2021 A/L Physics Past Paper · Question 37 (English Medium)

Answer: (1) Only (A)

What the question asks

Find which of the statements (A), (B) and (C) in the table are correct for both instruments.

Each statement gives a value for the telescope and a value for the microscope. A statement is correct only if both values are right.

The plan: check the telescope value and the microscope value of each statement.

Relevant theory

The astronomical telescope consist of an objective lens, which is a convex lens of long focal length, while the eye-piece too is a convex lens but of short focal length.

The lens closer to the object and known as the "objective" is a converging lens of very short focal length.

When the eye-piece is adjusted to form the final image at infinity, the telescope is in normal adjustment.

When the eye-piece is adjusted to have the final image at the near point, the microscope is in normal adjustment.

Reading the options

The options are combinations of (A), (B) and (C).

For the telescope, the angular magnification in normal adjustment is:

$$m = \dfrac{f_A}{f_B}$$

Here $f_A$ is the focal length of the objective and $f_B$ that of the eye-piece.

We test each statement on these facts.

Reasoning and calculation

The focal length of the objective lens is: large for the telescope, small for the microscope.

The telescope objective has a long focal length. The microscope objective has a very short one. Both entries in the table are right.

A

The final image at normal adjustment is: at infinity for both.

The telescope final image is at infinity. But the microscope final image is at the near point, not at infinity. The microscope entry is wrong.

B

The total angular magnification is: less than 1 for the telescope, greater than 1 for the microscope.

The telescope has a long $f_A$ and a short $f_B$. So $m = \dfrac{f_A}{f_B}$ is greater than $1$. The entry "less than 1" is wrong.

C

(A) is true. (B) is false. (C) is false.

So only (A) is correct for both instruments.

Why the other options fail

(B) is false, because the microscope image is at the near point.

(B) and (C) are both false.

(C) is false, because the telescope magnification $f_A/f_B$ is greater than $1$.

Conclusion

(A) is true, (B) is false and (C) is false.

Only (A) is correct for both instruments. The answer is (1).

Check the telescope and the microscope separately for each statement. Both must be right.

Remember: telescope, final image at infinity and $m = f_A/f_B$. Microscope, final image at the near point.

Exam tip