1) Rohan uses spectacles of power – 5 D to read the writing on the blackboard. His grandmother recently started using spectacles of power 2 D only for reading the newspapers. On the basis of this information, answer the questions given below:
i) What type of defect does Rohan have? ii) Name the type of lens in the spectacles used by Rohan
iii) What type of defect does Rohan’s grandmother have?
iv) Name the type of lens in the spectacles used by her?
v) Find the focal length of the lens in the spectacles used by Rohan?
Answers
Answer:
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Hint: Check the sign of the power. If it’s positive then we have a convex lens. If it's negative then we have a concave lens. Then depending on the lens used look for the defect.
Complete step by step solution
(A) The power of the lens is given as -0.5D Since, the power is negative, so the type of lens used is Concave. Concave lens is a Diverging lens. So diverging lenses have negative focal lengths and so do they have negative power.
(B) The defect of vision Aniket is suffering from is Myopic or Nearsightedness. Myopic type is the one in which close objects appear clearly. But for ones that don't. Glasses for correction of Myopia are Concave lenses (curved inwards), which moves the focus of the light to help you see clearly.
(C) Power of lens = 1F" role="presentation" style="box-sizing: border-box; font-family: "Open Sans", sans-serif; -webkit-tap-highlight-color: rgba(255, 255, 255, 0); display: inline; font-style: normal; font-weight: normal; line-height: 49px; font-size: 16px; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative;">Power of lens = 1FPower of lens = 1F
−0.5D=1F" role="presentation" style="box-sizing: border-box; font-family: "Open Sans", sans-serif; -webkit-tap-highlight-color: rgba(255, 255, 255, 0); display: inline; font-style: normal; font-weight: normal; line-height: 49px; font-size: 16px; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative;">−0.5D=1F−0.5D=1F
F=−1F" role="presentation" style="box-sizing: border-box; font-family: "Open Sans", sans-serif; -webkit-tap-highlight-color: rgba(255, 255, 255, 0); display: inline; font-style: normal; font-weight: normal; line-height: 49px; font-size: 16px; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative;">F=−1FF=−1F =−2metre" role="presentation" style="box-sizing: border-box; font-family: "Open Sans", sans-serif; -webkit-tap-highlight-color: rgba(255, 255, 255, 0); display: inline; font-style: normal; font-weight: normal; line-height: 49px; font-size: 16px; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative;">=−2metre=−2metre
So the focal length of the lenses used comes out to be -2m which is negative because the lens used is concave.
Explanation:
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