If the light in a room is extremely dim, we’ll probably notice if it gets a little brighter or dimmer. However, if a light is extremely bright, we probably won’t notice if the light becomes slightly brighter or dimmer. This is due to Weber’s Law, which states that our sensitivity to a change in a stimulus decreases as the magnitude of the stimulus increases. A term that is very important to understand when discussing Weber’s Law is the just noticeable difference (JND). The JND is the minimum amount that the magnitude of a stimulus has to be changed in order for a stimulus change to be noticeable. Returning to our initial example, if the light is dim (low magnitude), its JND is low. If the light is bright (high magnitude), its JND is high.

The ratio of the JND to the magnitude of the original stimulus is the Weber fraction, which is a constant that describes the percent change necessary to detect a change in a stimulus at all magnitudes of the stimulus. For example, if you have a 10 lbs backpack, and you require a 0.5 lbs weight to detect a change in weight, there is a 5% change in stimulus magnitude required for that change to be detected. In other words, the Weber fraction is 0.5/10, or 5%. Therefore, if we had a backpack of any weight, it would need to increase or decrease in weight by 5% before we noticed the change. For example, we would predict a 30 lbs backpack to require a 1.5 lbs increase or decrease in weight before a change is noticed.

So let’s say you have a person that is carrying a 10 lbs backpack, and you start adding weights to the backpack to find out how much weight you have to add in order for the person to be able to detect a change in the weight of the backpack. And let’s say, for instance, that 0.5 lbs weight is required to be added for the person to detect the change. This would mean that the JND is 0.5 lbs, and the Weber fraction is 0.5 lbs divided by 10 or 5%. This implies that a 5% change in the stimulus magnitude is required for that change to be detected.