Question

predict and write the reason why the value of the distance of the object u is always negative in the mirror equation​

Answer 1

Explanation:

According to sign convention the distances measured from the pole or optic centre, to the left of the mirror or lens is always negative and those measured to the right is always positive.

Conventionally objects are always placed to the left of the mirror or lens as light is said to approach from the left.

Answer 2

Answer:

Distances from the optical centre of the lens are interpreted as negative to the left and positive to the right according to the sign conventions used in optics ( you can compare this logic with number line).

Another internationally recognised practise for clarity is to always place the item to be understood to the left of the lens (whether it be concave, convex, or a combination of both, etc.), with the image to be generated to the right of the lens.

As a result, when performing computations, we enter the negative object distance into the formulas.

Explanation:

i) Direction of the incident rays is taken as positive (+ve).  

ii) Object distance is measured from the pole to the object in the opposite direction of incident rays.

iii) all distance should be measured from pole of mirror/lens.

iv) if direction of measurement of distance is opposite of direction ray then measured distance would be negative. That's why u is always negative and concave mirror has focal length negative

v) if you measure a distance in upward direction then it should be positive , other wise negative in case of measured in downward direction. So, when image is formed inverted the magnification is negative because object is in upward direction and image in downward direction.

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