Mammoth Memory

How much of an object can an observer see

In the following diagram how much of the arrow can an observer see.

Observer looking at an object in a mirror

To work this out we need to apply the law of reflection. The first stage of this is to draw an image behind the mirror by marking either end of the image as being equal-distance and perpendicular (at 90°) from the mirror.

Applying the law of reflection to an observer looking at an object in a mirror

We end up with a diagram as follows:

Image of an object in a mirror

The second stage is to draw a ray to the observer from the arrow end of the image and then join the reflected light ray from the mirror to the arrow end of the object.

Ray diagram of an image of an object in a mirror

Because the object is one continual line with an arrow at the end, the only way to find out how much of the line can be seen by the observer, you must reflect off the edge of the mirror, working back from the observer.

Ray diagram showing how much of the image an observer can see

Use the rule, "The angle of incidence equals the angle of reflection". We can draw a normal line and use a protractor to work out where the ray of light came from on the object.

Ray diagram showing the angle at which the last visible ray of light from the object reaches the observer

We can now show you what part of the object the observer can see reflected in the mirror. 

Ray diagram showing the part of the object an observer can not see in the mirror

The part circled in red cannot be seen by the observer.



Using a ray diagram is useful in showing why only a portion of the image of an object can be seen from a given location.

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