Video Transcript
The following figure shows a transparent object marked with opaque letters. The object is reflected in a plane mirror. The image of the object is seen by observer A. Which of the following shows the object as it would appear to observer B? (A), (B), (C), or (D).
In this question, we are shown how the image appears to observer A. And we must determine how the object is seen by observer B. The answer to this question is easier to understand if we imagine observer A and
observer B as being located along the same line and this line as being at an angle
of 90 degrees to the mirror. Two points on the object are shown from which light rays travel toward the mirror and
away from the mirror. The light rays that travel away from the mirror are seen directly by observer B. The light rays that travel toward the mirror reflect from the mirror and are seen by
observer A. Observer B sees the object directly; observer A sees the image of the object. Both observers are facing the mirror.
Let us now include in the diagram one letter in the word as it appears in the
image. We can see that the points on the letter in the image will be connected to the points
on the letter in the object by straight lines that are at an angle of 90 degrees to
the mirror. Knowing this, we can show the letter as seen on the object. To make the object letter and image letter easier to compare, we can now remove the
light rays.
The object letter and the image letter are the same as each other. We can imagine the observers moving in opposite directions parallel to the
mirror. If this happened, each observer would each see the letter turned slightly. The directions that the letter turned in would be opposite for the observers. Apart from the slight turning, the letters would appear the same. The letters would not be reversed in any way for observer B. We see then that the word ambulance would appear the same to observer B as to
observer A. And hence, the object will look the same as the image.
But if we now look at our answer options, we can see that options (B), (C), and (D)
are different compared to the image. So options (B), (C), and (D) are all incorrect. Option (A) shows the object as the same as the image, so option (A) is correct. Option (A) shows the object as it would appear to observer B.
