Questions about Physics

1. Is it necessary that optically denser medium must possess greater mass density? Explain.

Answer:  No. For example,kerosene oil and turpentine oil are lighter than water (as they float on the surface of water) and have less mass density than water. But refractive indices of kerosene oil (n_k = 1.44) and turpentine oil (n_t = 1.47) are greater than that of water (n_w = 1.33)

Q2. Why does sky appear to be blue during daytime to a person on the earth?

Answer: When visible light from the sun passes through the atmosphere, blue colour having smaller wavelength is scattered most. Hence, the sky appears blue.

Q3. Why is a normal eye not able to see clearly the objects placed closer than 25 cm?

Answer: Our eye lens can adjust its focal length due to the action of capillary muscles. This is called power of accommodation of the eye. However, focal length of the eye lens cannot be decreased beyond a certain limit. Therefore,  a person cannot see clearly objects placed closer than 25 cm.

Q4. A person, entering into a dark room or cinema hall, cannot see clearly for a few seconds. Why?

Answer: The iris regulates the amount of light entering into the eye by adjusting size of the pupil of the eye. It takes a few seconds to expand the iris so that proper amount of light can enter into the eye.

Q5. Why are coils of electric toasters and electric irons made of an alloy rather than a pure metal?

Answer: It is because

(i) resistivity of alloys are much higher than those of pure metals which form the alloys

(ii) resistivity of alloys changes less rapidly with change in temperature, and

(iii) they do not oxidize readily at high temperatures.

Q6. Why are copper and aluminium wires usually used for transmission of electricity?

Answer: Copper and aluminium metals have quite low resistivity. It means that resistance of a given copper or aluminium wire of given length and cross-sectional area will be much less and so, there will be very small power losses (I²Rt) during transmission. That is why wires of these metals are usually used for transmission of electricity.

Q7. Why does a compass needle get deflected when brought near a bar magnet?
Answer: A compass needle gets deflected when brought near a bar magnet because a compass needle is a small bar magnet. When two bar magnets are brought close to each other, like poles repel and unlike poles attract each other.
Q8. Why is biogas considered superior to animal dung fuel?
Answer: Biogas is considered superior to animal dung because it has high calorific value and leaves no residue after burning whereas the animal dung has low calorific value and leaves ash after burning.

Q9. Explain why is it difficult to use hydrogen a source of energy.

Answer: This is because hydrogen is highly combustible and it burns with an explosion. Also, hydrogen is difficult to store and transport.
Q10. Tap water conducts electricity whereas distilled water does not. Why ?
Answer: It is because tap water contains some ions of the dissolved salts whereas distilled water does not have any ion.