Important Physics GK Questions and answers for preparing competitive examinations.

1. When we step out of sunlight into a room, we cannot see clearly for some time. Why?
Ans. Iris controls the amount of light entering our eyes. When we are in sunlight the iris contracts the size of the pupil so that a minimum of light should enter our eyes as per the requirement. When we come to a room, the demand for light increases but the pupil takes some time to expand. Therefore, during this interval we cannot see clearly.

2. Ozone is found in the upper layer of atmosphere. Why?
Ans. The ultraviolet radiations coming from the sun, convert the oxygen present in the upper layer of the atmosphere into ozone. Since ozone has the property of screening ultraviolet rays, they are not able to come down further and form ozone in the lower layers of atmosphere.

3. Why is red colour chosen for the danger signals?
Ans. Red colour can penetrate through haze, mist, and frost. It is also more easily visible from a great distance. Hence the danger signals are always red as used on road crossing and on railways.

4. Why are ice-boxes made double-walled?
Ans. The space between the two walls of the box has vacuum so that, convection and conduction do not take place and therefore, heat cannot enter from outside. Hence, iceboxes are double walled.

5. Why do railway lines appear to converge at a distance, although their actual width is the same everywhere?
Ans. The apparent size of an object depends upon the size of the image formed on the retina of our eye. This image further depends upon the visual angle which is the angle subtended by the object at the eye. This angle goes on decreasing as the object is taken farther from the eye and vice versa. With the decrease of visual angle, the size of the image also appears to be diminished. For this reason, the railway lines appear to converge at a distance.

6. Why does a tennis ball bounce higher on hills than on plains?
Ans. When the ball bounces up, the force of gravity tries to pull it down. Since the value of ‘g’ is comparatively less at the hills than in plains, the tennis ball can bounce higher on a hill.

7. How does a submarine float and sink as desired?
Ans. The submarine has a chamber in which water can be filled in or pumped out as required. To make it sink into the water, the chamber is filled with water so that its weight exceeds the upthrust produced on the submarine by the displaced water. To bring the submarine up the water in the chamber is pumped out. Now the up thrust produced by the displaced water is greater than or equal to the weight of submarine and hence it can float.

8. When a mango tree is shaken, why do mangoes fall down?
Ans. At first, the tree and these mangoes both are in the state of rest. When the tree is shaken, according to Newton’s first law of motion, the mangoes tend to continue in the state of rest although the branches go in motion. The result is that the tension in the supporting twig increases due to which is breaks and hence the mangoes fall down.

9. Why does a straight stick look bent when partly immersed in water?
Ans. When a stick is partly immersed in water, the rays of light starting from the lower end of the stick after travelling through the water strike the surface of separation (water and air) and bend away from the normal because air is an optically rarer medium. Thus, the rays appear to diverge from a point higher than the actual position. Hence, when the stick is viewed from above the surface of the vessel or from the side through the water in the glass vessel, it appears to be broken at the surface of separation of water and air.

10. Explain why a mercury fluorescent lamp of same power gives more light than a filament lamp?
Ans. The inner part of mercury fluorescent lamp is coated with magnesium tungstate and zinc beryllium silicate. The mercury light produced by electric discharge contains a number of ultraviolet rays which are invisible. When these rays fall on the inner walls of the lamp magnesium tungstate produces blue fluorescence, while zinc beryllium silicate produces yellow orange fluorescence. The mixture of these two fluor spar produces white light. Such a process does not take place in filament lamps. Hence a mercury fluorescent lamp having same power produces nore light than a filament lamp.

11. Why does thunder produce vibrations?
Ans. Sound of thunder is reverberated by two clouds and the two sounds get mixed up and produce vibrations.

12. When an electric heater connected in series is switched on, the light gets dimmer, when heater is switched off the light gets brighter again. Why?
Ans. Due to very high resistance, an electric heater consumes a larger quantity of electricity. If a heater is in a series with other appliances, it will use most of the power supplied and as such the light will get dimmer. However, if the heater is placed in parallel with other appliances, no such effect will be observed.

13. What is Holography?
Ans. Holography is a technique of recording and reproducing three-dimensional images. This phenomenon is based on interference. A laser beam partly reflected from an object and partly from a minor produces interference fringes on a photographic plate which then becomes a hologram. When laser light is transmitted through the hologram, one can see a three dimensional virtual image of the object.

14. Why does sound travel faster in moist air than in dry air?
Ans. The density of moist air is less than that of dry air because the pressure of water vapours in air decreases its density. According to Laplance formula the velocity of sound in a gas is inversely proportional to the square root of density. Therefore its value decreases in moist air. Hence sound travels faster in moist air than in dry air.

15. Why are tides produced in the sea?
Ans. The moon and the sun attract the water of sea on the surface of earth and this causes tides. When attraction by the sun and the moon is in the same direction, a high tide is produced and when their attraction acts in opposite direction, low tide is produced.

16. A rope-dancer holds either an umbrella or a bamboo stick, while dancing on the rope. Why?
Ans. He holds them in order to change the position of the centre of gravity easily. By moving it to and fro, he makes the vertical line from his centre of gravity fall on the rope.

17. Why does land warm up and cool down faster than water?
Ans. This happens because land has much lower specific heat capacities than water. So water warms up and cools down slowly.

18. What do you mean by plasma state?
Ans. The fourth state of matter is called plasma state. The molecules in this state exist in the form of positive and negative ions. The number of positive ions is approximately equal to the number of negative ions so that the matter appears neutral, but it is full of charge and is highly conducting.

19. How is it possible for a deaf person to hear the sound of a watch held by his teeth?
Ans. Sound can travel faster in solids than in gases. Therefore in this case the sound is conveyed to the auditory nerves through the bony and other portions of the head which are solid.

20. An egg sinks in pure water but floats in salty water. Why?
Ans. An egg sinks in pure water but floats in salty water because salty water has more density than pure water, so salty water exerts more upward thrust.

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