2018 A/L Physics Past Paper · Question 05 (English Medium)

Optical instruments — normal adjustment

Which of the following statements made regarding optical instruments under normal adjustment is not true?

(1) In a simple microscope, the image of the object is virtual.
(2) When reading small letters using a simple microscope, a short-sighted person has an advantage over a long-sighted person.
(3) In a compound microscope, the eyepiece acts as a simple microscope.
(4) In a compound microscope, the final image is inverted.
(5) In an astronomical telescope, the object distance and the image distance are both considered to be very large.

Answer and explanation

Answer: (2) When reading small letters using a simple microscope, a short-sighted person has an advantage over a long-sighted person.

What the question asks

We want: which statement about optical instruments in normal adjustment is not true?

The key idea is the near point $D$ : the closest point a person's eye can see clearly.

Relevant theory

The point nearest to the eye in this range is called the "near point" of the eye and the distance to it from the eye is the "least distance of distinct vision".

The object to be viewed is placed in front of the lens within its principal focus to form a virtual, magnified, upright image.

This image I₁ will then act as the object to the eye-piece and would cause the formation of the final image which is further magnified, inverted and virtual.

When the final image is at infinity, the telescope is said to be in "normal adjustment".

Reading the options

In normal adjustment the image of a simple microscope is at the near point $D$ .

Then the magnification is

$m = 1 + \frac{D}{f}$

A short-sighted person has a small $D$ . So the magnification is small.

A long-sighted person has a large $D$ . So the magnification is large.

So the short-sighted person has no advantage. Statement (2) is not true.

Why the other options fail

This option says: in a simple microscope, the image is virtual.

The object is inside the focus. The image is virtual, upright and magnified. So this is true.

We want the statement that is not true. So it is wrong.

This option says: in a compound microscope, the eyepiece acts as a simple microscope.

The eyepiece looks at the first image $I_1$ , just like a simple microscope looks at an object. So this is true.

This option says: in a compound microscope, the final image is inverted.

The objective gives a real, upside-down image. The eyepiece keeps it upside down. So this is true.

This option says: in an astronomical telescope, both distances are very large.

The object is very far away. In normal adjustment the image is at infinity. So this is true.

Conclusion

$m = 1 + \frac{D}{f}$

A short-sighted person has a small $D$ , so a smaller magnification.

The answer is option (2): this statement is not true.

A person who sees near objects well gains more from a microscope

The magnification depends on $D$ . The person with the larger $D$ (long sight) gets the larger magnification.

தமிழ் · සිංහල