WHAT IS LIGHT?
Light is a form of energy to which human eye is sensitive. The whole electromagnetic spectrum ranges from cosmic rays on shorter wave length to radio waves on longer wavelength (Fig. 1.1). Ultraviolet rays, infrared rays and visible rays are clinically significant from ophthalmic point of view. Visible white light consists of seven colors ranging from violet (400 nanometer) to red (700 nanometer). Light with longer wavelength has lesser energy and light with shorter wavelength has more energy.
Optics is a branch of physics which deals with properties of light including its interaction with matter and construction of instruments that use or detect it.
ULTRAVIOLET RAYS
Ultraviolet rays are invisible rays, sunlight being the principal source. Depending upon their absorption spectrum, UV light has been divided into three bands:
- Ultraviolet A rays: This band of UV rays is absorbed by crystalline lens and thus retina is protected against their bad effects. Prolonged exposure to these rays causes cataract formation. IOLs implanted during cataract surgery have chromophores [inhibitors of UV rays] to protect retina against UV rays.
- Ultraviolet B rays: This band is responsible for snow blindness and photo keratitis caused by welding arc. Prolonged exposure to these rays can cause formation of pingicula and pterigium.
- Ultraviolet C rays: This band is blocked by the ozone layer of atmosphere.
VISIBLE RAYS
It consists of Violet, Indigo, Blue, Green, Yellow, Orange and Red light. Red color has longest wavelength. That is why traffic signals are made of red light so that it is visible from a long distance. Retina is most sensitive to yellow light in photopic conditions. In scotopic conditions it is most sensitive to blue light.
INFRARED RAYS
These rays are absorbed in anterior chamber and cause heating effect. They are also called as heat rays. They are further of three types:
- Infrared A rays are responsible for macular burn in solar eclipse [photo retinitis].
- Infrared rays B and C can cause corneal opacity and cataract formation on prolonged exposure.
PROPAGATION OF LIGHT
Light travels in all directions in straight lines from the source of light. It travels in form of waves which oscillates in all directions and also in form of tiny particles called photons. A photon is an elementary particle and the basic unit of light and all other forms of electromagnetic radiation. It has no rest mass. It shows dual nature, i.e. it exhibits properties of both waves and particles. For example a single photon is refracted by a lens and exhibits wave interference with itself. The modern concept of the photon was developed by Einstein to explain experimental observations that did not fit the classical wave model of light (Fig. 1.2).
Thus light has dual nature. The term RAY is used for the path along which the light travels. It is represented by a straight line. A bundle of rays is called a pencil or a beam of light.
Rays of light may have positive vergence, i.e. when they travel they converge at a point or negative vergence, i.e. when they travel they diverge from a point or they may have zero vergence, i.e. they are running parallel to each other (Fig. 1.3).
A medium through which light can pass uninterrupted is called transparent medium. A medium which offers some resistance to the passage of light is called translucent medium and a medium which does not allow passage of light through it is called the opaque medium.
SPEED OF LIGHT
Speed of light is the fastest anything has been observed to move. In vacuum, the speed is three lakh kilometers per second or one lakh eighty six thousand miles per second. At this 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.
HOW DO WE SEE?
Sun is the natural source of light. When light falls on a nonluminous body, following things can happen:
- Light strikes the surface, some part of it is absorbed and some part is reflected back. This reflected part of light enters our eyeball and stimulates our rods and cones which generate a visual impulse. This is carried via optic nerve to visual cortex where the signals are interpreted. Image formed on retina is very small and inverted which is reinverted and made of original size in the visual cortex. A red object appears red because it absorbs colors of all wavelengths and reflects red color which enters our eye to generate a sensation of red color.
- Whole of the light is absorbed and no color is reflected back, the object appears black or opaque.
- Whole of the light is reflected back and no color are absorbed, the object appears white.
HUYGENS’ PRINCIPLE
The Dutch physicist Christiaan Huygens and French physicist Augustin Jean Fresnel were the two scientists who gave this principle. It is used to analyze problems of wave propagation. According to this principle every point of a wave front may be considered as the source of secondary wavelets that spread out in all directions with a speed equal to the speed of propagation of the waves.
This means that each point of an advancing wave acts as a fresh source of waves creating a series of circular wave. Thus, as the wave advances, each advancing wave in turn creates next stream of successive waves and so on. It can be thought of as an example given below:
If two rooms are connected through an open door and you create a sound in the extreme corner of one room (farthest from the other room), to any person sitting in the second room, it will appear as if the sound has been created from the door (or starting point of the second room) itself. It is because when a person creates a sound in one room, the wave travels ahead and the next wave again creates the stream of waves. This continues and passes the person sitting in the second room. The person assumes as if the sound is created from the entry of the door itself.
