Interference and diffraction of mechanical waves of presentation. Presentation on the topic "Interference. Diffraction." Observing interference in laboratory conditions

• A thin film is applied to the glass surface

Coated optics

The reflection of light for the extreme parts of the spectrum - red and violet - will be less. The lens has a lilac tint.

• Deviation of the direction of wave propagation from straight line at the boundary of an obstacle (waves bending around obstacles)
• Condition: the dimensions of the obstacle must be comparable to the wavelength

Grimaldi experience

• In the mid-17th century, Italian scientist Francesca Maria Grimaldi observed strange shadows from small items placed in a very narrow beam of light. To the scientist’s surprise, these shadows did not have sharp boundaries, but for some reason were bordered by colored stripes.

Observation conditions

• - the size of the obstacle must be commensurate with the wavelength of the light
• - the distance from the obstacle to the observation point must be much greater than the size of the obstacle

As a result of diffraction, light waves coming from different points are superimposed (coherent waves), and it is observed interference waves

Diffraction manifests itself in a violation of the straightness of light propagation!

Huygens' principle Fresnel

• Each point on the wave front is a source of secondary waves, and all secondary sources are coherent.

• Fresnel proved the linear propagation of light and quantitatively examined diffraction by various types of obstacles.

Peculiarities

diffraction pattern

Explanation

Slit image dimensions

more sizes,

received through

geometric

constructions

Secondary waves go behind

edges of the slit

Peculiarities

diffraction pattern

Explanation

In the center of the picture appears

light stripe

Secondary waves in

direction,

perpendicular to the slit,

have the same

phase. Therefore, when they

superimposed amplitude

fluctuations increase

Features of diffraction

Explanation

Along the edges of the picture - alternation

light and dark stripes

Secondary waves interfere

in a direction at an angle to

perpendicular to the slot,

having a certain phase difference, from

which the resulting

vibration amplitude

• Diffraction prevents clear images of small objects because light bends around objects.
• Images appear blurry. This occurs when the linear dimensions of objects are less than the wavelength of light.

Resolution of microscope and telescope

If two stars are at a small angular distance from each other, then these rings overlap each other, and the eye cannot distinguish whether there are two luminous points or one.

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Slide captions:

Interference of mechanical waves and light. Physics teacher S.V. Gavrilova

Wave optics Wave optics is a branch of optics in which light is considered as an electromagnetic wave.

Repetition What do you know about electromagnetic waves? An electromagnetic field propagating in space. The speed in vacuum is the highest.

Repetition List properties electromagnetic waves. Are reflected; The law of rectilinear propagation is fulfilled; Refracted, reflected, absorbed; Plane polarized; Interference and diffraction;

interference of mechanical waves of light and sound

Waves that have the same frequencies and a constant phase difference are called coherent.

The phenomenon of interference is possible if the superposition of coherent waves Coherent waves Strengthening or weakening of waves in space The time-constant phenomenon of mutual amplification and weakening of oscillations at different points of the medium as a result of the superposition of coherent waves is called interference. Interference Conditions

Conditions of interference maxima and minima Maximum condition A light band d 2 is observed, d 1 geometric path of the rays; d=d 2 -d 1 geometric path difference - the difference in distances from the wave sources to the point of their interference; Δ d = d∙n - optical path difference – geometric path difference multiplied by the relative refractive index of the medium. Maximum condition Condition max - the amplitude of oscillations of particles of the medium at a given point is maximum if the difference in the paths of two waves exciting oscillations at a given point is equal to an integer number of wavelengths.

Conditions for interference maxima and minima Minimum condition Minimum condition A dark band is observed Condition min - the amplitude of oscillations of particles of the medium at a given point is minimal if the difference in the paths of two waves exciting oscillations at this point is equal to an odd number of half-wavelengths

Distribution of energy during interference Waves carry energy During interference, energy is redistributed Concentrated in maxima, does not enter minima

History of the discovery of the interference of light The phenomenon of interference of light was discovered in 1802, when the Englishman T. Young, a doctor, astronomer and orientalist, a man with very diverse interests, conducted the now classic “two-hole experiment”. June 13, 1773 – May 10, 1829

Interference of light Light waves from different sources (except lasers) are incoherent. Coherence is achieved by dividing light from one source into parts. Interference of light is the phenomenon of superposition of light beams, which results in a pattern of alternating light and dark stripes.

