We often hear or read the word “spectrum.” “Political spectrum” is one, which describes the broad range of political views on a scale from ultra-left to ultra-right, from nationalists to imperialists, from ultra-conservatives to ultra-liberals, etc. Electromagnetic spectrum” is another, which describes the electromagnetic radiations emitted or absorbed by a body at different wavelengths. This word came from Latin words which means “form” or “appearance.”
It was Isaac Newton who once used this word to describe the rainbow-like image that resulted when a beam of sunlight passed through a glass prism. With the publication of his book “Principia” in the 1680s, astronomers were provided an understanding of how heavenly bodies move and were of fundamental importance in unraveling the celestial mechanics of our solar system. But other data had to wait another 200 years or more to be discovered. In the early 1800, the French philosopher Auguste Comte argued “humanity would never know the nature and composition of the stars. Because they are so far beyond our earthly grasp, these remote celestial worlds would forever remain unfathomable and mysterious.”
This probably would have not been the case had Newton and the rest of his contemporaries discovered in time the implications in understanding the nature of the electromagnetic spectrum that ultimately became one of the medium in understanding the cosmic messages from outer space.
But people in the scientific community during those earlier times, as we are today, and in particular to some of you in the broad “spectrum” of membership in PAS, never remained perpetually satisfied with merely identifying the objects of admiration in the evening sky. It was never enough to have the basic skill to locate its position and to predict its where and when in the celestial sphere. It was but a natural tendency to seek a deeper understanding of the object he sees in particular; how it evolved and came into reality, to learn what they are made of and what its physical/chemical state is.
And in only a few years after the above bold pronouncement by Comte, scientists began discovering the basic properties through the studies of electromagnetic radiations that unlocked some of the mysteries and secrets from outer space.
It soon became clear that by analyzing “starlight,” we could learn many things not only about the stars but other matters concerning the evolution of the Universe.
Importance of Study
Seeing objects in space is one thing, understanding their physical properties such as temperature, mass, size, chemical composition, and direction of their motion is something else together.
These information and knowledge about distance of planets, stars, and galaxies come from analysis of the radiation they sent to us. An understanding of these radiations including how atoms emit and absorb radiation is therefore fundamental to the subject of Astronomy.
A detailed study of electromagnetic radiations of any celestial object would reveal the temperature, pressure, turbulence, and the physical state of the gases and chemical substances in that object and also, how fast that particular object is approaching or receding away from us
Also great advances in Astronomy are now attained through observations at different wavelengths other than visible light that bring to our knowledge the existence of some deep sky objects ( black holes, quasars, etc. ) never known before.
Therefore, the study, understanding, and application of electromagnetic radiations at different wavelengths (spectrum ) is the most powerful means in obtaining data about the Universe.
Nature and Property
All forms of electromagnetic radiations (EMR) are the same basic kind of energy and could be thought of as different kinds of light (visible and invisible). Both theory and available evidence shows that all EMR propagates in a vacuum with a speed of 2.9979290 x 1010 cm / sec or 300,000 km/sec. Any object for as long as its temperature is not at absolute 0º K (-273º C or -459.67º F) emits radiation at particular wavelength. Furthermore, as an object grows hotter and hotter or otherwise, the chief type of electromagnetic waves it radiates will change. The various EMR propagates in “wave” motions, consisting of varying electric and magnetic fields, but they can also be considered as being made up of “particles” or quanta of energy called photons.
The Universe is filled with this countless tiny packages of energy (photons), each of them traveling at the speed of light. There are roughly a billion photons today in the microwave background for each proton and neutron in the Universe.
The study of electromagnetic radiation of all wavelengths has in recent years been used to provide more accurate picture of the structure and evolution of the Universe. All forms of EMR have certain characteristics in common : their wave/particle method of propagation and their speed.
The different kinds of electromagnetic radiation are as follows:
1.0 Radio waves have the longest wavelength at 10-1 to 108cm.. Traveling outward at the speed of light, the expanding wave front of radio/tv signals transmitted on Earth since about 1950 has now reached approximately 400 stars, carrying information to their inhabitants, if any, about our civilization.
2.0 Microwave is the band with wavelength between 1 mm to 30 cm which lies immediately beyond the infrared region. It resembles radio wave but are more difficult to generate. Water and organic molecules when subjected to microwaves of certain frequency can vibrate very energetically which generates heats rapidly and so cooks the food.
3.0 Infrared radiations have wavelength at 10-4 to 10-1cm that can be detected with thermocouples, lead sulfide cells, and gas bulbs. The warmth that you feel when you place your hand near a glowing light bulb is primarily the result of infrared radiation emitted from the bulb and absorbed by your hand.
A simple devise for detecting infrared radiation is a container of water. As the water absorbs the radiation, its temperature rises.
This is also the kind of radiation that no humans can avoid radiating into space. Hence, a sizable military industry in the US has sprung up to detect the infrared waves that human emit to “see” the enemy.
Visible light with wavelength at 8x10-5 to4x10-5 cm. is the radiation most common to us. Fortunately or not, our human eyes are only sensitive to this wavelength and objects radiating at that range are the only one we can see. Our eyes are not capable of capturing what it looks like, even if you used the biggest optical telescope ever made on Earth if the object radiates below or above this range. The window to see the Universe in terms of wavelength is open to the entire spectrum from 10-16 to 106 meters which is so wide for us to see how the Universe really looks like. Unfortunately or not, with or without optical telescope, our eyes are operating only in the range of 0.00000000009% or 9x10-11 % of the entire spectrum which is extremely limited .
