The method of testing for the absolute threshold is similar for different sensory systems. Thus, the tester can briefly present a light or a sound (or any other kind of stimulus) at different, low intensities until the observer is unable to detect the presence of the stimulus. In such a task, the person may undergo thousands of trials before the researcher can determine the threshold.

While the absolute threshold is a useful concept, it does not exist in reality. That is, on one occasion, an individual might be unable to detect a certain faint light

difference threshold - the smallest change in stimulation that a person can detect

Just Noticeable DifferenceScientific calculation of the average detectable difference between two measurable qualities, such as weight, brightness of light, loudness of sound.

Weber’s law, also called Weber–Fechner law, historically important psychological law quantifying the perception of change in a given stimulus. The law states that the change in a stimulus that will be just noticeable is a constant ratio of the original stimulus. It has been shown not to hold for extremes of stimulation.

Parts of Human Eye & Their Functions:

The human eye is a ball about 2.5 cm in diameter. It consists of the following parts:

1) Sclerotic: Sclerotic is the outer coating of the eye which is white in colour, that protects the interior of the eye and provides the shape to the eye.

2) Cornea: The front part of sclerotic is transparent to light and is termed as cornea. The light coming from an object enters the eye through cornea.

3) Iris: Iris is just at the back of cornea. This controls the size of the pupil. It acts like a shutter of a photographic camer and allows the regulated amount of light to enter the eye.Parts of Human Eye & their Functions (4-8)

4) Eye Lens: Eye lens is a double convex lens with the help of which image is formed at retina by refraction of light.

5) Ciliary Muscles: The eye lens is held by ciliary muscles. Ciliary muscles help the eye lens to change its focal length.

6) Pupil: At the centre of the iris there is a hole through which light falls on the lens, which is called pupil.

7) Aqueous humour : The space between cornea and eye lens is filled with a transparent fluid called aqueous humour.

8) Vitreous Humour: The space between eye lens and retina is filled with a jelly like transparent fluid called vitreous humou

9) Retins : Retina serves the purpose of a screen in the eye, wherethe images of the objects are formed. Retina is at the back of the eye lens. Retins is made of light sensitive cells, which are connected to the optical nerve.

10) Optic Nerve: Optic nerve carries the information to brain.

11) Principal Axis

12) Blind Spot : The region of eye containing the optic nerve is not at all sensitive to light and is called blind spot. If the image of an object is formed in the blind spot, it is not visible.

13) Yellow Spot : The central part of retina lying on the optic axis of eye is most sensitive to light and is called yellow spot.

14) Eye Lids: Eye lids are provided to control the amount of light falling on the eye. They also protect the eye from dust particles etc.