Where must you place an object in front of aconcave mirror with radius R so that

Question

Where must you place an object in front of aconcave mirror with radius R so that the image is erectand 2 1/2 times the size of the object? A) .7 R behind B) .7 R infront C) .3 Rinfront D) 1.4 R infront The image is ___ the mirror? A) .7 behind B) .75 infront of C) 1.75behind D) .75 behind Where must you place an object in front of aconcave mirror with radius R so that the image is erectand 2 1/2 times the size of the object? A) .7 R behind B) .7 R infront C) .3 Rinfront D) 1.4 R infront The image is ___ the mirror? A) .7 behind B) .75 infront of C) 1.75behind D) .75 behind

Explanation / Answer

mirror formula 1/f = 1/u+1/v f is focal length u = object distance v = image distance For concave mirror f is negative and u is negative magnification m = -v/u 2.5 = -(v/-u) v = 2.5 u so mirror equation becomes -1/f = -1/u + 1/2.5u u = 0.6f f = R/2 u = 0.6*R/2 = 0.3R So object must be placed in front of the mirror at 0.3R ( c isthe correct answer) v = 2.5u = 2.5*0.3R = 0.75R So the image is at 0.75R behind the mirror

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