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The below image shows the formation of an image with an optical instrument.

The below image shows the formation of an image with an optical instrument. CBSE 2026 Science Class 10 Sample Question Paper Solved.

(a) Identify the optical instrument (shown schematically as a rectangle) in the image.

(b) What type of image is formed in this case?

(c) Based on the measurements given in the image, calculate the focal length of the instrument.

Refraction Lens

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Answer

(a) It’s a concave (diverging) lens since the emergent rays diverge and their backward extensions meet on the same side as the object.

(b) Virtual image is formed.

(c) From the figure,

  • Object distance (u\text u) = -20 cm
  • Image distance (v\text v) = -10 cm

Let, focal length of the lens be 'f'.

By using the lens formula,

1f=1v−1u⇒1f=1(−10)−1(−20)⇒1f=−110+120⇒1f=1−220⇒1f=−120⇒f=−20 cm\dfrac{1}{\text f} = \dfrac{1}{\text v} - \dfrac{1}{\text u} \\[1em] \Rightarrow \dfrac{1}{\text f} = \dfrac{1}{(-10)} - \dfrac{1}{(-20)} \\[1em] \Rightarrow \dfrac{1}{\text f} = -\dfrac{1}{10} + \dfrac{1}{20} \\[1em] \Rightarrow \dfrac{1}{\text f} = \dfrac{1 - 2}{20} \\[1em] \Rightarrow \dfrac{1}{\text f} = -\dfrac{1}{20} \\[1em] \Rightarrow \text f = -20 \text { cm}

So the focal length is −20 cm where negative sign accounts for a concave lens.

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