Flash of lightning is seen before the sound of thunder. Why

   Both sound and light energy are released at the same time. In air the speed of light is 300000000 m/s and speed of sound is 340 m/s. Hence the light reaches the observer instantly and sound after some time.

    For example, if an electric discharge  between the clouds takes place at a distance of 1km from the observer then lightning flash is seen instantly but sound of thunder will be heard after about 3 seconds.

Freezer compartment of a refrigerator is placed at its top portion. Why?

Cold air from the freezing compartment settles down owing to its higher density and keeps the temperature of the contents below it low.

Infrared rays are used for thermal photography?

Infrared rays are invisible rays whose wavelength is more than that of red colour.They produce heating effect. As they have long wavelength, they are least scattered by the fog, mist and smog and dust particles. As they effect a specially treated photographic plate due to the heating effect, they are used in thermal photography.

THE DIFFERENCE BETWEEN AC AND DC.

                        ALTERNATING CURRENT
1. Its value changes continuously.
2. Its polarity changes periodically.
3. It can be transformed by a transformer.

                         DIRECT CURRENT

1. Its value doesn't change, it has  a constant magnitude.
2. Its polarity doesn't  change.
3. It can not be transformed by transformer.

The temperature is lower in shade than in direct sunlight, why ?

In the direct sunlight there are Infrared Rays. Infrared rays have strong heating effect. But in shade there is no direct sunlight with these heating Rays. Only reflected or diffused light is there. So, temperature is lower in  shade than in direct  sunlight

Efficiency of a machine is always less than 100%. Why?

Efficiency is the ratio of work output and work input. As work output is always less than work output for the following reasons, the efficiency is less than 1 or 100%.

1. The movable parts of a machines are not weightless.
2.  The movable parts are not frictionless.
3.  The parts of the machines are not rigid.
4. The strings or belts are not perfectly elastic in nature.

WHY IS EARTHING OF AN APPLIANCE DONE?

Earthing is required in a circuit when
1. the appliance draws high current. High power devices draw large current.(Current =power/voltage).
2. the appliance has outer metallic case.