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LIGHT

Introduction:
    Light is a transverse, electromagnetic wave that can be seen by humans. The wave nature of light was first illustrated through experiments on diffraction and interference. Like all electromagnetic waves, light can travel through a vacuum. The transverse nature of light can be demonstrated through polarization.

 

Light is produced by one of two methods…
    Incandescence is the emission of light from "hot" matter.
    Luminescence is the emission of light when excited electrons fall to lower energy levels (In matter that may or may not be "hot").

 

Sources of Light
  There are two general sources of light.
    Natural source and Artificial source.

 

(I) Natural Sources: Our most important natural source of light is the sun. Nearly all the natural light we receive comes from the sun; moonlight is sunlight reflected from the surface of the moon. Distant stars provide an extremely small amount of light.
 

(II) Artificial Sources: There are several ways of producing artificial light. In general, artificial light source can be divided into three categories.
 

(i) Thermal Sources: Examples of thermal source are incandescent lamp, burning candle, etc. When object is heated until it glows or becomes incandescent, it emits all visible wavelengths along with large quantity of infrared radiation. Hence, as producers of visible radiation (i.e. luminous energy), they have a low efficiency. Generally, the efficiency of such light sources improves as the operating temperature is increased.
 

(ii) Gas Discharge Sources: Examples of gas discharge source are neon lamp, sodium lamps, etc. In this case, light is obtained by maintaining electric current in a gas at low pressure. Such a source emits only a few wavelengths. The color and intensity of light depends upon the nature of gas or vapor only. It may be noted that in case of light emitted by a thermal source, the spectrum is continuous. However, when light is obtained from a gaseous discharge, the spectrum is discontinuous i.e. it consists of one or more colored lines. For examples, in the case of sodium lamp, the spectrum consists mainly of two yellow lines very close together with wavelengths of 5890Ao. These wavelengths are so close to each other that light from a sodium lamp is said to be monochromatic i.e. a light having only one wavelength.
(iii) Luminescent Sources: The familiar example of such a source is the fluorescent tube. A fluorescent tube consists of a thin-walled glass tube with fluorescent substance coated on the inside of the tube. An electric current is maintained in mercury vapors at low pressure. It emits visible radiation as well as ultraviolet radiations (invisible). The fluorescent material absorbs ultraviolet radiation and re-emits them at longer wavelengths of the visible spectrum.

 

Transparency and translucency
     Transparency is the physical property of allowing light to pass through the material without being scattered. On a macroscopic scale the photons can be said to follow Snell's Law.
Examples: Air, water, and clear glass

 

Snell's law
      Snell's law is a formula used to describe the relationship between the angles of incidence and refraction, when referring to light or other waves passing through a boundary between two different isotropic media, such as water, glass, or air.
     Translucency is a super-set of transparency it allows light to pass through, but does not necessarily follow Snell's law; the photons can be scattered at either of the two interfaces where there is a change in index of refraction, or internally.
Examples: Frosted glass, oil paper, some plastics, and ice.

 

Opaque
    An opaque object is neither transparent (allowing all light to pass through) nor translucent (allowing some light to pass through).
Examples: Wood, wall, hat, clothes, towel, box, paper, bag and book.

 

Light Measurement
      The branch of optics which deals with the quantitative study of light energy is called photometry.
     The following three quantities are generally measured in practical photometry: Luminous flux, Luminous Intensity and Illumination.

 

Luminous Flux:
      The luminous flux from a light source is the luminous energy emitted per second by the source. It is denoted by . The unit of luminous flux is lumen, symbol lm. A lumen is a unit of energy per second or power, so it must be related to watt.

