"The term affordance refers to the relationship between a physical object and a person." — Don Norman Sometimes it can be difficult to know how we can interact with an object if its affordances are invisible. Some affordances are perceivable; others are invisible. Affordances exist even if they are invisible – it's your job as a designer to make them visible. Let's look into invisible affordances; let's look into *glass*.
Glass affords transparency – we can see through glass. Glass affords seeing through. We can pour water into glass – glass affords containing water. We can use glass as a table because glass affords support. That's because glass blocks the passage of air and physical objects. Well, okay, atomic particles can pass through glass, but that's not our focus here.
It's an *anti-affordance* that water, fingers, air and all other physical objects can't pass through glass. Glass prevents that interaction. When you design, it's your job to ensure that affordances and anti-affordances are discoverable and understandable. People have to be able to perceive them with their senses. If not, people will not know what the design affords. It can be tricky to create a design where the affordances
and anti-affordances are *discoverable*. Let's take a simple example. One of the reasons we like glass is that it's almost invisible. However, since it's almost invisible Birds often try to fly through windows. But glass does not afford passage – in that manner, glass is an anti-affordance. Anti-affordance: Define what actions are impossible. Glass does not afford passage of objects.
Affordance: Define what actions are possible. Glass affords seeing through. When birds and people don't discover the anti-affordance, it's a messy affair. "The presence of an affordance is jointly determined by the qualities of the object and the abilities of the agent that is interacting." Now, back to your job – if people cannot perceive the affordance and anti-affordance, it's your job to design some kind of *signifiers* to help people understand that it's there.
Signifiers help people perceive the affordances and anti-affordances. Here, you see that some designers put arrows to signify the anti-affordance of the glass door. The arrows signify that the glass door blocks the passage when it's closed. Likewise, when you design apps, products and services, it's your job to help people discover the affordances and anti-affordances.
Where can – and should – they press, tap and scroll to fulfill their goals? It's your job to make it as easily discoverable and understandable as you possibly can. Interaction Design Foundation