Applications
- Diffraction refers to various phenomena which occur when a wave encounters an obstacle. It is described as the apparent bending of waves around small obstacles and the spreading out of waves past small openings. Now, this can be explained through Huygens’ principle. When the wave hits an obstacle, the points where it touches the obstacle through the slit, start creating waves in all directions. Waves moving in the same direction are added together. Hence, it appears as if waves are spreading out of small opening.
- Reflection and refraction: When a ray (or wave) hits a surface, the point at which it hits, starts creating waves. If the upper medium has different refractive index than the lower medium, we see the size in the waves as different. The tangent of these waves explains the angle of reflection and refraction.
PROPERTIES OF LIGHT
Until the middle of 1800s, light was taken to be a stream of tiny particles. This was advocated by Newton. However, by the late 1800s, the particle theory was replaced by the wave theory. This was because light exhibited certain properties that could only be explained by the wave theory. Now the most accepted view is that light exhibits dual nature. Different properties of light are:
- Reflection
- Refraction
- Dispersion
- Diffraction
- Polarization
- Interference.
Reflection
When a ray of light strikes a polished surface, it bounces back in a particular direction. This is known as reflection of light. Light ray falling on the surface is called incident ray, the ray that bounces back is called the reflected ray and a line drawn at right angle to the surface is called normal (Fig. 1.4). This phenomenon 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. This is because the image is laterally reversed.
Laws of Reflection
The laws of reflection can be summarized as:
- The incident ray, the reflected ray and the normal at the point of incidence, all lie in the same plane.
- The angle of incidence is equal to the angle of reflection.
Refraction
When a ray of light passes from one medium (say air) to another medium (say glass) of different optical density, it deviates from its original path. This is called refraction. When it passes from denser to rarer medium it deviates away from normal and when it passes from rarer medium to denser medium it deviates towards normal. The greater the density difference between the two media, the more the light bends. This property is used in optical lenses used to correct refractive errors and to make different ophthalmic instruments (Fig. 1.5).
Refraction Through a Plate of Glass
When a ray of light passes through a denser medium its speed is slowed down. It does not deviate from its path if it strikes the denser medium at right angle to it. But if it strikes the medium obliquely, it deviates from its path as shown in Figures 1.6 and 1.7.
When we look into the surface of a lake or pond while fishing, the fish we catch seems larger when under the water than when we actually land it. This is due to refraction. Since the air is less dense than water, the light bends away from the normal as it emerges out of water to enter our eyes. This is the reason that stars twinkle and if a wooden stick is half dipped in water, the underwater portion of stick appears broken.
Laws of Refraction
- The incident ray and the refracted ray are on the opposite sides of the normal at the point of incidence and all three lie in the same plane.
- The ratio of sine of angle of incidence to the sine of angle of refraction is constant. This is known as Snell's Law. The value of this constant is known as refractive index of the medium. Refractive index of water is 1.33, crown glass is 1.52, flint glass is 1.65 and air is 1.00.
Total Internal Reflection
Another property that combines both refraction and reflection is total internal reflection. If for an incident ray of light angle of incidence is increased, its angle of refraction also increases. A limit comes when the refracted ray travels parallel to the surface. This angle of incidence for which angle of refraction is 90° is known as critical angle. If angle of incidence is increased further, the refracted ray bends and travels in the same medium. This is known as total internal reflection (Fig. 1.8). This phenomenon is made use of in making optical fibers. Diamond shines even in dark because of this phenomenon. 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.
Dispersion or Polychromatic Effects of Light
It refers to the ability to break white light into its constituent colors. White light consists of seven colors. If white light enters a prism it splits into seven colors with violet light having shortest wavelength suffers maximum deviation and red light with longest wavelength suffers minimum deviation (Fig. 1.9).
Rainbows are natural phenomena that exemplify all of the above properties of light. They use refraction, dispersion, and internal reflection to produce their amazing hues.
White light enters raindrops from the sun it gets dispersed and refracted inside the raindrops. When the dispersed light hits the back of the raindrop, it gets internally reflected, and when it emerges it gets dispersed even more.
The color you see most vividly in a rainbow depends on the angle of your eye. Generally, you must look higher in the sky to see the red, and lower to see the blue. What you actually see is the red on the top and the blue on the bottom, with all of the other colors in between. The arc of the rainbow depends on the angle that your line of sight makes relative to the sun behind you.
