightedness: eyes focusingtoo loose use convex lens to

enhance focusing.


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Next

Nature of light.

20_a wave diffraction interference refraction_pre

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