# Example Of Light Diffraction

The light waves that are visible to the naked eye are in the wavelength range. In the second part of the video(single slit diffraction), a narrow slit. ββββ Answer: 2 πππ question β Red and blue light are simultaneously passed through a diffraction grating. The first-order maxima for the red light is located 20 cm from middle of the central bright.

Diffraction is a process by virtue of which a system of waves spreads out after passing through narrow gaps. The system of waves includes sound waves, light waves, electromagnetic waves, water waves, etc. Diffraction, in general, is the bending of waves around a small aperture. The process of diffraction was first observed by Francesco Maria Grimaldi, an Italian mathematician and physicist. His work was officially published in the year 1665. Diffraction is quite a versatile process that can be seen in a lot of daily practices.

For instance, the following are some real-life examples of diffraction:

## 1. Compact Disk

Compact Disks are most susceptible to the process of diffraction. The surface of the compact disk is shiny and consists of a number of grooves. When light falls on the top of a CD, a part of it gets diffracted while some part of the light gets reflected. This is the reason why we see a rainbow-like pattern on a compact disk.

## 2. Hologram

Hologram, the word has been derived from two Greek words, βholoβ means whole, and βgramβ means a message. It is one fine technology that promises us an incredible future. Hologram basically makes use of diffraction to generate a 3D impression of the image. Different versions of the image get diffracted and reach the lens from multiple sides, all together forming an interference pattern. This pattern is then made to fall on the holographic plate. Finally, providing us with a 3-Dimensional experience.

## 3. Light entering a dark room

Suppose, there is a room with no light source, plus the light from the door is forbidden to enter the room as it is closed, and when someone opens the door partially, you can observe that the light gushes inside with a bend across the edges and around the corners of the door. The door acts as an obstacle in the path of light, therefore the light bends. This bending, undoubtedly, is known as diffraction.

## 4. Crepuscular Rays

You must have seen this breathtaking view for at least once in your life. These magnificent looking rays are known as crepuscular rays or God rays. When the light rays from the sun try to reach the ground but are blocked by the clouds, the light waves get diffracted and deviated. This deflection of light, due to the presence of a barrier in its normal pathway, is nothing but diffraction. The next time you see such a stunning view, you can share the reason behind it.

## 5. X-Ray Diffraction

In x-ray diffraction, the sample is kept in an instrument and is illuminated with x-rays. The x-ray tube and detector move in a synchronized motion, the observed signal is then recorded and studied. This phenomenon is most widely used in the determination of the distance between two consecutive atoms of an element. The process of X-Ray Diffraction is very important in meteorological, pharmaceutical, chemical, and other related industries as whenever the researchers come across some unidentified elements, they need to configure out the details about its structure, beginning with the alignment, distance, and other characteristics of its atoms.

## 6. Water passing from a small gap

The flowing water of a river when confronts a small slit, it tends to break its normal flow. The water waves undergo bends at the other side of the slit. This bending is yet another example of diffraction.

## 7. Solar/Lunar Corona

In meteorological terms, the term corona describes the ring of light around the sun or the moon that is formed when the sunlight or moonlight gets diffracted by small water vapours or ice crystals. The halo of the moon is known as the lunar corona and that of the sun is known as the solar corona.

## 8. Sound

If someone calls your name loudly, you are able to hear it. If they hide behind a tall tree and call your name with the same intensity, would you be able to hear that? The answer is yes, but how come the sound is not blocked even when a huge tree is present in its pathway. The reason being sound travels and reaches your ear through the process of diffraction.

## 9. Ring of light around the source

Look at any source of light around you right now, you may observe the light does not directly get transmitted in the forward direction, a small part of the light energy is diffracted around the source. This diffraction of light occurs due to the presence of dust and gaseous particles present nearby.

## 10. Signal Propagation

The process of diffraction is significantly used in long-distance radio signal propagation. Due to the curved surface of the earth and huge obstacles present on it, line of sight propagation for long-distance is not possible. Which is why we make use of multi-level diffraction for a signal to reach its destination. The signal keeps striking the obstacles while being amplified simultaneously with the help of boosters again and again till it reaches the destination. Diffraction is responsible for the phone calls youβre able to attend.

## Serendipitous observation

One fine day, I was looking at my mobile screen sipping my tea under a tree. At one particular angle, I could see a pattern of colours reflected from my mobile screen. My mind was filled with questions 'What could be the reason? , Why? , why? '

After pondering for a few days, I can up with an explanation. The display in my phone is made of tiny physical elements called 'Pixels'. These pixels diffract the incident light to form a pattern. Later that night I noticed a lot of phenomena occurring around me are attributed to Interference and diffraction effects. Their effect is ubiquitous. In this article, I will first explain theory behind these effects followed by specific examples that we encounter in our everyday life.

## What are Waves?

Waves are disturbances that carry energy, information etc from one place to another. For example, a sound wave carries that travels from a source (speaker) to a listener

The most common of them are sound and light waves. The light waves that are visible to the naked eye are in the wavelength range of 400nm to 700nm and sound waves audible to the human ear are in the range 17mm to 17m. (Note the stark contrast between wavelengths of the two waves)

## Interference And Diffraction:

### Interference:

Interference is a phenomenon that occurs when two or more coherent( same frequency and constant phase difference) waves interact with each other. They redistribute energy in space that results in a pattern.

In the first part of the above video, you can see that when two point sources interfere, they redistribute energy. You can see the energy redistributed alternatively with concentration in few areas. In areas where energy is maximum, the two waves are interfere constructively and in areas where energy is minimum they interfere destructively.

