Reflection — Set 4
Physics · परावर्तन · Questions 31–40 of 70
Diffuse reflection is most likely from a surface that is?
Correct Answer: A. Rough and irregular
A rough surface has many tiny facets with different normals. Each small part reflects light according to the laws of reflection, but in different directions. This scattering makes the reflection diffuse.
If speed of sound is 330 m/s, the minimum distance of a wall from a person for a distinct echo (minimum delay 0.1 s) is about?
Correct Answer: B. 16.5 m
For an echo, the sound must travel to the wall and back in at least 0.1 s. So d = v × t / 2 = 330 × 0.1 / 2 = 16.5 m. If the wall is closer, the reflected sound merges with the original sound.
The normal to a plane mirror at a point is a line that is?
Correct Answer: B. Perpendicular to the mirror surface
The normal is defined as a perpendicular line to the surface at the point of incidence. Reflection angles are measured from this normal. This definition applies to any reflecting surface, not just mirrors.
A ray of light incident along the normal to a plane mirror is reflected?
Correct Answer: A. Back along the same path
If the incident ray is along the normal, its angle of incidence is 0°. The angle of reflection equals the angle of incidence, so it is also 0°. Therefore the ray retraces its path after reflection.
Which statement about diffuse reflection is correct?
Correct Answer: B. It follows the laws at each point but scatters due to varying normals
Even on a rough surface, each tiny region reflects light with equal incidence and reflection angles. But the surface normals vary from point to point. So the reflected rays spread out and no clear image is formed.
SONAR works mainly on the principle of?
Correct Answer: B. Reflection of ultrasonic waves and echo timing
SONAR sends ultrasonic waves and receives their reflections from objects. The time delay between emission and echo gives the distance. It is widely used for underwater detection because ultrasound travels well in water.
An object is 15 cm in front of a concave mirror of focal length 10 cm. The magnification of the image is?
Correct Answer: A. -2
Use the mirror formula to find image distance: 1/f = 1/v + 1/u. With f = -10 cm and u = -15 cm, we get v = -30 cm and then m = -v/u = -(-30)/(-15) = -2. A negative magnification indicates the image is inverted and twice the size.
Compared to a soft curtain, a hard wall generally reflects sound with?
Correct Answer: D. Greater intensity
Hard and smooth surfaces reflect a larger fraction of sound energy. Soft and porous materials absorb more sound and reduce reflection. This is why curtains are used to control echoes in rooms.
A plane mirror is moved 2 cm toward a fixed object. The image shifts by?
Correct Answer: C. 4 cm toward the object
In a plane mirror, the image is always the same distance behind the mirror as the object is in front. When the mirror moves 2 cm toward the object, the image position changes twice as much. So the image shifts 4 cm toward the object along the normal direction.
The angle between incident and reflected rays is 120°. The angle of incidence is?
Correct Answer: D. 60°
The angle between incident and reflected rays equals 2 times the angle of incidence. So 2i = 120° gives i = 60°. This result assumes both angles are measured from the normal.