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| Mirror Formula (Concave Mirror) |
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| Mirror formula is the relationship between object distance (u), image distance (v) and focal length. |
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| The figure shows an object AB at a distance u from the pole of a concave mirror. The image A1B1 is formed at a distance v from the mirror. The position of the image is obtained by drawing a ray diagram. |
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| Consider the D A1CB1 and D ACB |
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[when two angles of D A1CB1 and D ACB are equal then the third angle  |
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| But ED = AB |
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| From equations (1) and (2) |
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| If D is very close to P then EF = PF |
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| But PC = R, PB = u, PB1 = v, PF = f |
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| By sign convention |
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| PC = -R, PB = -u, PF = -f and PB1 = -v |
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 Equation (3) can be written as |
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| Dividing equation (4) throughout by uvf we get |
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| Equation (5) gives the mirror formula |
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