Q1. What are the optical densities of air and diamond?
The optical density of a medium is measured in terms of its refractive index:
Refractive index of air = 1.0003
Refractive index of diamond = 2.42
Q2. Define one diopter of power of lens.
The power of lens is one diopter if focal length of the lens is one meter (1m) or 100 cm.
Q3. A student has difficulty in reading the blackboard while seating in the last row. What could be the defect the child is suffering from? How can it be corrected?
The student is suffering from myopia as he could not see the writing on the blackboard clearly from large distance. This defect can be corrected using concave lens of suitable power.
Q4. Why sky appears as blue during daytime if a person is on the earth?
When visible white light from the sun passes through the atmosphere, in the spectrum of colors blue color having smaller wavelength is scatters the most. Hence, the sky appears blue.
Q5. What are the advantages of connecting electrical devices in parallel with the battery instead of connecting them in series?
When the electrical devices are connected in parallel, the overall resistance of the circuit decreases. Thus, more current flows in different circuits depending upon the resistance of electrical device in the circuit. Hence, optimum amount of current flows through each device. Moreover, the potential difference across the ends of each device remains the same (equal to battery e.m.f.).
On the other hand, when all the electrical devices are connected in series, overall resistance of the circuit increases and so a small constant current flows throughout the circuit. So, if we connect all the electrical devices in series, they may not operate properly as each device needs different current to operate properly.
Q6. What is meant by power of accommodation of the eye?
The ability of the eye lens to adjust its focal length so that it can see objects lying at all distances between near point and far point, is called power of accommodation of the eye.
Q7. Why is the tungsten used almost exclusively for filament of electric lamps?
The tungsten is used almost exclusively for filament of an incandescent lamp because of the following reasons:
- Tungsten has a fairly good resistivity (5.2 x 10-8 $\omega$ m). So, electric energy easily changes into heat and light energy.
- Secondly, the melting point of tungsten is very high (3642 K). So, on getting heated to high temperature, it produces incandescent light.
- Thirdly, tungsten can be drawn into very thin wires, which in turn offer high resistance.
Q8. What will happen when a fuse of capacity 6A is used to feed power to an oven and a microwave system?
A fuse of higher rating should be used to feed power to an oven and a microwave system. Lower rating fuse of capacity 6A will be unable to prevent flow of large current through the electric circuit. This may cause short circuiting and then may cause fire. To avoid this, it will be better if we use a fuse of capacity 15A to feed power to an oven and a microwave system.
Q9. What precaution should be taken to avoid the overloading of domestic electric circuits?
To avoid the overloading of domestic electric circuits, electric fuses of correct ratings should be used in different sets of appliances. Fuse of higher rating should be used in the circuits containing electric iron, electric geyser, immersion heater, etc., whereas a fuse of lower rating is sufficient in circuits containing low power rating appliances such as fan, electric bulb, florescent tubes, etc. These days Miniature Circuit Breakers (MCB) are used to avoid the overloading of electric circuits.
Q10. Explain, why fossil fuels are classified as non-renewable sources of energy?
Formation of fossil fuels like coal, petroleum and natural gas have occurred over millions of years due to certain very slow changes under special circumstances. Since these conditions are not prevailing now, the fossil fuels are being depleted rapidly. These sources of energy are called non-renewable sources.
Q11. "Wind energy or water energy is an indirect form of solar energy". Comment.
Unequal heating of different landmass and water bodies by the solar energy causes movement of air. This is the cause of wind, so wind energy or water energy is an indirect form of solar energy.
Q12. What are the advantages and disadvantages of using a solar cooker? Are there places where solar cookers would have limited utility?
- The food cooked by using solar cooker saves fuel.
- Solar cookers do not produce smoke.
- They do not cause pollution in the environment.
- While cooking the food in the solar cookers, its nutrients do not get destroyed as the food is cooked at relatively lower temperature.
- More than one food can be cooked in solar cooker at the same time.
- No fuel or electricity is required to use solar cookers.
- These cannot be used to cook the food during night time.
- Food cannot be cooked, using a solar cooker when the sky is covered with clouds.
- The direction of plane mirror, i.e., reflector has to be changed with the change in the direction of sun rays.
- Box type solar cookers cannot be used to bake chapatis or frying of food.
- Initial cost of solar cookers is very high.
- Solar cookers would have limited utility at those places where sun shines very less.
Q13. What is Tyndall effect?
The phenomenon of scattering of light by colloidal particles is called Tyndall effect. The effect can be seen when sunlight passes through a canopy of dense forests.