 

Luminous Intensity:
    The luminous intensity of a light source is the light radiating capacity of the source in a given direction.
It may be defined as the luminous intensity of a light source in any direction is the luminous flux emitted by the source per unit solid angle in that direction. It is denoted by I. If a uniform light source emits luminous flux of ∅ lumens within a solid angle of ω steradian, then luminous intensity I of the light source is given by:
   Luminous intensity I =  ∅/ ω Lumen / Steradian or lm / sr
   The unit of luminous intensity is lm/sr or candela (cd).
   1 cd = 1 lm/sr

 

Illumination or Illuminance:
    When luminous flux falls on a surface, it is said to be illuminated. The illumination of a surface is measured by the normal luminous flux per unit area received by it. It is denoted by E. If ∅ lumen is the flux incident normally on a area A m2, then illumination of the surface E is given by,
          Illumination E = ∅ / A lumen / m2 or meter candle
    One lux is equal to the illumination produced at the inner surface of a sphere of radius 1 m when a source of 1 cd is placed at the centre of the sphere.

 

Theories of light
     In the seventeenth century two rival theories of the nature of light were proposed, the wave theory and the corpuscular theory.

    The Dutch astronomer Huygens (1629-1695) proposed a wave theory of light. He believed that light was a longitudinal wave, and that this wave was propagated through a material called the 'aether'. Since light can pass through a vacuum and travels very fast Huygens had to propose some rather strange properties for the aether for example; it must fill all space and be weightless and invisible. For this reason scientists were sceptical of his theory.                                
     In 1690 Newton proposed the corpuscular theory of light. He believed that light was shot out from a source in small particles, and this view was accepted for over a hundred years.
     The quantum theory put forward by Max Planck in 1900 combined the wave theory and the particle theory, and showed that light can sometimes behave like a particle and sometimes like a wave. You can find a much fuller consideration of this in the section on the quantum theory.

 

Wave theory of Huygens
    Huygens published his theory in 1690, having compared the behaviour of light not with that of water waves but with that of sound. Sound cannot travel through a vacuum but light does, and so Huygens proposed that the aether must fill all space, be transparent and of zero inertia. Clearly a very strange material.
    Even at the beginning of the twentieth century, however, scientists were convinced of the existence of the aether. One book states 'whatever we consider the aether to be there can be no doubt of its existence'.
    We now consider how Huygens thought the waves moved from place to place. Consider a wave front initially at position W, and assume that every point on that wave front acts as a source of secondary wavelets. The new wave front W1 is formed by the envelope of these secondary wavelets since they will all have moved forward the same distance in a time t.


    There are however at least two problems with this idea and these led Newton and others to reject it:
   (a) the secondary waves are propagated in the forward direction only, and
   (b) they are assumed to destroy each other except where they form the new wave front.

 

Corpuscular theory of Newton
      Newton proposed that light is shot out from a source as a stream of particles. He argued that light could not be a wave because although we can hear sound from behind an obstacle we cannot see light - that is, light shows no diffraction. He stated that particles of different colours should be of different sizes, the red particles being larger than the blue.
    Since these particles are shot out all the time, according to Newton's theory, the mass of the source of light must get less.
    We can use Newton's theory to deduce the laws of reflection and refraction.

 

Newton’s Particle Theory of Light
     Newton proposed that light consists of little masses. This means that a horizontal beam of light near the earth is undergoing projectile motion, and forms a parabola. The straight line we observe is due to the fact that the speed of the particles is so great.
In one microsecond, light travels 300 m. In that time it should fall a distance
y = 1/2gt2 = 5 × 10-12 m, much too small to be seen.
      Many known properties of light could be explained easily by a particle model. For example it was known that when light reflects from a smooth surface, the angle of incidence is equal to the angle of reflection. This is also how an elastic, frictionless ball bounces from a smooth surface.

 

Particle Theory of Refraction
      Newton imagined that matter is made of particles of some kind (today we would call them molecules or atoms). When a light particle is deep within a medium, such as water or glass, it is surrounded on all sides by equal numbers of these particles. Suppose there is an attractive force between the light particles and the matter particles. Then deep within a medium, these forces cancel each other out and there is no net force on the light particle. Then, according to Newton’s first law, the light particle will continue moving in a straight line since no net force acts on it. Near an interface the situation is different. Now there are more matter particles on one side than the other, and the light particle can experience a net force. It would experience a brief attractive force towards the medium with more matter particles.

