Prisms

If you were to take a square like the red one below

And a congruent square like this blue one

And then took four rectangles with the same widths as the squares like these grey and white ones below

Then you would have all of the shapes you need to make a prism. Below the two squares are arranged so that the edges of the squares are parallel. I have marked the bottom edge of both squares with arrowheads to show that these edges are parallel. The top edges are also parallel and so are the left edges and the right edges.

The red and blue squares are congruent and parallel.

By joining a rectangle between the bottom sides of both squares we can start to build a prism

By joining a rectangle between each pair of sides, we finish the prism

The word prism comes from a Greek prisma meaning something sawed.

If you were to saw through the prism with the saw parallel to the square faces then you would create two smaller prisms. Imagine we saw through the prism along the plane of the dotted yellow square.

We would end up with two smaller prisms like these

The yellow area and any similar area along the length of the prism is called the cross section. In a prism, the cross section is the same all the way along the length of the prism

The prisms we have seen so far are called square prisms but there are also triangular prisms, heptagonal prisms and others. The name of the prism comes from the shape at the ends of the prism.

To be a prism

• the cross section is the same all along its length
• the sides are parallelograms

The sides of the prism have to be parallelograms but they don’t have to be rectangles. If they are rectangles then the two ends of the prism will be directly above each other.