All on one flat sheet

The idea

The incident ray, the normal to the mirror at the point of incidence, and the reflected ray all lie in the same plane.

Bend the paper

A torch beam travels along a sheet of paper to a mirror. The reflected beam is also seen on the sheet. Now you bend the part of the sheet with the reflected beam.

What do you expect to see?

• The reflected beam disappears from the sheet
• The reflected beam bends along with the paper
• Nothing changes

Bend and flatten

Bend the right part of the sheet down, along the normal. Look for the reflected beam. Then flatten the sheet again.

A second law of reflection

The incident ray, the normal to the mirror at the point of incidence, and the reflected ray all lie in the same plane.

A flat sheet is one plane. Bending it makes a new plane, and the reflected beam does not follow it. This is another law of reflection.

• The sheet | Reflected beam on the sheet?
• Flat | Seen
• Right part bent | Not seen
• Flattened again | Seen again

Notes

The incident ray, the normal to the mirror at the point of incidence, and the reflected ray all lie in the same plane.

Check yourself

What does the bent-sheet activity show?

Which three lie in the same plane?

The bent part of the sheet is flattened again. What happens?

• The mirror stops reflecting when the paper is bent. The mirror still reflects. The reflected beam is simply not on the bent paper.
• The reflected ray bends along with the paper. If it did, we would still see it on the bent part. It disappeared.
• The incident ray, the normal and the reflected ray lie in the same plane — correct. Yes. This is a law of reflection.
• Only the incident ray and the reflected ray; the normal is in another plane. The normal lies on the sheet too, along the fold line. All three share one plane.
• The incident ray, the normal at the point of incidence and the reflected ray — correct. Yes. All three are on the same flat sheet.
• The incident ray, the reflected ray and the back of the mirror. The back of the mirror is not part of this law. The third one is the normal.
• The reflected beam is seen on the sheet again — correct. Yes. The sheet is back in the same plane as the incident ray and the normal.
• The reflected beam stays hidden. Once the sheet is flat again, it is back in the plane of the reflected beam.
• The incident beam disappears. Nothing happened to the incident beam's part of the sheet.