171. Oil spreads on water surface because-

1. Oil is denser than water
2. Oil is less dense than water
3. Surface tension of oil is more than water
4. Surface tension of oil is less than water

Option “4” is correct.
Oil spreads on water surface because the surface tension of oil is less than water.
172. A sudden fall in barometer reading indicates that the weather will be:

1. Stormy weather
2. Rainy weather
3. Cool dry weather
4. Hot and humid weather

Option “1” is correct.
A sudden fall in barometer reading indicates that the weather will be stormy.
173. The sun is visible to us a few minutes before the act sunrise because of:

1. Scattering of light
2. Diffraction of light
3. Total internal reflection of light
4. Refraction of light

Option “4” is correct.
Due to the refraction of light, the sun is visible to us a few minutes before sunrise and a few minutes after sunset.
174. The mirror used in motor vehicles to see the rear view is-

1. Biconcave
2. Convex
3. Plane
4. Concave

Option “2” is correct.
The convex mirror produces an upright image of any object and offers a wide viewing area as compared to the plane mirror. The usage of this type of mirror can be perceived on the car rear-view mirror.
175. The change of focal length of an eye lense is caused by action of the-

1. Pupil
2. Retina
3. Ciliary muscles
4. Iris

Option “3” is correct.
The eye lens is composed of a fibrous, jelly-like material. Its curvature can be modified to some extent by the ciliary muscles. The change in the curvature of the eye lens can thus change its focal length. When the muscles are relaxed, the lens becomes thin. Thus its focal length increases. This enables us to see distant objects clearly. When you are looking at objects closer to the eye, the ciliary muscles contract. This increases the curvature of the eye lens. The eye lens then becomes thicker. Consequently, the focal length of the eye lens decreases. This enables us to see nearby objects clearly.
176. The force which opposes the relative motion between different layers of liquid or gases is called-

1. Critical Velocity
2. Streamline Flow
3. Terminal Velocity
4. Viscous Force

Option “4” is correct.
The viscous force is the force between a body and a fluid (liquid or gas) moving past it, in a direction so as to oppose the flow of the fluid past the object.
177. Fathometer is used to measure:

1. Earthquake
2. Rain
3. Depth of sea
4. Sound intensity

Option “3” is correct.
Fathometer is an instrument used to determine the depth of water or a submerged object by means of ultrasound waves.
178. Which one of the following is the effect of the flight of supersonic jet?

1. Air pollution
2. Eye disease
3. Depletion in ozone layer
4. None of these

Option “3” is correct.
A jet engine is a machine for turning fuel into thrust. The thrust is produced by action and reaction also known as Newton’s third law of motion. Those jet planes which are able to fly faster than the speed of sound are called supersonic jet planes. Jet engines are responsible for depletion of ozone. Flying at stratosphere height, they emits nitrogen oxide which has the potential to destroy significant quantities of ozone in the stratosphere.
179. The incident ray, the _________ at the point of incidence and the reflected ray all lie in the same plane.

1. Surface
2. Tangent
3. Normal
4. Angle of reflection

Option “3” is correct.
The law of reflection states that the incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane.
180. Earth’s escape velocity is-

1. 15.0km/sec.
2. 21.1km/sec.
3. 7.0km/sec
4. 11.2km/sec

Option “4” is correct.
The escape velocity of earth is 11.2 km/sec.
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