Red Lights

Light and Vision

An 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.

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.

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.

Infrared (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.

Ultraviolet (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

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 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.

The 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 field is the total extent in space that can be seen when looking in a given direction.

Acuity is the ability to distinguish between details that are very close together. This ability increases as the amount of available light increases.

Adaption 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.

Contrast is the difference in brightness or colours between two parts of the visual field.