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Nature Of Light
Refraction - The Nature Of Light
The nature of light is such that it bends when passing from one transparent medium to another.
Identifying one medium from another is by the differences in their densities.
An example would be a ray of light passing through a block of glass as shown in diagram 01.
Glass has a density different from air and thus air and glass are two different mediums.
When a ray of light hits a block of glass or another medium at right angles, it will pass right through without any deviation at all.
However if the ray enters the block of glass at an angle, it does change its direction as it enters the glass medium. It actually bends towards an imaginary line called the "normal".
This particular behaviour of light is called refraction.
The "normal" is at right angles to the surface of the medium, and passes
through the point at which the ray of light comes into contact with the
It is necessary to imagine the existence of this line for the purpose of relating something to the angle with which the ray of light hits the surface of that particular medium.
I have drawn diagrams using MS Paint which I believe would help to understand the idea.
As shown in diagram 01, R1 is ray of light entering the block of glass which is shaded blue. R2 is the refracted ray, where i is the angle of incidence and r is angle of refraction.
When this angle of incidence is divided by the angle of refraction, the value thus obtained is known as the refractive index of the substance in which the ray of light undergoes refraction.
This value is represented by the greek letter "µ." The refractive index of a substance is shown by the formula µ=i/r.
Total Internal Reflection - Nature Of Light
Since glass is denser than air, a ray of light entering a glass block bends towards the normal.
Conversely, a ray of light originating from a point inside the glass block will bend away from the normal when it exits.
The angle at which the ray of light hits the surface in relation to the normal is known as the angle of incidence.
The angle in relation to the normal after refraction is called the angle of refraction.
When the angle of incidence is divided by the angle of refraction, a value is obtained.
This value is referred to as the refractive index of the substance through which the light is passed.
The higher the refractive index the greater the ability of the substance to bend light.
The ability of glass to cause light to bend is what enables optical instruments to function.
A ray of light originating from within a dense medium, when emerging into a less dense medium, tends to move away from the normal.
Irrespective of whether the ray of light bends towards or away from the normal, which in turn is determined by where the beam of light originates, the deviation is always referred to as refraction.
As shown in diagram 02, if the angle of incidence keeps on increasing, at one point the "emerging" ray will be along the surface which separates the two mediums.
When this occurs, the angle of refraction will be ninety degrees. That means the angle between the normal and the emerging ray will be a right angle.
It is at this point that the angle of incident gets callled the critical angle. From this moment the refractive index of the other medium no longer needs be considered.
If the angle of incidence is further increased, beyond the critical angle, the light actually begins to reflect off the separating surface.
This is called TOTAL INTERNAL REFLECTION.
Referring diagram 02 C, B and A are rays of light originating within the block of glass which is shaded yellow. The beam C, represented by a blue line, undergoes deviation and moves away from the normal on emerging from glass to air.
The beam B represented by a red line deviates in proportion to the angle of incidence. The angle of incidence is always relative to the direction of the light beam. The deviation that the red beam undergoes is such that it grazes the surface which separates the two mediums.
The beam of light A, represented by the green line hits the separating surface at an angle which is sufficient to cause TOTAL INTERNAL REFLECTION. Examining the diagram will make things clear.
Optical Fiber Technology
Optical fiber technology works on the principle of total internal reflection.
An optical fiber cable is somewhat similar to a current carrying wire.
The only difference being the metal cord within the insulated wire is replaced by a substance that enables light to pass through.
An optical fiber resembles a current carrying wire, is flexible and permits light to travel through it just like a pipe allows water to flow from one end of it to the other.
The light that is transmitted from one end of the optical fiber to the other does not undergo any signal loss as all the light that is transmitted via the fiber is kept within the cable itself.
This of course is due to total internal reflection.
Total internal reflection is what enables the continuity of the flow of light even when the cable undergoes bending.
No energy is lost as the light is contained within the cable.
Materials which allow light to pass through are called transparent.
Some substances allow light to pass through but causes diffusion such that nothing could be viewed through it. An example would be frosted glass.
Substances that block light and do not allow it to pass through are called opaque.
So you need to remember the terms transparent, translucent, and opaque.
The Spectrum - Nature Of Light
Quite apart from this, white light, or light from the sun is actually made up of several colors.
