A concave mirror forms a virtual image three times the object height. If |f|=12 cm, the object distance magnitude is:
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A concave mirror forms a virtual image three times the object height. If |f|=12 cm, the object distance magnitude is:

G1 Physics · Accepted Answer

For a virtual erect image m=+3. With Cartesian signs u<0, v>0? For a concave mirror f<0 and virtual image v>0; m=−v/u=3 gives v=−3u. Substituting in 1/f=1/v+1/u gives |u|=8 cm.