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You look at a clock in a plane mirror, and the image shows the time 2:25. What is the real time?
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
You look at a clock in a plane mirror, and the image shows the time . What is the real time?
Need a reminder? The key ideaShow
The angle of incidence equals the angle of reflection, both measured from the normal. A plane mirror gives a virtual, upright image of the same size, as far behind the mirror as the object is in front, with left and right swapped.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. The rule. Reflecting the clock face in the line through and turns a time into minus that time.
Step 2. Subtract. : write as . Then .
Answer: the real time is .
Check. At the hour hand is past , and the minute hand is at . In the mirror, goes to where is and goes to where is: about .
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