1. Why is rainbow seen after rain?
- After a rainfall, the clouds break and lie hanging in the sky and the sun is also visible. The Sun’s rays, falling on the water drops, are dispersed, totally reflected at the back of the drops and then again refracted into the eye of the observer with his back towards the sun. These dispersed rays which have suffered deviation give rise to a concentrated beam of light indicating all the colours of light.
2. Ice wrapped in a blanket or packed with saw dust does not melt away quickly. Why?
- Ice wrapped in blanket does not melt, because blanket being bad conductor, cut off heat rays. Saw dust is also a bad conductor of heat. It protects the ice from the external heat and prevents it from melting away.
3. Petrol fire cannot be put with water. Why?
- Petrol has less density than water. So when water is poured the petrol floats on it and keeps on burning. The temperature of the burning petrol is so high than the water and it would be evaporated before it can extinguish the fire.
4. Fuse wires are always provided in electrical installations.
- Fuse wire is a high resistance wire and when it is put in the series of electrical installations under strong currents, the wire melts away avoiding damage to main installations.
5. Copper wire cannot be used as heating element in electric heaters.
- Copper melts at 1083°C and also reacts with air to form a black powder.
6. Glass when heated cracks while metal does not.
- Glass is a bad conductor of heat. On heating, only the upper layers of the glass expand while lower ones remain unaffected, hence it cracks due to unequal expansion. Metal is a good conductor of heat and so all its bulk gets uniformly heated up. Thus the expansion also has uniform surface strain, hence no cracking.
7. Why a cyclist has to apply a greater force at the start than when the cycle is in motion?
- A cyclist has to apply a greater force at the start in order to produce momentum, but once a suitable momentum has been gained only then a smaller force is required to balance the frictional forces.
8. Why sea-water is saline?
- Because the rivers falling into the sea, bring deposits of salt along with them during their journey from mountains and plains and this process has been going on for ages
9. How do you convert Centigrade reading to Fahrenheit? With the help of the following formula:-
10. The sky appears blue, Give reasons, why?
- The blue colour of the sky is due to the scattering of light by dust particles or air molecules. This scattering is inversely proportional to the fourth power of wavelength. Consequently the shorter wavelengths are profusely scattered and when we look at a portion of the sky away from sun, we receive this scattered and re-scattered light which is rich in blue (short wavelength).
- The bulb has a partial vaccum. When it is broken the air rushes in to fill the vaccum and thus a noise is produced.
12. How is it that food gets cooked quicker in a pressure cooker than in an ordinary vessel?
- In a pressure cooker, on account of the increase of pressure, the boiling point is raised, so the food gets cooked quicker than in an ordinary vessel. A pressure cooker is very useful in high mountains where, on account of low pressure and hence low boiling point, the vegetables cannot be cooked properly.
13. A burning candle gets extinguished when covered with a tumbler. Why?
- The tumbler cuts off the oxygen supply which is necessary for its burning.
- River water during its course in the mountainous terrain dissolve the calcareous matter or lime compounds and thus becomes hard. While during evaporation only the molecules of water go up and minerals are left behind; hence the rain water is soft.
15. Why is water from hand pump warm in winter and cold in summer?
- In winter, the temperature outside is low and the unexposed water underground is at a higher temperature. In summer, the outside temperture is higher than the temperature inside. Hence water is warm in winter and cool in summer.
16. An iron nail floats on mercury but sinks in water.Give reasons, why?
- An iron nail floats on mercury, because the weight of mercury displaced by it and hence the upward thrust is more than its weight, the density of mercury and iron being respectively 13.6 gm per c.c.