did you know?

eception and absolute
threshold
The second important element in the eye detecting the light stimulus is that
the light energy that falls within the visible light spectrum must be intense
enough for the human eye to see. In other words, it must reach absolute
threshold. The absolute threshold is the minimum amount of light energy
needed for an observer to perceive a stimulus, in ideal conditions, 50 per cent
of the time.
One method psychologists use to measure absolute threshold is by
presenting light stimuli at different intensities to see what level of intensity
is needed for a person to detect the light. If that person detects it during the
experiment about 50 per cent of the time at a particular intensity (the point
at which they actually perceive it), then absolute threshold has been reached.
Absolute threshold for the senses are:
> Hearing: ticking of a watch six metres away.
> Smell: one drop of perfume in a large house.
> Taste: one teaspoon of sugar dissolved in 10 litres of water.
> Touch: the wing of a fly falling on the cheek from a height
of one centimetre.
> Vision: the flame of a candle 50 kilometres away on a dark, clear night.
These are based on sound scientific research but may not be the same for
everyone and can vary depending on a range of environmental factors (noise,
amount of light) and psychological factors (fatigue, motivation, stress,
expectations). For example, if a person has had someone break into their
house, they will be more highly attuned to sounds at night, and this may
affect their usual absolute threshold for sound.
the role of the eye in visual
perception
As already mentioned, light enters the eye through the cornea, a tough
transparent tissue covering the front of the eye. It then passes through the
pupil, the hole in the middle of the iris. The lens then focuses the light onto
the retina, which contains photoreceptors (light-sensitive cells).
The retina is nerve tissue that covers more than 50 per cent of the
inner surface of the back of the eye. The retina contains two types of
photoreceptors: rods and cones.
Rods: There are 125 000 000 in each eye.
> They are responsible for vision in low light (that is, they are very
sensitive to light).
> They are responsible for peripheral vision (out of the corner of the eye).
They are concentrated at the edges of the retina.
> They have low visual acuity (they can’t register detail).
> They can register only in black and white.
> They are most sensitive to light of approximately 500 nm wavelength.
fig 6.6>> Your eyes can detect a
candle flame up to 50 km away on a
clear dark night.
fig 6.7>> Your tastebuds can detect one
teaspoon of sugar in 10 litres of water.
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