 

Maxwell's Electromagnetic Theory
     Electromagnetic theory of light was put forward by James Clerk Maxwell in 1873. According to this theory, light consists of fluctuating electric and magnetic fields propagating in the form of electromagnetic waves. But this theory failed to explain the photoelectric effect.

 

Planck's Quantum Theory
     According to Max Planck's Quantum theory, radiation is not continuous but is made up of tiny packets of energy called photons. However, this theory could not explain other optical phenomena.
     From all the theories it is clear that certain optical phenomena can be explained clearly only if light is considered to be made up of particles, while certain other phenomena can be explained only if we consider light as a wave. Thus light appears to have a dual nature.

 

Characteristics of Light
* Light is a form of energy produced by luminous objects.
* Light can travel through vacuum.
* Light can penetrate through transparent materials but cannot pass through opaque objects.
* Light travels in a straight line in an optically homogeneous medium.
* Light bounces back when made to fall on polished surfaces such as mirrors or metal surfaces. This bouncing back of light is described as reflection.
* The change in the velocity of light when it travels from one transparent medium to another is described as refraction.
* Light takes the path of least time in passing from one point to the other. This is nothing but Fermat's principle. The shortest distance between any two given points is a straight line. Thus Fermat's principle proves the rectilinear propagation of light.
* Light appears to have a dual nature. During propagation, light exhibits wave characteristics but when it interacts with matter, it behaves like particles.

 

Properties of Light
     One of the properties of light is that it reflects off surfaces. Among other things, this reflection allows us to see images in mirrors. We see the images in mirrors as apparently coming from behind the mirror because our eyes interpret it in this manner. But when we see ourselves reflected in the mirror and raise our left arm, the image apparently raises its right arm.
     Another property is the speed of light, which is the fastest anything has been observed to move. In a vacuum, the speed is 300 million meters per second. At that speed, it takes light one ten thousandth of a second to travel around the earth. When light enters a material, it slows down. The amount depends on the material it enters and its density. For example, light travels about 30% slower in water than it does in a vacuum, while in diamonds, which is about the densest material, it travels at about half the speed it does in a vacuum. This slowing down of light plays a role in another property, refraction. Refraction means that light bends when it passes from one medium to another. When light enters a denser medium from one that is less dense, it bends toward a line normal to the boundary between the two media. The greater the density difference between the two media, the more the light bends. This property is used with respect to optical devices such as microscopes, corrective lenses for vision, magnifying lenses, and so on.
     Another property that combines both refraction and reflection is total internal reflection. This is an interesting concept. When light coming from the air strikes water, part is reflected and part is refracted. When the angle of incidence of the light striking the water is large enough, it gets totally reflected and in fact cannot leave the water. Fiber optics uses this property of light to keep light beams focused without significant loss, as long as the bending of the cable is not too sharp. TV and telephone cables use fiber optic cable more and more since it is much faster and more efficient than electrons in an electric current.
     Diffraction is yet another property of light. This term refers to the fact that light bends as it goes through an opening. While it is hard to give an everyday example of this, the closest would be when there is a light source shielded by a door such that only a limited amount of light can get through the opening. However, even the area shielded is a little brighter, reflecting some actual reflection and diffraction as well. An easier example is with another wave form, sound. When someone speaks from in front of an open door, a person standing way around the corner from the door will still hear the diffracted sound waves.
     Interference is another property of light. It is a phenomenon that occurs when two beams of light meet. Depending on both the nature of the two beams and when they meet, they can either merge and enhance one another and give a brighter beam, or they might interfere in such a way as to make the merged beam less bright. The former is called constructive interference, and the latter is destructive interference.