Jung's classic experiment “I made a small hole in a window shutter and covered it with a piece of thick paper, which I pierced with a thin needle. I placed a strip of paper about one-thirtieth of an inch wide in the path of the sun's ray and observed its shadow either on the wall or on a moving screen. Next to the colored stripes on each edge of the shadow, the shadow itself was divided by identical parallel stripes of small sizes, the number of stripes depended on the distance at which the shadow was observed, the center of the shadow always remained white. These stripes were the result of the connection of parts of the light beam that passed on both sides of the strip and inflected, rather diffracted, into the shadow region.” T. Jung proved the correctness of this explanation by eliminating one of the two parts of the beam. The interference fringes disappeared, although the diffraction fringes remained. This experiment clearly proved that light is not a stream of particles, as was believed since the time of Newton, but a wave. Only waves, folding in different ways, are capable of both amplifying and canceling each other - interfering.

Interference pattern: alternating light and dark stripes Classical Young experiment Waves interfere in the overlap region Condition max: Condition min: d - optical difference in wave path - wavelength

color Wavelength, nm Frequency, THz red 760-620 385-487 Orange 620-585 484-508 yellow 585-575 508-536 green 575-510 536-600 blue 510-480 600-625 blue 480-450 625- 667 Violet 450-380 667-789 By studying interference fringes, Young first determined the length and frequency of light waves of different colors. Modern meanings are given in the table.

With the help of his theory of interference, Jung was for the first time able to explain a well-known phenomenon - the multi-colored coloring of thin films (oil films on water, soap bubbles, dragonfly wings...)

Interference in thin films Coherent light waves reflected from the top and bottom surfaces interfere. The result of interference depends on the thickness of the film, the angle of incidence of the rays and the wavelength of the light. In white light, the film has a rainbow color because the thickness of the film is not the same and interference maxima for waves of different lengths are observed in different places of the film

Newton's rings. Waves 1 and 2 are coherent. Wave 1 is reflected from the glass-air boundary. Wave 2 is reflected from the air-glass boundary. An interference pattern appears in the air layer between the glass plates

Thank you for your attention D.Z. §67- 69

Interference of mechanical waves. Wave addition
What happens to sound waves when
a conversation between several people while an orchestra is playing,
sings a choir, etc.?
What do we observe when we enter the water at the same time?
two stones fall
or drops?

Let's trace this on a mechanical model

Interference.

Coherent waves.

Interference of light

Interference in thin films.

Jung's experiment diagram

Observing interference in laboratory conditions

Interference maxima and minima

Bubble

Newton's rings

Diffraction is a deviation from the rectilinear propagation of waves.

Diffraction of light waves

Fresnel's theory.

The distance through which the lines on the grating are repeated is called the period of the diffraction grating. Denoted by the letter d. If

“Refraction of Light” - Refraction of light in various liquids and glass. Path of light rays Light rays and Fermat's principle. The geometric locus of all such foci of non-homocentric beams is called caustic. Mercury was poured onto the foil, which formed an amalgam with tin. Characteristics of light. A set of close rays of light can be considered a beam of light.

“Propagation of Light” - The path of rays in a thin lens. Optical instruments. 2. A ray of light falls on the surface of the water at an angle of 300 to the horizontal. If the image: -imaginary f< 0 -действительное f >0 If the lens: -converging F > 0 -diverging F< 0. D - расстояние от предмета до линзы. Линзы. Образование тени и полутени.

“Physics of Mirages” - Here is one of the paintings that can be seen. Performer: 9th grade student Vitaly Sergeevich Remeshevsky. Astigmatism. Mixed illusions. Levitation. Head: physics teacher Tatyana Gennadievna Dolmatova. Natural, or created by nature (for example, mirage); Changelings. The result is two images.

“Diffraction of Light” - Lesson Plan: Diffraction gratings are used to separate electromagnetic radiation into a spectrum. Diffraction of mechanical waves. Conditions for the coherence of light waves. Thus, after passing through the slit, the wave expands and deforms. Experience of T. Jung. 1802 Diffraction of light is accompanied by interference.

“The Human Eye” - What is the aurora? Why do we sometimes see things that are not really there? Rainbow formation area. And the circles, after all, are completely motionless. sin? /sin? = n1 / n2. Therefore, the observer sees the image distorted. The law of light refraction. Conclusion: 90% of information enters our brain through the eyes. We learned that the laws of optics are described using trigonometric functions.

“Interference and diffraction” - Fresnel biprism. A) from a thin wire; b) from a round hole; c) from a round opaque screen. Each point of the wave surface is a source of secondary spherical waves. Coating of optics n (film)

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