since a concave lens cannot kind a real picture of a actual object, it is complicated to measure up its focal size precisely. One an approach uses a second, convex, lens to create a digital object for the concave lens. Under the appropriate conditions, the concave lens will form a real photo of the online object! A college student conducting a laboratory task on concave lenses provides the adhering to observations: when a lamp is put 42.0 centimeter to the left that a specific convex lens, a genuine (inverted) picture is formed 37.5 cm to the best of the lens. The lamp and the convex lens are kept in location while a concave lens is mounted 15.0 centimeter to the appropriate of the convex lens. A real picture of the lamp is now created 35.0 cm to the appropriate of the concave lens. What is the focal size of each lens?
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Borys Shumyatskiy

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We need a formula which develops a relation between a lens" focal size `F,` a distance `d` from an object to the lens and also a street `f` from photo to a lens.

For a convex (collecting) lens and a real picture the formula is


for a concave...

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We require a formula which develops a relation between a lens" focal size `F,` a distance `d` from an object to the lens and a distance `f` from photo to a lens.

For a convex (collecting) lens and a real picture the formula is


for a concave (diverging) lens and a virtual photo the formula is



Please look at the photo attached. The focal length of the convex lens is

`F_(convex)=(42*37.5)/(42+37.5) approx 19.8 (cm).`

For the concave lens the thing is the actual image created by the convex lens. Therefore the distance between the lens and also the object is 37.5-15=22.5(cm). The street from the concave lens to its real image is given (35 cm). Thus its focal size is

`F_(concave)=(22.5*35)/(22.5-35) approx -63(cm).`

Yes, the is negative because the lens is diverging.

The answer: the focal size for the convex lens is 19.8 cm and for the concave lens is 63 cm (or -63 cm).

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