Free Tutorial in Basic Physics - the entity of Light


Light is the source of all life forms on Earth and we are able to see things with the help of it. The primary source of light on Earth is Sun. This article gives basic scientific information and concepts about this entity known as light.

Introduction

Light is an entity in presence of which we are able to see things. It is of course a form of energy found at so many places in the universe and the stars are the primary sources of it. Our Sun is also a star but it is so near to us in comparison to other stars that we see it like a yellow whitish disc hanging in the sky and providing us so bright a light. Light from Sun is a very important energy source for us as life on Earth can survive only if light is available to us continuously. Light from other stars like Sun in the universe is very feeble by the time it reaches Earth and that is the reason we cannot see them in the daytime when sunlight is obscuring them and we can only see these stars in a clear night sky.

Nature of light

As we said that light is a form of energy and our eyes can detect it even in minute form and that is the reason why some people are able to make out their surroundings even in dim light. Our eyes are the biological miracles which have the property to recognise light coming from any source or even from a secondary source which does not have its own light but it can reflect some light falling on it. For example, Moon has no light of its own. It is the light of the Sun that falls on it and then we see the reflected light. It is obvious that the intensity of light after reflection will be reduced to a great extent and that is the reason that we find moonlight so subtle and cool.

Scientists and researchers have done a lot of studies and experiments to know the nature of light and it was found that it has a dual nature. Sometimes it behaves like a wave known as an electromagnetic wave while other times it behaves like a beam of energy particles known as photons. There are some effects of lights that can be explained by their particle nature while some are explained by its wave nature.

Speed of light

Unlike sound energy, light does not require any medium to travel. It is an energy that can travel trough the vacuum and space in the universe which is predominantly a vacuum barring the cosmic material here and there. It propagates having its own electric and magnetic field associated with its wave nature. One of the most important thing about the light is that it has a very big speed with which it travels from one place to another. The speed of light is a mind-boggling number. In MKS units it is equal to 300000 km/second. That is a huge number. If we compare it with the speed of sound that is 0.332 km/second we can see the difference. From this information about the speeds of these two entities sound and light, we can find out the difference in seeing the cloud thundering light and hearing its sound by a few seconds.

Let us for an example say that there are clouds about 1 km far from our place and thundering takes place. The light from the thundering will reach us in a very small time equal to 1/300000 seconds which is practically zero time. But the sound from the thundering reaches us after 1/0.332 = approximately 3 seconds. So that is reason as why we hear the sound of thundering after sometime of seeing it. Understanding the speed of light is very essential for understanding the huge distances in the universe. Sun is very far from us about 150 million Km. Light from Sun takes 150000000/300000 = 500 seconds = about 8.3 minutes to reach Earth. Using this analogy for the distant stars we can find the time that it takes the light from that star to reach Earth. It is no surprise if that time is in lakhs or millions of years. Stars are actually situated at a very very far distance from Earth and what we are seeing today is the light that left it so many years ago. So, the interesting thing is that we are not seeing the star as on today but as it was existing so many years ago!

In the universe the bodies are present so much distance apart that we use a different unit for measuring the distance between them. That is called light year and one light year is the distance that the light travels in one year. So if a star is 75 light years away then its distance is the length that the light travels in 75 years. That comes to 75x 365 x 24 x 60 x 60 x 300000000 km which is a huge number about 7.1 x 10^17 km. The nearest star from the Earth is Proxima Centauri and is 4.24 light-years away. A light-year is 365 x 24 x 60 x60 x 300000000 equal to 9.46 x 10^15 km (which is actually 9.46 trillion km).

The great scientist Einstein while announcing his theory of relativity told us with mathematical proofs that light velocity is the maximum that is attainable by a wave or energy and that is the ultimate limit of the speed that can be achieved. He also proved that the mass of a body starts increasing when its speed increases and becomes very high when its speed reaches values comparable to the speed of light. That is something unbelievable for a layman but Einstein has given all justifications and theoretical background and calculation for that and so far that is the accepted theory.

In the next paragraph we would be learning about the interaction of light with materials. Light can pass through some of the material like glass, water, thin sheets of material etc and the speed of light is slightly reduced inside these material depending upon the optical characteristics of the material.

Interaction of light with a medium

When light from a light source falls on a body then three things happen. A part of light is reflected by the body and we see the body because of that reflected light. Second is that some light is absorbed by the body and it heats up because that absorbed light energy. The third thing happens when the body is partially or fully transparent and allows the light through it and the light emerges out of it from the other side. This is known as refraction. So there are practically three things happening in the interaction of light energy with any body - reflection, absorption, and refraction. Depending on the type of body a combination of the above three things would happen. For example when light falls on a mirror then a good part of it is reflected and we can even illuminate the other objects with that reflected light. When it falls on a glass plate then a part of it is reflected and a part passes through it and emerges from other side. The window glass is one such example. If there is a wooden log then it reflects a little light because of which we see it and it also absorbs some light energy due to which it slightly heats up as we can feel it like by touching a wooden log kept under Sun. But it does not allow any light to pass through it. So it is an opaque medium. Another example is a glass filled with water which passes the light as well as reflecting it and also a little part being absorbed by the glass and water.

