Red Lights
Light and Vision
Scientific and technical nature of light and human visionAn introduction to the nature of light and human vision including: electromagnetic waves, white light, monochromatic light, infra red, ultra violet, human eye, rod vision, cone vision, vision sensitivity, vision terminology
The aim of this knol is to give you enough knowledge to appreciate aspects of light, lighting and vision that are important in our everyday lives. Purposely, there is not enough technical detail here to gain you top essay marks in medical or physics studies!
NATURE OF LIGHT
Light is energy in the form of electromagnetic radiation. This energy is radiated by processes in the atomic structure of different materials and causes a wide range of effects. The different forms of electromagnetic radiation all share the same properties of transmission although they behave quite differently when they interact with matter.
Light is that particular electromagnetic radiation which can be detected by the human sense of sight. The range of electromagnetic radiation to which the eye is sensitive is just a very narrow band in the total spectrum of electromagnetic emissions, as is indicated in the diagram.
Diagram: Spectrum of electromagnetic radiation (Nasa)
Electromagnetic waves
The transmission of light energy can be described as a wave motion or as packets' of energy called photons. The two theories co-exist in modern physics and are used to explain different effects. The most convenient theory for everyday effects is that of electromagnetic wave motion. This can be considered as having the general properties listed below.
- The energy resides in fluctuations of electric and magnetic fields, which travel as a transverse wave motion.
- The waves require no medium and can therefore travel through a vacuum.
- Different types of electromagnetic radiation have different wavelengths or frequencies.
- All electromagnetic waves have the same velocity, which is approximately 300, 000, 000 metres per second (186, 000 miles per second) - in a vacuum.
- The waves travel in straight lines unless affected by effects such as reflection, refraction or diffraction.
Reflection is reversal of direction which occurs at a surface. Examples include mirrors and coloured surfaces.
Refraction is deflection that occurs at the boundaries of different materials. For example: prism effects occur at the edge of air and glass, red sunsets are caused by differing layers of the atmosphere.
Diffraction is deflection that occurs at apertures, at edges and in thin layers. Examples include coloured patterns in thin layers of oil, and coloured spectrums caused by narrow slits.
Visible radiation
The wavelengths of electromagnetic radiation that are visible to the eye range from approximately 380 to 760 nanometres ( 1 nanometre = 1/1000, 000, 00 metre) . If all the wavelengths of light are seen at the same time, the human eye cannot distinguish the individual wavelengths and the brain has the sensation of white light.
White light is the effect on sight of combining all the visible wavelengths of light.
The definition above is necessarily human-oriented and extra-terrestrial beings with scientific knowledge would not agree with it!
White light can be separated into its component wavelengths. One method, made famous by Isaac Newton, is to use the different degrees of refraction of light that occur in a glass prism. The result is a spectrum of light, which is traditionally described in the seven colours of the rainbow although, in fact, there is a continuous range of hues (colours) whose different wavelengths cause different sensations in the brain.
Diagram: Dispersion of white light (Nasa)
Monochromatic light is light of one particular wavelength and colour.
If the colours of the spectrum are recombined then white light is again produced. Varying the proportions of the individual colours can produce different qualities of white' light.
Non-visible radiation
Electromagnetic radiation with wavelengths outside the range of visible wavelengths cannot, by definition, be detected by the human eye. However, there are wavelengths emitted by the sun which are adjacent to the visible range of wavelengths, and although they cannot be seen they are relevant to lighting processes.
InfraredInfrared (IR) radiation has wavelengths slightly greater than those of red light and can be felt as heat radiation from the sun and from other heated bodies. Infrared radiation is made use of in radiant heating devices, for detecting patterns of heat emissions, for seeing' in the dark, and for communication links.
UltravioletUltraviolet (UV) radiation has wavelengths slightly less than those of violet light. It is emitted by the sun and also by other objects at high temperature. Ultraviolet radiation helps keep the body healthy but excessive amounts can damage the skin and the eyes. The composition of the earth's atmosphere normally protects the planet from excessive UV radiation emitted by the sun.
