Reflection — Set 7
Physics · परावर्तन · Questions 61–70 of 70
The magnification produced by a convex mirror is always?
Correct Answer: D. Positive and less than 1
A convex mirror forms a virtual and erect image, so magnification is positive. The image is always diminished for a convex mirror, so the magnitude is less than 1. This is why it provides a wide field of view.
The unit of reflectance (reflectivity) is?
Correct Answer: C. No unit (dimensionless)
Reflectance is a ratio of reflected power to incident power. Ratios of the same physical quantity have no unit. It is often expressed as a fraction or percentage.
A good reflector of sound is generally a surface that is?
Correct Answer: C. Hard and smooth
Hard and smooth surfaces reflect sound waves effectively. Soft and porous materials absorb sound and reduce reflection. This is why stone or concrete walls can cause strong echoes.
Ultrasound used for echo-based detection has frequency generally?
Correct Answer: C. Above 20 kHz
Ultrasound refers to sound waves with frequencies above the upper limit of human hearing. That limit is about 20 kHz. High-frequency waves can reflect well from small objects, giving better detection detail.
An echo is heard 0.2 s after a shout. If speed of sound is 340 m/s, the distance to the reflecting wall is?
Correct Answer: A. 34 m
Echo time includes the trip to the wall and back, so d = v × t / 2. Here d = 340 × 0.2 / 2 = 34 m. The division by 2 is needed because the sound covers twice the wall distance.
Reverberation time is commonly defined as the time taken for sound intensity in a hall to fall to about?
Correct Answer: D. One millionth of its initial value
Reverberation time is typically defined using a 60 dB decay of sound level. A drop of 60 dB corresponds to intensity becoming one millionth of the initial value. This definition helps compare acoustic quality of rooms.
Which material is most effective for absorbing sound to reduce echoes?
Correct Answer: D. Foam or thick carpeting
Soft and porous materials trap air and convert sound energy into heat. Foam and thick carpets are good examples of sound absorbers. Hard surfaces like glass and steel reflect more sound instead of absorbing it.
The focal length of a plane mirror is considered to be?
Correct Answer: A. Infinite
A plane mirror has zero curvature, so its radius of curvature is infinite. Since f = R/2 for spherical mirrors in magnitude, this implies f is also infinite for a plane mirror. This matches the fact that plane mirrors do not converge or diverge parallel rays.
A concave mirror forms a diminished real image when the object is placed?
Correct Answer: C. Beyond the center of curvature
When the object is beyond the center of curvature, the image forms between the focus and center. This image is real, inverted, and smaller than the object. Diminished real images are typical for objects placed far from a concave mirror.
What type of image is formed by a plane mirror?
Correct Answer: B. Virtual and erect
The correct answer is Virtual and erect. A plane mirror always forms a virtual, erect, and laterally inverted image of the same size as the object. The image appears to be behind the mirror at the same distance as the object is in front. This topic is frequently tested in competitive examinations such as RRB NTPC, SSC, and UPSC.