CLASS 10 SCIENCE · LIGHT: REFRACTION

Lens formula (image distance)

1/v − 1/u = 1/f for thin lenses. u is negative; f is positive for a convex lens and negative for a concave lens.

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Worked examples

Example 1 · level 1

An object is placed 35 cm from a convex lens of focal length 10 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/(+10) + 1/(−35)
v = uf/(u + f) = +14 cm.
Positive v: real image on the other side of the lens.

Answer: 14 cm

Example 2 · level 2

An object is placed 90 cm from a convex lens of focal length 15 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/(+15) + 1/(−90)
v = uf/(u + f) = +18 cm.
Positive v: real image on the other side of the lens.

Answer: 18 cm

Example 3 · level 3

An object is placed 65 cm from a convex lens of focal length 40 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/(+40) + 1/(−65)
v = uf/(u + f) = +104 cm.
Positive v: real image on the other side of the lens.

Answer: 104 cm