Ultraviolet radiation can be used to kill harmful bacteria in kitchens and in hospitals. Certain chemicals can convert UV energy to visible light, and the effect is made use of in fluorescent lamps.
NATURE OF VISION
The portion of the electromagnetic spectrum known as light is of environmental interest to human beings because it activates our sense of sight, or vision. Vision is a sensation caused in the brain when light reaches the eye. The eye initially treats light in an optical manner, producing a physical image in the same way as a camera. This image is then interpreted by the brain in a manner which is psychological as well as physical.
The eye
The diagram shows the main features of the human eye with regard to its optical properties. The convex lens focuses the light from a scene to produce an inverted image of the scene on the retina. When in the relaxed position, the lens is focused on distant objects. To bring closer objects into focus the ciliary muscles increase the curvature of the lens, a process called accommodation. The closest distance at which objects can be focused, called the near point, tends to retreat with age as the lens become less elastic. Diagram: Structure of the eye
The amount of light entering the lens is controlled by the iris, a coloured ring of tissue, which automatically expands and contracts with the amount of light present. The retina, on which the image is focused, contains light receptors which are concentrated in a central area called the fovea, and are deficient in another area called the blind spot. Comparison
| Eye | Camera |
| Lens | Lens |
| Iris | Aperture - f stop |
| Retina | Film or CCD sensor |
Operation of vision
The light energy falling on the retina causes chemical changes in the receptors, which then send electrical signals to the brain via the optic nerve. A large portion of the brain is dedicated to the processing of the information received from the eyes, and the eyes are useless if this sight centre in the brain is damaged.
The initial information interpreted by the brain includes the brightness and colour of the image. The stereoscopic effect of two eyes gives further information about the size and position of objects. The brain controls selection of the many items in the field of view and the sense of vision greatly depends on interpretations of images learned from previous experience.
Sensitivity of vision
The light-sensitive receptors on the retina are of two types. These receptors respond to different wavelengths of light in the manner shown in the diagram, and they give rise to two types of vision: cone vision and rod vision. Diagram: Sensitivity of human vision to different colours
Cone vision The cones are the light receptors that operate when the eye is adapted to normal levels of light. The spectrum appears coloured. There is a concentration of cones on the fovea at the centre of the retina and these are used for seeing details.
Rod visionThe rods are the light receptors that operate when the eye is adapted to very low levels of light. The rods are much more sensitive than the cones but the spectrum appears uncoloured. The colourless appearance of objects in moonlight or starlight is an example of this vision. There is a concentration of rods at the edges of the retina, which cause the eyes to be sensitive to movements at the boundary of the field of view.
Terminology of vision
Visual fieldVisual field is the total extent in space that can be seen when looking in a given direction.
Visual acuityAcuity is the ability to distinguish between details that are very close together. This ability increases as the amount of available light increases.
AdaptionAdaption is the process occurring as the eyes adjust to the relative brightness or colour of objects in the visual field. The cones and the rods on the retina take a significant amount of time to reach full sensitivity.
ContrastContrast is the difference in brightness or colours between two parts of the visual field.
******* Further sources of information: Companion Knols: Lighting and its measurement Text: Environmental Science in Building Attribution All text is personal copyright. Diagrams are personal copyright or public domain
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A ray of red light, for which n = 1.54 , and a ray of violet light, for which n = 1.59, travel through a piece of glass. They meet right at the boundary between the glass and the air, and emerge into the air as one ray with an angle of refraction of 22.5 degrees.
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Blue light is refracted more than red light, but which of the following statements are true?
1.) The wavelength of blue light is shorter in the droplet than in the air.
2.) The wavelength of red light is longer in the droplet than in the air.
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5.) Blue light travels faster than red light in the droplet.
6.) Both blue and red light travel at the same speed in the droplet.
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