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CBSE Class 10 Science Question 11 of 31

Light - Reflection and Refraction — Question 11

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Question 11

A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object placed from the lens? Draw the ray diagram.

Answer

f = -15 cm [∵ focal length of concave lens is negative]

v = -10 cm [∵ image distance of concave lens is negative]

u = ?

According to the lens formula,

1v\dfrac{1}{v} - 1u\dfrac{1}{u} = 1f\dfrac{1}{f}

Substituting the values we get,

1−10−1u=1−15⇒1u=1−10+115⇒1u=−3+230⇒1u=−130⇒u=−30 cm\dfrac{1}{- 10} - \dfrac{1}{u} = \dfrac{1}{- 15} \\[0.5em] \Rightarrow \dfrac{1}{u} = \dfrac{1}{- 10} + \dfrac{1}{15} \\[0.5em] \Rightarrow \dfrac{1}{u} = \dfrac{-3+2}{30} \\[0.5em] \Rightarrow \dfrac{1}{u} = - \dfrac{1}{30} \\[0.5em] \Rightarrow u = -30\text{ cm}

The object is placed 30 cm in front of the concave lens.

A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object placed from the lens? Draw the ray diagram. NCERT Class 10 Science CBSE Solutions.