 

Speed of light
      Speed of light, speed at which light waves propagate through different materials. In particular, the value for the speed of light in a vacuum is now defined as exactly 299,792,458 meters per second.
    The speed of light is considered a fundamental constant of nature. Its significance is far broader than its role in describing a property of electromagnetic waves. It serves as the single limiting velocity in the universe, being an upper bound to the propagation speed of signals and to the speeds of all material particles. In the famous relativity equation, E = mc2, the speed of light (c) serves as a constant of proportionality linking the formerly disparate concepts of mass (m) and energy (E).

 

Convex and Concave Mirrors
      Convex and concave mirrors are known collectively as spherical mirrors, since their curved reflecting surfaces are usually part of the surface of a sphere. The concave type is one in which the midpoint or vertex of the reflecting surface is farther away from the object than are the edges. The center of the imaginary sphere of which it is a part is called the center of curvature and each point of the mirror surface is, therefore, equidistant from this point. A line extending through the center of curvature and the vertex of the mirror is the principal axis, and rays parallel to it are all reflected in such a way that they meet at a point on it lying halfway between the center of curvature and the vertex. This point is called the principal focus.
      The size, nature, and position of an image formed by a concave spherical mirror depend on the position of the object in relation to the principal focus and the center of curvature. If the object is at a point farther from the mirror than the center of curvature, the image is real (i.e., it is formed directly by the reflected rays), inverted, and smaller than the object. If the object is at the center of curvature, the image is the same size as the object and is real and inverted. If the object is between the center of curvature and the principal focus, the image is larger, real, and inverted. If the object is inside the principal focus, the image is virtual, erect (right side up), and larger than the object. The position of the object can be found from the equation relating the focal length f of the mirror (the distance from the mirror to the principal focus), the distance
d o of the object from the mirror, and the distance d i of the image from the mirror: 1/ f = 1/ d o +1/ d i . In the case of the virtual image, this equation yields a negative image distance, indicating that the image is behind the mirror. In the case of both the real and the virtual image, the size of the image is to the size of the object as the distance of the image from the mirror is to the distance of the object from the mirror.
    In a convex spherical mirror the vertex of the mirror is nearer to the object than the edges—the mirror bulges toward the object.
The image formed by it is always smaller than the object and always erect. It is never real because the reflected rays diverge outward from the face of the mirror and are not brought to a focus, and the image, therefore, is determined by their prolongation behind the mirror as in the case of the plane mirror.

 

Uses of Convex Mirrors:
Convex mirrors are used inside buildings: Large hospitals, offices or stores sometimes make use of convex mirrors in order to let people see what is around a corner to avoid people running into each other and prevent minor/major collisions.

 

They are used in sunglasses: Convex mirrors are also used in making lenses of sunglasses. This is done to help reflect the light of sun away from the eyes of the person wearing the sunglasses.
 

Use in magnifying glass: Two convex mirrors are placed back to back in order to make a magnifying class.
 

Use of convex mirror in securities: Convex mirrors are placed near ATM’s so as to allow the bank customers to check if someone is behind them. This is a measure of security taken which helps in keeping customer’s using ATMs safe from robberies of cash withdrawals or other valuables or even their cards along with the pin number. This also keeps the identity of ATM user’s secure.
* Convex mirrors use also used as street light reflectors because they are able to spread light over a bigger area.
* They are also used for inspection purposes for places where it is difficult to reach. For this, the mirrors are mounted on an appropriately sized rod and are extended with lights under the object that is to be viewed, common examples for this are appliances or car repair, clocks etc.
* They are also put on corners of roads so that you can see any cars coming to avoid collisions.
* They are also used in telescopes.
* Another use of convex mirrors is as ceiling dome mirrors.