A color could be described as light of a particular frequency.
An example could be found when comparing a red beam of light to a blue beam of light. The red beam is of a different frequency than a blue beam.
Likewise each color is identified by a frequency typical of its own.
It also happens that a red ray of light will undergo a deviation different to that of a blue ray of light when it passes from one medium to another.
The angle of refraction of a red beam of light will be different to the angle of refraction of a blue beam of light. Each color has its own angle of deviation.
Sir Isaac Newton performed an experiment in which he passed a ray of sunlight through a glass prism.
The different colors which make up the white light, each of them vibrating at different frequencies, deviated at different angles and emerged revealing the true identity of each color.
As shown in diagram 03, when passed through a glass prism, the beam of white light split into the primary colors that white light is made of.
Since the color red deviates most and blue the least, it was found that when a screen was placed on the other side of the prism to capture the colors, red was found at the very bottom, and blue, the least deviating of the colors, at the top, the other colors occupied the middle range in an order typical of their respective frequencies.
The Electromagnetic Spectrum - Nature Of Light
This experiment performed by Sir Isaac Newton also proved that white light is made up of seven primary colors, namely violet, indigo, blue, green, yellow, orange and red.
These are the colors of the rainbow. This combination of colors is known as the visible spectrum.
Further proof comes along by replacing the screen with an inverted prism.
The inverted prism causes the colors entering it to converge back to form a beam of pure white light.
An extremely important thing to note here is that the human brain is tuned to identify only colors which have a frequency range that matches red and blue and all the other colors in-between. There are other "colors" that we really cannot see.
These are beyond the extremes occupied by red and blue on the visible spectrum.
Refraction Via A Prism
Light - A Form Of Energy
Light is a form of energy and can be generated by altering the orbital patterns of electrons in atoms.
Intense heat can cause this to happen. For example the filament in an incandescent light bulb offers resistance to the flow of electrical current through it.
This resistance generates heat and the heat in turn is what generates light.
The process involved in causing the filament to get hot enough to generate white light is known as heating a substance to incandescence.
Most importantly this needs to be done in the absence of air, otherwise the filament or the substance involved will "burn out."
Nearing Black Holes
Experiments have shown that light also has a tendency to bend when in the vicinity of strong gravitational or magnetic fields.
Scientists have discovered that certain areas in space in which high gravitational fields exist, appear totally black.
This is because all the light in the vicinity have been "sucked" in towards the source of the gravitational fields.
Therefore these areas appear to be dark patches in space. They are also known as black holes. More about them later!
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Summary On The Nature Of Light
Light bends when passing from one medium to another. If the medium which it is entering is denser than the medium in which it originated, the ray of light bends towards the normal.
The normal is an imaginary line which is at right angles to the surface which separates the two mediums, and passes through the same point at which the ray of light hits the surface of the second medium. This deviation is called refraction of light.
Total internal reflection is caused when the refraction of light exceeds a certain limit.
The limiting factor occurs when the angle of incident exceeds what is known as the critical angle. The critical angle is dependant on the density of the medium concerned.
It is beyond this limiting point that light reflects from the border that separates the two mediums, bouncing back within the medium in which it originates.
Optical fiber technology relies on both refraction as well as reflection of light and cannot function if one does not transform into the other.
White light is made up of the colors that are identified in the rainbow. Passing white light through a glass prism will cause the white light to split into the colors of the rainbow which would be visible on a screen.
If the screen is replaced by an inverted prism, these colors would converge back into a beam of white light.
Visible light is generated when something is heated to incandescence. An example would be an incandescent light bulb in which the filament is heated in the absence of air, by passing an electric current through it.
Light is sensitive to strong gravitational fields and has been known to bend towards sources which generate intense gravity.
Black holes in space are known to display very strong gravitational fields and any light originating in the vicinity of black holes is sucked into them.
Black holes do not allow light or any other electromagnetic waves to travel past them. They are sucked in.
Light waves are also electromagnet waves and are similar to radio waves, micro waves, and x-rays.
Light, does not need a medium to travel in. Just like radio waves and x-rays light can travel in the absence of air.
So ... such is the nature of light! I shall attempt to explain black holes in another article. Stay tuned!
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