What areFresnellights used for

Most Fresnel lenses look like a beehive or barrel; most contain from two to twenty-four different panels. A clock type mechanism, which had to be wound by hand every few hours before automation, was used to make the revolving lenses rotate around the lamp itself to produce the flash. The movement of the lens is timed precisely so the panel will pass by when a flash is due.

The capacity of an optical instrument to show separate images of very closely placed two objects is called resolving power.

The reciprocal of the smallest angle subtended at the objective lens of a telescope by two-point objects which can be just distinguished as separate is called the resolving power of a telescope.

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As explained above Diffraction of light is the phenomenon of bending light from the sharp corners of a slit or obstacle and spreading into the region of the geometrical shadow.

The Rayleigh criterion specifies the minimum separation between two light sources that may be resolved into distinct objects.

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The resolving power of a microscope is the ability of the microscope to show as separate images of two-point objects lying close to each other.

Rayleigh scattering can be considered to be elastic scattering since the photon energies of the scattered photons are not changed.

Atmosphere consists of many different particles and molecules of gases and dirt, suspended or in relative motion. When sunlight passes through the atmosphere, it hits these particles and gets diffracted. This diffraction also scatters light. For example, Light can enter the room through a small hole, get diffracted and fill the room.

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Francesco Grimaldi made the first observation of diffraction in 1665. When light waves were forced to pass through a slit, he observed that they dispersed. Later, it was discovered that diffraction happens everywhere light waves bend around a corner, not just in tiny slits or holes.

The slits are separated by opaque spaces. When a wavefront is an incident on a grating surface, light is transmitted through the slits and obstructed by the opaque portions. Such a grating is called transmission grating.

Diffraction is the slight bending of light as it passes around the edge of an object. The amount of bending depends on the relative size of the wavelength of light to the size of the opening.

The most common example of diffraction occurs with water waves that bend around a fixed object. Light bends similarly around the edge of an object.

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The resolving power of a diffraction grating is defined as its ability to form separate diffraction maxima of two closely separated wavelengths.

A distinctive element of the diffraction pattern is the centre maxima, also referred to as the central fringe. A distinctive element of the diffraction pattern is the centre maxima, also referred to as the central fringe. The secondary minima and maxima, which encircle the centre maxima, are made up of dark and bright lines.

Now if d is the separation between two slits, N is the number of diffractions grating and θ is some angle for the first maxima then the angular width can be expressed as

Diffraction can occur only when the wavelength of light is comparable to the size of the obstacle or the width of the slit.

Based on Rayleigh scattering, the Rayleigh criterion for the diffraction limit is used to define the resolution. It states that two images which are just resolvable when the centre of the diffraction pattern of one is directly over the first minimum of the diffraction pattern of the other.

Although light is typically thought to move in straight lines, when it comes close to a barrier, it tends to bend around the barrier and disperse out. The dispersion of waves as they go through or around a barrier is referred to as diffraction. Diffraction of light, more precisely, happens when a light wave passes by a corner or through an opening or slit that is physically as tiny as, or even smaller than, that light’s wavelength.

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Fresnel lenses may be fixed, showing a steady light all around the horizon, or revolving, producing a flash. The number of flashes per minute is determined by the number of flash panels and the speed at which the lens revolves. A unique flash pattern for each light is produced by varying the amount of light and dark periods. For example, a light can send out a flash regularly every five seconds. Alternatively, it might have a ten second period of darkness and a three second period of brightness, or any number of other combinations. The individual flashing pattern of each light is called its characteristic. Mariners consult a light list or a maritime chart that told what light flashes that particular characteristic, and what color the light is. This allowed them to determine their position at sea in relation to the land.

Now if the width of each slit is finite, we shall observe a combined effect of diffraction (through each slit) as well as interference pattern as we can see in the figure below.

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The Fresnel lens (pronounced "Frey Nel"), as it came to be known, represented a monumental step forward in lighthouse lighting technology, and therefore also in maritime safety. In a Fresnel lens, hundreds of pieces of specially cut glass surround a lamp bulb. This design intensifies the glow from the light, focusing rays of light that would normally scatter into a single, intense beam of light, which shines out in a specific direction. The lens could produce an unlimited number of flashing combinations and intensified the light so it could be seen at greater distances, allowing mariners a greater deal of safety in their navigations near shore.

Where λ is the wavelength of the light, n is an integer value, a is slit width and D is the distance of the screen from the slit.

Fresnel lenses came in several sizes, or orders, from the largest, the Hyper-Radial, to the smallest, the eighth order. Not all orders were used in the United States. Large first order lenses, such as those still in place at the Fowey Rocks lighthouse in Biscayne National Park or the Bodie Island lighthouse in Cape Hatteras National Seashore, were usually used on major seacoasts, with a more powerful beam that shines up to twenty-one miles out to sea. Fifth or sixth order lights, the smallest used in the United States, were used in smaller bodies of water, such as bays or rivers. The Jones Point lighthouse on the Potomac River in Alexandria, Virginia, used a fifth order lens for the comparatively smaller distances it had to cover, but it was nonetheless essential to the hundreds of merchant, passenger, fishing, and naval vessels that traveled the waters around Washington, DC daily.

Nearly all lighthouses in National Park units originally had a Fresnel lens, though many of them have been removed and/or replaced with more modern lighting mechanisms. However, the lenses’ beauty and their pivotal place in lighthouse history has ensured their preservation in many instances. Some are in museums associated with the historic lighthouse itself; others are in museums away from the lighthouse. And of course, there are many, many more Fresnel lenses in American lighthouses that are not part of National Parks. The United States Lighthouse Society maintains a large amount of information about Fresnel lens history and technology, as well as lists of current and operational Fresnel lenses in the United States.

The use of lenses in lighthouses began in England in the 18th century, and was adopted in the United States by 1810. These early lenses were thick, excessively heavy, and of poor quality glass. Therefore, they were not very effective and prone to losing the light through the thick glass. In 1811, the French Commission on Lighthouses established a committee to investigate improvements in lighthouse illumination. Among the committee members was Augustin Fresnel, who in 1822 completed the design of his flashing lens using thin bull’s eye shaped panels, which refracted the light both horizontally and vertically, producing a much stronger beam of light.

Now at any point on the screen, the rays reaching from the first and second slit will have a path difference, where ‘b’ is the separation between the center of two slits.

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A diffraction grating is an extremely useful device, and one of it consists of a large number of narrow slits side by side.

where, λ is the wavelength of light (or other electromagnetic radiation) and D is the diameter of the aperture, lens, mirror, etc., with which the two objects are observed

We hope that the above Physics Optics detailed notes helped you to understand the importance of the Diffraction of Light. Do practice it now on the Testbook App through the free mock tests.

In Young’s double-slit experiment, an interference pattern comprising alternate dark and bright fringes on-screen was observed. However, in that experiment, we have assumed each slit as behaving like a point source thus we did not consider the width of the slit, as a result, no diffraction pattern was observed due to either slit.