 

Uses of Concave Mirror
     
 * One of the important applications of a concave mirror is in satellite dishes. These antennas are designed to first receive and then amplify (increase) the weaker signals that are sent in space via communication satellites. As the distance is very large, the waves reaching the earth from the antennas are in parallel. Thus when these parallel waves strike a concave surfaced mirrored antenna dish, they all get reflected by the focus of the concave mirror. At that focus the receiver for the signals is placed. Here, the purpose of mirror is to gather weaker signals falling over a bigger area and them concentrate or focus them on one point or spot.
* Another major application of concave mirrors is in headlights of the car. A powerful source of light in a smaller size is placed at the focus point of the concave mirror which is placed at the back of the headlight. Then any light from the focus that will strike the mirror will get reflected in parallel to the axis of the concave mirror. Thus the beam of headlight gets focused in this way.
* One more application of concave mirrors is in telescopes used for astronomical studies. All telescopes may them be large or small, use concave mirror in accordance to the size of the telescope. If telescope is large a bigger mirror is used, for smaller ones small mirror is used.
* Dentist and ENT doctors also use concave mirrors in their examinations procedure to obtain a larger image than the original of teeth, ear or skin etc.
* Concave mirrors are also used in torches on the same principle as they are used in vehicles headlights.
* They are also used in solar powered gadgets. The parallel rays of the sun are focused at the focal point of the mirror and then the reflected rays are used for heating purposes like cooking, heating water, recharging power backups etc.
* They are used in electron microscopes and magnifying glasses to get a larger view of smaller objects under study.
* Another use of concave mirror is in visual bomb detectors.
* The flash light mirror of camera also makes use of concave mirrors.

 

Defects of Vision and their Correction
    There are four types of defect of the Eye: Myopia, Hypermetropia, Presbyopia and Astigmatism. Below are given the nature of the defect, its causes and corrective measures:-

 

Myopia:
     Nearsightedness, also called myopia is common name for impaired vision in which a person sees near objects clearly while distant objects appear blurred. In such a defective eye, the image of a distant object is formed in front of the retina and not at the retina itself. Consequently, a nearsighted person cannot focus clearly on an object farther away than the far point for the defective eye.

 

Causes:
     This defect arises because the power of the eye is too great due to the decrease in focal length of the crystalline lens. This may arise due to either
       (I) Excessive curvature of the cornea, or
       (II) Elongation of the eyeball.

 

Correction:
     This defect can be corrected by using a concave (diverging) lens. A concave lens of appropriate power or focal length is able to bring the image of the object back on the retina itself.

 

Hypermetropia:
     Farsightedness, also called hypermetropia, common name for a defect in vision in which a person sees near objects with blurred vision, while distant objects appear in sharp focus. In this case, the image is formed behind the retina.

 

Causes:
   This defect arises because either
(I) The focal length of the eyelens is too great, or
(II) The eyeball becomes too short, so that light rays from the nearby object, say at point N, cannot be brought to focus on the retina to give a distinct image.

 

Correction:
    This defect can be corrected by using a convex (converging) lens of appropriate focal length. When the object is at N’, the eye exerts its maximum power of accommodation. Eyeglasses with converging lenses supply the additional focusing power required for forming the image on the retina.

 

Presbyopia:
     Presbyopia, progressive form of farsightedness that affects most people by their early 60s. The power of accommodation of the eye decreases with ageing. Most people find that the near point gradually recedes.

 

Cause and cure:
     It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the crystalline lens. Simple reading eyeglasses with convex lenses correct most cases of presbyopia.
     Sometimes, a person may suffer from both myopia and hypermetropia. Such people often require bi-focal lenses. In the bi-focal lens, the upper portion of the bi-focal lens is a concave lens, used for distant vision. The lower part of the bi-focal lens is a convex lens, used for reading purposes.

 

Astigmatism:
     Astigmatism, a defect in the outer curvature on the surface of the eye that causes distorted vision. In astigmatism, a person cannot simultaneously focus on both horizontal and vertical lines.

 

Causes:
     This defect is usually due to the cornea that is not perfectly spherical. Consequently, it has different curvatures in different directions in vertical and horizontal planes. This results in objects in one direction being well-focused, while those in a perpendicular direction not well focused.

 

Correction:
    This defect can be corrected by using eyeglasses with cylindrical lenses oriented to compensate for the irregularities in the cornea.

Posted Date : 03-02-2021

 

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