Questions 41-43 concern the reflection of sound, uses of radioactivity, and convex lenses. Provide the answers to these questions.

Figure 7 shows sound waves being produced and then reflecting off a large reflecting wall. The students are likely measuring the time taken for the sound to travel to the wall and back, which is used to determine the speed of sound.

When sound waves bounce off a surface, the phenomenon is called reflection.

The correct option is C.
C) Reflection

• A. Radioactivity is used in determining the thickness of films (e.g., using beta radiation to monitor paper thickness). This is a valid use.
• B. Refrigerators operate based on thermodynamic principles (refrigerant cycles, compression, expansion) and do not use radioactivity in their primary action.
• C. Radioactive tracers are widely used in medicine (e.g., PET scans) and industry to track substances. This is a valid use.
• D. Gamma radiation is used for sterilizing medical equipment and food products. This is a valid use.

The action of a refrigerator does not involve radioactivity.

The correct option is B.
B) It is used in the action of a refrigerator

Figure 2 shows a convex lens forming an image of a candle. The object (candle) is at 30 cm, and the image is formed at 45 cm. The image is real (formed on a screen), inverted, and larger than the object (magnified).

• A. A projector uses a convex lens to produce a real, inverted, and magnified image of a slide or film onto a screen.
• B. A magnifying glass produces a virtual, upright, and magnified image.
• C. A simple camera produces a real, inverted, and diminished image.
• D. A photocopier typically produces an image of the same size or slightly reduced/enlarged.

The correct option is A.
A) Projector

The image formed is larger than the object (magnified). For a convex lens, a real, inverted, and magnified image is formed when the object is placed between the focal point ($F$) and twice the focal length ($2F$). In this case, the image is formed beyond $2F$ on the other side of the lens.

Since the image is magnified, the object must be between $F$ and $2F$. Consequently, the image is formed beyond $2F$.

The correct option is A.
A) Beyond 2F

Object distance, $u = 30$ cm.
Image distance, $v = 45$ cm.
Since the image is real, $v$ is positive.

The lens formula is:

Substitute the values:

Find a common denominator, which is 90:

The focal length is 18 cm.
A. 30 cm
B. Greater than 30 cm
C. 60 cm
D. Less than 30 cm

18 cm is less than 30 cm.

The correct option is D.
D) Less than 30 cm

Object distance, $u = 30$ cm.
Image distance, $v = 45$ cm.

Magnification is given by:

Substitute the values:

The correct option is C.
C) 1.5

Figure 8 shows a spring with a mass attached, causing it to extend. The original length and the extended length are shown. This setup demonstrates how a spring stretches under an applied force.

Hooke's law states that the force required to extend or compress a spring is directly proportional to the distance of extension or compression, within its elastic limit ($F = ke$).

The correct option is D.
D) Hooke's law

For the experiment to work correctly, the spring must stretch when a force is applied and return to its original length when the force is removed.