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📘 Name the three distances

A lens equation problem starts with three distances measured from the optical center of the lens. The object distance, d_o, runs from the object to the lens. The image distance, d_i, runs from the lens to the image. The focal length, f, run

3
lessons
~15 min
to learn
Adults
level
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What you’ll learn

  1. Foundations
  2. Solve
  3. Check

Questions this course answers

Which equation is the standard thin-lens equation?

The reciprocal focal length equals the sum of the reciprocal object and image distances.

A converging lens has f = 10 cm and d_o = 30 cm. What is d_i?

d_i = f d_o/(d_o - f) = 10 times 30 divided by 20 = 15 cm.

What does a negative image distance mean in the common convention?

A negative d_i represents a virtual image formed by backward extensions of diverging rays.

If d_o = 75 cm and d_i = 150 cm, what is f?

1/f = 1/75 + 1/150 = 1/50, so f = 50 cm.

What does m = -0.50 tell you?

The negative sign indicates inversion and the magnitude 0.50 gives half the object height.

What happens when a converging-lens object is placed exactly at f?

At d_o = f, the denominator in d_i = f d_o/(d_o - f) is zero, representing parallel outgoing rays.

Grounded in trusted sources

  • [object Object]
  • [object Object]
  • [object Object]
  • [object Object]
  • [object Object]

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