Predict the path of a light beam from a mirror

Stand a small mirror upright along a line and mark a right angle at the beam contact point. Use a narrow ordinary flashlight beam on the paper surface.

Draw a ninety-degree reference through the point where a narrow flashlight beam reaches a mirror. Repeat a simple comparison and keep the starting conditions as similar as the setup allows. Where a ruler, timer, or camera is involved, read it in the same way each time. A handful of consistent observations is easier to interpret than a long list collected under changing conditions.

Two measured angles should be close when both are measured from the normal. Sketch your apparatus before you start and use the sketch to identify where each observation will be made. Write down the starting state before changing it. If another person repeats the trial, knowing the relative positions can matter more than the brand or appearance of the materials.

Mark two points along each visible path. Remove the equipment and draw lines through the marks, then measure both angles from the normal.

A back-silvered mirror may show a faint second reflection near the strong one. End with a short account linking the changed condition, the observation, and its likely explanation. Include one uncertainty that would matter if you repeated it. This separates the physical idea explored by the demonstration from limits created by a household setup and gives another person a way to test the same claim.

Mark the same part of the reflected beam at both points along its trace. Write one prediction alongside the setup. It helps to describe what would be observed if that prediction were right, including a direction, a clearer pattern or a measurable difference. This makes room for the outcome to disagree with what you expected and gives you a better question for a second run.

Check a second incoming angle without moving the marked normal. Make a simple model of what you believe happens between the starting and final positions. Then check one point at which that model gives a visible consequence. A single carefully observed intermediate position can be more revealing than comparing two widely separated endpoints with an unknown change between them.

Write a row for each run and keep apparently odd results in the record. Mark things that went wrong, such as a shifted surface, uneven starting point or missed time, before judging the pattern. Changing how you record the behaviour halfway through can make two otherwise similar trials difficult to compare. Explain the outcome in a sentence that a reader can check against the figures or sketch. Use a second sentence for a proposed cause. If you can imagine a different cause that fits the same observations, describe a small test that would help distinguish the two. Comparing those predictions can make a follow-up clearer than simply repeating the same demonstration. Use a short surface tracing and draw the full lines after removing the mirror. The ruler can obstruct the beam if you try to trace the whole path at once. Use the empty setup once before introducing the change you want to study. A baseline can show motion from a loose support, light falling on the screen from another direction or variation in the way the sample starts. Record what you see at that stage as part of the experiment.

Try a different incoming angle and record how the reflected path changes. If the results vary, look for a second feature that moved along with the one you meant to change. That can be the distance to an observer, the angle of a support or the way the material moves between runs. Isolate it in a small follow-up trial when possible.