Things to remember:

• Interfering waves should be monochromatic and must have a constant phase difference.
• Different colors/wavelength will have different conditions for interference. A maximum for one color might be a minima for another.

## Diffraction:

Have you ever wondered whether you can bend a wave, deviate from its path? If you have thought about it, you are in for a thrill. So why a light would bend? Imagine this, a small particle and a big hole. Do you think the big hole will hinder small particle when you drop inside? Obviously it won't. But what if the hole's size was comparable to the size of the particle? Yes, definitely it would obstruct and deviate its path. This is exactly what diffraction is about.
The bending of waves around the small corners/apertures (of the size of wavelength) is called diffraction.

In the second part of the video(single slit diffraction), a narrow slit comparable to the wavelength of incident wave, bends and spreads after passing through the slit.

### Diffraction grating:

A diffraction grating is a periodic structure with varying refractive index as depicted in the figure below. When light is incident on a grating, light is diffracted in different directions depending on their wavelength. When white light is incident on a grating all the colours are split and they are reflected and transmitted in different directions. Similar to a prism, one can obtain a rainbow pattern.

### Anti-reflection coating in spectacles:

Light suffers a reflection loss whenever it travels from one medium to another. For example, you can see yourself when you stand in front of a glass door, the reflection of sky your car's windshield, etc.

Similarly, one of the main use of glass in our daily life is spectacles. When light travels through our spectacles to reach our eyes, some amount of light is lost at the glass-air interface. In order to minimise the reflection loss, the opticians coat a thin layer called the 'antireflection coating' above the lens.

Now, how does it work?

An 'anti-reflection coating' is essentially a thin film coated on top of the glass. The thin film is designed in such a way that the reflected rays from air/thinfilm interface and thinfilm/glass interface interfere destructively. Thereby it reduces reflection loss and more light enters the spectacles increasing the visibility.

## Soap Bubbles:

Soap bubbles exhibit rainbow colours. Is it because of chemical constitution of the soap bubbles? Well no, the reason is similar to the phenomena happening in antireflection coatings, you just learned. The soap molecules attach themselves to the surface of water forming a thin film in circular shape(owing to minimum surface tension).

The soap film acts as a thin film. The reflected waves from the top and the bottom surfaces interfere. Hence we see soap bubble exhibiting different colours in different directions.

### Hologram:

A hologram is a photographic film with a recorded inteference pattern. The true object picture is reproduced with paralax effect. This is possible because, the film stores the phase information along with the intensity values. There are two kinds of holograms, viz reflection and transmission hologram. A transmission hologram formed by interfering light reflected off from and a reference beam on a photographic film. When this film is illuminated with a coherent source(laser), the original object is reproduced. In a reflection hologram the data is stored layer wise. When light is incident on a reflection hologram, light is reflected from different layers of the film. Similar to an anti-reflection coating, different wavelengths are diffracted in different directions. That's the reason hologram in a security card or credit card exhibits different colors in different directions.

### Peacocks and butterflies:

Peacocks feather demonstrate brilliant colours. A common misconception is that the colors are due to pigments. The actual reason is due to the presence of periodic small grooves (diffraction gratings) present in peacock's feather. The periodicity of these grooves differs from one region to another. Each set of groove is responsible for the reflection of a particular colour. When sunlight is shined over the feather the light interacts with the grooves resulting in the spectacular colors.

Some butterflies display shiny blue colour. Wings of this butterfly have diffraction grooves with a particular period that permits constructive interferece .

### Displays:

Displays are made up of minute periodic physical elements called 'pixels'. Each pixel is made up of the primary colors red, green and blue. High-resolution displays have high pixel density. These pixels are arranged with a periodicity comparable to the wavelength of visible light. Hence, when sunlight or a flashlight is shined upon the display, the light is diffracted resulting in a beautiful pattern.

### CD ROMS:

Have you ever wondered why the back side of a CD ROM exhibits shiny colors? Why Blu-Ray discs have a superior storage in comparison to CDs?

The data in compact discs are stored and written in small groves of the size comparable to the size of visible wavelength. Hence, when light is incident on them, the grooves diffract light in different directions.

What's special about Blu-ray discs? In Conventional CDs, reading data is done using a red laser. The wavelength of red color is around 680nm. Thereby the data storage is limited by red colour. In Blu-ray disc technology, Blue(wavelength) laser is used to read data.

Now that we read about light waves, let's turn our attention towards sound waves.

### Sound waves diffracting through corners

The sound waves audible to the human ear are in the range 17mm to 17m. This enables sound waves to diffract through the edges or corners of a door. When a music is played with the door slightly opened, it is possible to hear sound from any direction near the door.This is because sound waves are of wavelength comparable to the width or aperture near the door enabling them to diffract and spread out.

### Tuning musical instruments:

Have you ever wondered how a musician uses a tuning fork to tune his or her equipment?

Musical instruments like piano, guitar etc produce sounds at definite frequencies. For example, the musical note A4 has a frequency of 440Hz. A tuning fork upon excitement emits a sound for a certain frequency. When two identical tuning forks and are placed next to one another, striking one of them causes other to resonate at the same frequency. If they are at slightly different frequencies, they cannot interfere and leads to a phenomenon called 'beats'. The musical instrument is tuned until beats are removed ensuring the instrument is emitting the same frequency.

References:

### Example Of Light Wave Diffraction

1. http://hyperphysics.phy-astr.gsu.edu/hbase/sound/diffrac.html
2. http://www.asknature.org/strategy/1d00d97a206855365c038d57832ebafa