Class 12th Physics 2017 Set3 Delhi Board Paper Solution

Question 23

Mrs. Rashmi Singh broke her reading glasses. When she went to the shopkeeper to order new spects, 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 than 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.

(i) Write two qualities displayed each by Anuja and her mother.

(ii) How do you explain this fact using lens maker's formula?

Answer

(i) Mrs. Rashmi is curious and observant in nature.

Anuja is well aware of the working of the lenses and she is explainer too.

(ii)) Lens maker’s formula is 1 /f = ( µ -1) x 2/r.  

r is the radius of curvature of lens. Plastic has more refractive index (µ) than glass, so in order to make lens of matching focal length,r needs to be increased. Hence, lens has to be thicker. 

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