The eye undoubtedly has evolved to be sensitive to just this region for a combination of reasons: (1) the sun radiates most strongly in visible wavelength, and (2) the Earth's atmosphere block out many wavelength of radiations but visible light lies in one of the wavelength regions that can pass through the ground. Therefore, it was advantageous for creatures dwelling on Earth’s surface to develop the ability to see the radiation at this particular wavelength.
4.0 Ultraviolet light whose wavelength at 4x10-5 to 10-6cm is short to be visible to our eyes. Brief exposure to this radiation causes skin sunburn but long-term exposure can lead to more serious effects.
5.0 X-ray radiation whose wavelengths at 10-6 to 3x10-10cm correspond roughly to the spacing between the atoms in solid. It can easily penetrate soft human tissues but are stopped by bone and other solid matter
This portrait of St. Sebastian by the 15th century shows x-ray photograph (right) revealing that the artist changed his mind and repainted the head, inclining it to the left of the picture instead of the right. Such technique using x-ray radiation allow paintings and other inanimate objects to be examined internally.
4.0 Gamma rays with wavelength at 10-10 to 10-13cm. is the radiation of the shortest wavelength that are generated in the deep interiors of stars. They are the most penetrating of electromagnetic radiation, and exposure to intense gamma radiation can have a harmful effect to human body.
The array of radiation of all wavelengths from radio waves to gamma rays is called the electromagnetic spectrum.
Different Types of Radiation Using Different Methods of Observation
Radiation from the Sun and Outer Space
If we could pass outward in the Sun from its center to its surface, we would find mostly gamma rays and x-rays at the center, mostly x-rays and ultraviolet in its middle region, and mostly ultraviolet and visible light near the surface. Finally, from the regions close to the Sun’s surface, photons can escape. The photons that do escape are ultraviolet and visible light photons.
The Earth’s atmosphere is relatively transparent and allows certain type of EMR to penetrate into it. Visible light and radio waves pass through freely to reach ground-based telescopes sensitive to these forms of electromagnetic radiation. Infrared radiation has a limited penetration through the Earth’s atmosphere. We detect this radiation as heat. Likewise, ultraviolet radiation also has a very limited penetration through the atmosphere. This radiation causes tanning and sunburns.
The Earth’s atmosphere is completely opaque to other types of EMR - meaning, they do not reach the Earth’s surface.
Gamma rays and x-rays do not come through the Earth’s atmosphere. Ozone, a molecule of 3 atoms of oxygen (O3 ) is located in a broad layer between 20 to 40 kilometers in altitude. The ozone prevents all radiations at wavelengths less than approximately 10-6 cm. from penetrating. Observations of these radiations must be performed high in the atmosphere, or ideally, out in space.
What if our eyes evolved to see through other wavelength?
The Universe as seen from Earth plus all its terrestrial surroundings would look very dark if our eyes are only sensitive to short wavelengths such as gamma rays, x-rays, and ultraviolet rays since these wavelength cannot penetrate the atmosphere.
In contrast, although radio waves can easily pass through the atmosphere having the longest wavelength, it would require our eyes to evolve 10,000 times larger due to the need for a larger sensor that has to evolve with nature.
To see in the infrared wavelength, our eyes would need to be 5 to 10 times bigger to catch as much photons as it can plus a bigger built in sensor to acquire enough sensitivity.
How will it be if an astronomical observation is conducted on lunar surface?
Since the Moon lacks significant atmosphere and ionosphere in particular, it can permit unimpeded observation over the entire electromagnetic spectrum with a resolution that is limited only by the characteristic size of the telescope. The lunar nights provide thermal stability that is important for maintaining the precise alignment of a large telescope. Also, the 70º K temperature in lunar night can greatly improve infrared sensitivity of the telescope.
On the other hand, cosmic rays and the solar wind strikes directly on the lunar surface uncontrolled by the absence of any magnetosphere.
Contamination of optical and mechanical components by lunar dusts everywhere is also a potential problem.
Numerous Deep-Sky Objects Not Known Before Are Now Visible Through Other Forms of Electromagnetic Radiation
Earth-based optical telescope can only resolve and magnify objects through visible light with wavelength of 0.000016 to 0.000028 inch. Celestial objects radiating at wavelength longer or much shorter than these are invisible to any form of optical observations. Through the use of telescope coupled with instrumentations that reflect and absorb magnification of objects radiating through other part of the spectrum, numerous celestial bodies not known before are now resolved that has made a great leap in the study of the Universe.
Some of the most significant astronomical observations made through other wavelength of the electromagnetic spectrum are as follows:
Composition of the Universe
Latest cosmological estimate by astrophysicists of the composition of the Universe shows data as follows :
Dark Energy 73 %
Dark Matter 23 %
Visible Matter(stars, galaxies,etc.) 4 %
Total 100 %
In conclusion, if there is anything the data above indicate assuming these are more or less close to reality, and considering the nature of electromagnetic radiation as discussed, anyone can say we really have not seen it all, or perhaps, we never ever will, if and only if, we continue to lock ourselves and be contended with the limit of our vision and fail to appreciate, understand, and put into application the physics behind the entire electromagnetic spectrum.
(For more information and other details regarding the subject matter, please see PAS structured program re Astrophysics 201 and 202.)