The Description of Light - Report Example

Rainbows Light which is normally referred to as the visible light is an electromagnetic radiation that can be detected andperceived by the human eye. The wavelengths range of visible light is usually 400 to 700n nanometres (nm). This is a very narrow band located to the left of ultraviolet region and to the right of infrared region. The most dominant source of visible light in the universe is the sun which provides the energy used by the green plants to manufacture the sugars in a process referred to as photosynthesis. The aim of the paper is to understand the description of light and how it is perceived by the human eye. Light: Photons and Waves Sir Isaac Newton, while carrying out optical experiments discovered that when light from the sun is passed through a prism, it was split into different colours. The colours produced were arranged in a precise array and followed the order of red, orange, yellow, green, blue, indigo and violet. The order of the colors was always constant and the colors were continuous without boundaries forming a spectrum. Each color in the spectrum represents a wavelength in the visible light region. It was only after less than 100 years after Newton’s discovery that another scientist, James C. Maxwell brought forth the electromagnetic theory of light. Maxwell demonstrated that light was an electromagnetic radiation. An electromagnetic radiation consists of self sustaining oscillations of electric and magnetic fields that are at right angles to one another and to the direction of propagation. These radiations do not require a supporting media and can travel in a vacuum at the speed of light (Brill, 452). The radiations consists of radio waves, infrared, visible light, ultraviolet, x-rays, and gamma rays as shown in Figure 1. Figure 1. The electromagnetic spectrum, which includes the visible region of light, extends from gamma rays with a wavelength of 0.01nm to radio waves with a wavelength of 1m or more. Visible region forms a very small part of the spectrum. The radiations emitted by the sun falls within the visible region and extend into the infrared region beyond the red wavelength and ultraviolet region. As an electromagnetic wave, the each colour can be identified by its wavelength. Light can as well be described as stream of energy packets called photons which create an energetic electromagnetic disturbance. Photons of different colours differ in energy and therefore can be distinguished from each other. The most convenient unit to measure the energy of light is the electron volt (eV). An electron volt is the energy that is acquired by a single electron that moves across a positive voltage of one volt (Smith, 322). Photons of visible light have energy content that range from 2 to 3 eV. The energy of light is inversely proportional to the wavelength, meaning that as the energy increases, the wavelength decreases and vice versa as shown in Figure 2 below. Figure 2. Diagram showing the visible region of the electromagnetic spectrum in terms of wavelength and corresponding energies It is clear from Figure 2 above that photoreceptors in the human eye can only be triggered by energy of 1.8 to 3.1 eV. Wavelengths of lower energies can only be detected by special infrared detectors while those with higher energy can be detected by photographic film or other special apparatus. White light consists of all the colours of the rainbow since it contains all the wavelengths and therefore it is described as polychromatic light. However monochromatic light such as one form a laser only produces one colour because it contains one wavelength. For example, light produced by a laser of wavelength of 650 nm appears red. Sources of Light: The sun and the Lamps The visible spectrum lies in the middle range of wavelengths between 400 – 700 nm. The photoreceptors in the eye can only perceive light in the visible region. The radiation from the sun contains the visible spectrum as shown in Figure below. Some of the radiations particularly the ultraviolet and infrared are absorbed in the earth’s atmosphere Figure 3. The region of the suns spectrum that spans the range from ultraviolet to infrared Light can also be produced by heating an electric filament until it glows. Light produced in this manner is called incandescent light for example light produced by a candle or a tungsten lamp. When candle wax is melted by the flame, it vaporises and as the combustion raises the temperature of the carbon particles, incandescence occurs resulting in emission of yellow colour. The filament in a tungsten lamp becomes hot as electric current passes through it an radiates thus producing light. Fluorescent lamps also produce light though on a different principle from incandescence. A glass tube is filled with mercury or sodium vapour and its electrodes connected to as source of alternating current (AC). The atoms in the tube are ionized by the electric current and emit light mostly in the ultraviolet region. The phosphor coating the inside of the tube absorbs the UV light and produces visible light. Therefore fluorescent tubes produce broad spectrums that are similar to tungsten lamps. The colour of objects Colour can only come about when three basic components are present: a viewer, an object and light. Everything in the universe has colour of some sort. There are numerous reasons as to why the objects appear coloured. All objects are composed of atoms and electrons. When irradiated with light, the manner in which it interacts with the electrons and atoms dictates how the object will appear. The object can reflect or scatter the light, absorb or transmit or even refract all or part of the wavelength of the light (Albers, pp. 97-115). For example of why objects have colour is the picture of a ripe tomato in Figure 4. A ripe tomato contains a photochemical substance called lycopene which absorbs most of the wavelengths in the visible region of the spectrum and reflect red colour to the viewer hence the ripe tomato appears red in colour. Pure white light appears colourless but it actually contains all the wavelengths of the spectrum. Objects that reflects all the light appears white in colour while objects which absorb all the wavelengths of white light appears black since there is no colour reflected to the viewer. Figure 4. A ripe tomato is a good example of reflective light (the shiny bit) and Absorbed light (the red colour) The eye and colour sensation Perception of colour depends on the energy of the photons and composition of the light that is entering the eye. The inner surface of the retina at the back of the human eye (Figure 5) contains photosensitive cells called rods and cones. It is the rods that are responsible for distinguishing different colours (Nassau, 1983, pp.47). The rods are used for perception in dim light and they perceive changes in light intensity rather than the energy of the photons. Figure 5. A cross-sectional representation of the eye showing light entering through the pupil. The human eye is referred to as trichromatic because it has three types of cones depending on the cone pigment present. The three colour sensitive pigments present in the rods are the red sensitive pigment, green sensitive pigment and the blue sensitive pigment (Rogers,577). Each cone cell contains one of the pigments and so is sensitive to that colour only. For any other colour reaching the eye, a response from some ratio of the three types of cones is triggered and that permits the perception of that colour. Works Cited Albers J., The Interaction of Colour. New Haven, NH: Yale University Press, 1975 Brill T. B., Light, Its Interaction with Art and Antiquities. New York, NY: Plenum Press, 1980. Nassau K., The Physics and Chemistry of Color. New York, NY: John Wiley, 1983 Rogers K. The Eye: The Physiology of the Human Perception. New York, NY: Britannia Educational Publishing, 2011. Smith G. S., Introduction to Classical Electromagnetic Radiation. New York, NY: Cambridge University Press, 1997. Read More