Two parallel plane mirrors M₁ and M₂ are separated by 40 cm. An object is 10 cm from M₁. Starting with reflection in M₁ and then alternating between mirrors, where is the third image measured from M₁ along the normal, taking the region toward M₂ as positive?
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Question

Two parallel plane mirrors M₁ and M₂ are separated by 40 cm. An object is 10 cm from M₁. Starting with reflection in M₁ and then alternating between mirrors, where is the third image measured from M₁ along the normal, taking the region toward M₂ as positive?

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G1 Physics · Accepted Answer

Let M₁ be at x=0, M₂ at x=40 and object at x=10. First image: −10. Reflect it in M₂: x=2(40)−(−10)=90. Reflect that in M₁: x=−90.