Diffraction
Diffraction refers to the fact that light bends as it goes through an opening. It is difficult to give an everyday example of this; 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. Phenomenon of diffraction is more if the size of aperture is small and vice versa. This is the reason that diffraction is less if the pupil is dilated and it is more when the pupil is constricted.
Polarization
Polarization is another property of light. Since a light wave's electric field vibrates in a direction perpendicular to its propagation motion, it is called a transverse wave and is polarizable. A sound wave, by contrast, vibrates back and forth along its propagation direction and thus is not polarizable. Light is unpolarized if it is composed of vibrations in many different directions, with no preferred orientation. Many light sources (e.g., incandescent bulbs, arc lamps, and the sun) produce unpolarized light (Fig. 1.10).
A common example of the use of polarization in our daily life is found in polarizing sunglasses. The material in the lenses passes light whose electric field vibrations are perpendicular to certain molecular alignments and absorbs light whose electric field vibrations are parallel to the molecular alignments. The major component of light reflecting from a surface, such as a lake or car hood, is horizontally polarized, parallel to 9the surface.
Thus, polarization in sunglasses, with the transmission axis in a vertical direction, rejects horizontally polarized light and therefore reduces glare. However, if you consider a sunbather lying on his or her side, wearing such sunglasses, the usual vertical polarization (transmission axis) will now be at 90° and parallel to the surface and will therefore pass the horizontally polarized light reflected off the water or the land.
Interference
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 (Figs 1.11 to 1.13).
One experiment used to demonstrate how light signals can interfere with one another is called Young's double slit experiment after the physicist who used it for demonstrating the interference phenomenon (Fig. 1.14).
He set up a screen with two small slits and behind it set up another screen some distance away. When he subjected the first screen to a single light source, he found that there were alternate light and dark spots on the distance screen, corresponding to points where light rays coming from the two different slits underwent constructive and destructive interference. This is only possible when we think of light in terms of waves.
One situation that is illustrative of interference is where there is oil or gasoline floating on the surface of a puddle. Sometimes, you will see a brilliant pattern of colors given off by the oil or gas, even when the gas or oil is subjected to white light. What happens is that different potions of the film cause different colors in the white light to interfere constructively or destructively, depending on the thickness of the film. One region of the film might look red because the red light bouncing off the top of the film interferes constructively with red light passing through the film and is then reflected back off the water below it.
We can see this more clearly with sound. When you are in the back of an auditorium, sound can reach you in different ways.11
It can take a direct path, be reflected off a ceiling, or walls, or the floor. All of these will reach you at slightly different times, and sometimes not at all. They can actually cancel each other out and you hear nothing when you sit in one area (also called dead zone), and sitting in another, you can hear an abnormally loud sound. These are examples of destructive and constructive interference and the reason that modern auditoriums use sound absorbing materials on ceilings, walls and floors.
LASER INTERFEROMETER
Laser interferometer is an equipment meant clinically (Fig. 1.15) to determine outcome of a cataract surgery especially where cataract is mature or hyper mature. In this type of cataract, retinal details cannot be seen hence in spite of best surgery, outcome may not be rewarding due to associated macular pathology. Laser interferometry done prior to surgery, tells us how much vision should be expected. It is based on the principle of interference. Patient sees stripes whose size can be varied as desired. Depending upon size of stripe appreciated patient's vision can be assessed. However this test is more of academic importance and has not been able to gain any clinical significance because of its limitations.
LAW OF INVERSE SQUARE
This law applies to light, sound, electricity, gravitation, etc. It states that the intensity of light, i.e. luminance radiating from a point source is inversely proportional to the square of the distance from the source. So an object twice as far away receives only one quarter the energy in the same time period. In other words, we can say that the intensity of a spherical wavefront varies inversely with the square of the distance from the source assuming that there is no loss caused by absorption or scattering. For example, the intensity of radiation from Sun is 9140 watts per square meter at the distance of Mercury (0.387AU) but only 1370 watts per square meter at the distance of the Earth (1AU), i.e. a threefold increase in distance results in a nine fold decrease in intensity of radiation. For a point source, the equation is 12
E = 1/d2; where | E = Energy at one particular point |
d = Distance of point from the source of light |
Clinical Application
This law is made use of by photographers and theatrical professionals to determine optimum location of the light source for proper illumination of the subject. This law can be used only in case of a point source of light. Fluorescent lamp is not a point source of light. A point source is like a light from a distant star seen through a small telescope or light passing through a pinhole or other small aperture viewed from a distance much greater than the size of the hole.