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Ray Optics and Optical Instruments — Physics STD 12 Science — Question

Gujarat BoardEnglish MediumSTD 12 SciencePhysicsRay Optics and Optical Instruments1 Mark
Question
Two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below.
  1. Both A and R are true and R is the correct explanation of A.
  2. Both A and R are true but R is not the correct explanation of A.
  3. A is true but R is false.
  4. A is false and R is also false.
Assertion: The focal length of an equiconvex lens placed in air is equal to radius of curvature of either face. Lens is made up of material of refractive index of 1.5.
Reason: For an equiconvex lens, radius of curvature of both the faces is same.
✓

Answer

  1. Both A and R are true but R is not the correct explanation of A.

Explanation:

For an equiconvex lens, R1 = R2 = R

From $\frac{1}{\text{f}}=(\mu-1)\Big(\frac{1}{\text{R}_1}-\frac{1}{\text{R}_2}\Big)$

Using lens maker's formula for glass, $\mu = 1.5$ placed in air $\frac{1}{\text{f}}=(1.5-1)\frac{2}{\text{R}}\Rightarrow\text{f}=\text{R}.$

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