What is Weber’s law in Psychology?

Weber’s law: Weber’s Law states that the size of the difference threshold is proportional to the intensity of the standard stimulus. It is a constant ratio. The size of the difference threshold, a constant ratio of the standard stimulus, is referred to as the Weber’s fraction. Each noticeable stimulus increment is a constant fraction of the stimulus to which it is added.

Sensations go up by arithmetic series and stimulus goes up by geometric series. Weber’s Law states that the ratio of the increment threshold to the background intensity is a constant. So when you are in a noisy environment you must shout to be heard while a whisper works in a quiet room. And when you measure increment thresholds on various intensity backgrounds, the thresholds increase in proportion to the background.

The following example of the law (numbers used are not factual):

If we lift up and hold a weight of 2.0 kg, we will notice that it takes some effort. If we add to this weight another 0.05 kg and lift, we may not notice any difference between the apparent or subjective weight between the 2.0 kg and the 2.1 kg weights. If we keep adding weight, we find that we will only notice the difference when the additional weight is equal to 0.2 kg. The increment threshold for detecting the difference from a 2.0 kg weight is 0.2 kg. The just noticeable difference (jnd) is 0.2 kg.

Now lets start with a 5.0 kg weight. If we add weight to this, we will find that the just noticeable difference is 0.5 kg. It takes 0.5 kg added to the 5.0 kg weight for us to notice an apparent difference.

For the weight of magnitude, I, of 2.0 kg, the increment threshold for detecting a difference was a ΔI (pronounces, delta I) of 0.2 kg.

For the weight of magnitude, I = 5.0 kg, the increment threshold ΔI = 0.5 kg.

The ratio of ΔI/I for both instances (0.2/2.0 = 0.5/ 5.0 = 0.1) is the same. This is Weber’s Law.