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Question : 23
Total: 26
SECTION -D
Mrs. Rashmi Singh broke her reading glasses. When she went to the shopkeeper to order new spectacles, he suggested that she should get spectacles with plastic lenses instead of glass lenses. On getting the new spectacles, she found that the new ones were thicker that the earlier ones. She asked this question to the shopkeeper but he could not offer satisfactory explanation for this. At home, Mrs. Singh raised the same question to her daughter Anuja who explained why plastic lenses were thicker.
(a) Write two qualities displayed each by Anuja and her mother.
(b) How do you explain this fact using lens maker's formula?
Solution:
(a) Two qualities each of Anuja and her mother
(b) Explanation, using lens maker's formula
(a) Anuja : Scientific temperament, co-operative, knowledgeable (any two)
Mother: Inquisitive, scientific temper/keen to learn/has no airs (any two)(or any other two similar values)
(b)
= (
− 1 ) (
−
)
As the refractive index of plastic material is less than that of glass material therefore, for the same power( =
)
Therefore plastic lens is thicker.
Alternatively, If student just writes that plastic has a different refractive index than glass, award one mark for this part.
(b) Explanation, using lens maker's formula
(a) Anuja : Scientific temperament, co-operative, knowledgeable (any two)
Mother: Inquisitive, scientific temper/keen to learn/has no airs (any two)(or any other two similar values)
(b)
As the refractive index of plastic material is less than that of glass material therefore, for the same power
Therefore plastic lens is thicker.
Alternatively, If student just writes that plastic has a different refractive index than glass, award one mark for this part.
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