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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: By increasing the diameter of the objective of telescope, we can increase its range.
Reason: The range of a telescope tells us how far away a star of some standard brightness can be spotted by telescope.
✓

Answer

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

The light gathering power (or brightness) of a telescope is directly proportional to the area of the objective lens.

i.e., light gathering power $\propto\pi\text{r}^2=\frac{\pi\text{D}^2}{4},$ where D is the diameter of the objective. Thus telescope will have large light gathering power if the diameter of the objective lens is large. So by increasing the objective diameter even far off stars may produce images of optimum brightness.

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