When light enters a medium then there is a change in its direction just at the boundary of the two mediums and then it starts moving in that new straight direction. This bending of light at the boundaries is a very interesting phenomenon and the reason why we observe from outside a body under water as shallower than its actual depth can be explained with it.

Another interesting thing about he visible light is that it is not a single frequency or wavelength but is comprised of a spectrum starting from violet light at one end and ending with red light at the other end (VIBGYOR - Violet, Indigo, Blue, Green, Yellow, Orange, and Red). We also see those colours in the rainbows during rainy seasons when sunlight refracted from the tiny water drops create majestic rainbows. The spectrum of visible light comprises of above colours and so many colours that we are seeing around are produced due to this spectrum. What happens is that when light falls on a body then it reflects some of these spectrum colours but absorb some others and what we see by our eyes is nothing but what the body reflects. So a piece of red cloth is reflecting only the red light and other colours are absorbed by it. This is a very interesting observation and all the colours in the nature can be explained with this understanding.

The frequencies and wavelengths of these different colours vary in this visible band and from violet to red light their wavelengths are in the range 400 to 780 nano meters (in frequency terms this would be from 750 to 380 tera Hertz (1 tera Hertz = 10^12 Hertz)).

Electromagnetic spectrum

The visible part of spectrum of the electromagnetic waves is actually light. So visible light is only a small part of it. Electromagnetic spectrum is very wide and there are various types of waves in it including the light waves. All these electromagnetic waves travel with the same velocity as that of light.

The energy of light waves (or photons) depend on its frequency. More the frequency, more the energy. At the same time wavelength is inversely proportional to the frequency. So we can now understand the complete spectrum of electromagnetic waves in that order starting from lower frequencies and going to higher ones. Each group will have more frequency and more energy than the preceding group but less wavelength as we go ahead. Readers should note that there would be some little overlap between the adjacent groups of electromagnetic waves shown in the following manner and the nomenclature for the different waves is -
  • Radio waves: Low frequency - Low energy - High wavelength.
  • TV waves: Frequency lightly more than Radio waves - Slightly more energy - Slightly reduced wavelength than Radio waves.
  • Microwave and Radar waves: More frequency than Radio and TV waves - Slightly more energy - Slightly less wavelength.
  • Infrared rays: More frequency than Microwave and Radar waves - Slightly more energy - Slightly less wavelength.
  • Visible light: Band of light from Red to Violet having more frequency than Infrared - Slightly more energy - Slightly less wavelength.
  • Ultraviolet light: More frequency than visible light - Slightly more energy - Slightly less wavelength.
  • X rays: More frequency than ultraviolet light - Slightly more energy - Slightly less wavelength.
  • Gamma rays: More frequency than X rays - Slightly more energy - Slightly less wavelength.
From the above it is very clear that X rays and Gamma rays are having high energies and it is obvious that we have to protect ourselves from them as they would do maximum harm to us if we get exposed to them.

Conclusion

Light is the most amazing part of the electromagnetic spectrum with the help of which our eyes are able to see and make out the shape and colour of things around us. If there is no light there is no life as all life forms and vegetation on Earth is dependent on sunlight that is available to us on a perpetual basis.

Frequently Asked Questions

If an object is very far like a distant star then are we seeing it in real time?
No. We are seeing the image of the star only because the light that we are observing today is the light that left the star a long time back.
Can we travel back in time if we move with more speed than light?
As per the theory of relativity, speed more than light is not possible so thinking so is a fantasy only.
Does light wave have a mass?
No. Whether we consider it a wave or photon, it has no mass.
How the light from Sun is different than the light from artificial sources?
Whatever light we see is in the visible part of electromagnetic waves. Only difference is in their spectrum where each source has a characteristic spectrum differentiating it from others.


Comments

Author: DR.N.V. Srinivasa Rao29 Aug 2022 Member Level: Platinum   Points : 3

An elaborate article on Light. If there is no light on this earth there would be no life on the earth. The light we are getting is from the Sun and that is why we say Sun is a God whom we can see. He is the only God visible to us.
We all know that white light is a combination of 7 colours. This fact is demonstrated to all of us during our high school days. When we take a disc on which all the 7 colours are painted one after the other and it is stationary we will be able to see all the colours separately. But when the same disc is rotated at a high speed we will see only the white colour. These colours are very well known as VIBGYOR. Light travels faster than sound.

Author: Umesh01 Sep 2022 Member Level: Platinum   Points : 2

The constituent colours of light can be observed using a prism which is a specially designed glass slab and based on the angle between its faces it refracts the different colour lights in different angles and one can observe the conspicuous 7 colours.
During the rainy season the small drops of water hanging in the air work like a prism only and create a beautiful rainbow arc using the Sun light in the sky.



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