CLASS 10 SCIENCE · LIGHT: REFLECTION

Mirror formula (image distance)

1/v + 1/u = 1/f for spherical mirrors. With the sign convention, u is negative; f is negative for a concave mirror and positive for a convex mirror.

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Worked examples

Example 1 · level 1

An object is placed 35 cm in front of a concave mirror of focal length 30 cm. Find the image distance v in cm, with sign. Use the New Cartesian sign convention (distances measured from the pole/optical centre; along the incident light positive).

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1/v = 1/f − 1/u = 1/(−30) + 1/35
v = uf/(u − f) = −210 cm.
Negative v: the image is real, in front of the mirror.

Answer: -210 cm

Example 2 · level 2

An object is placed 80 cm in front of a convex mirror of focal length 20 cm. Find the image distance v in cm, with sign. Use the New Cartesian sign convention (distances measured from the pole/optical centre; along the incident light positive).

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1/v = 1/f − 1/u = 1/(+20) + 1/80
v = uf/(u − f) = +16 cm.
Positive v: the image is virtual, behind the mirror.

Answer: 16 cm

Example 3 · level 3

An object is placed 30 cm in front of a concave mirror of focal length 5 cm. Find the image distance v in cm, with sign. Use the New Cartesian sign convention (distances measured from the pole/optical centre; along the incident light positive).

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1/v = 1/f − 1/u = 1/(−5) + 1/30
v = uf/(u − f) = −6 cm.
Negative v: the image is real, in front of the mirror.

Answer: -6 cm