Questions OBJECTIVE - II and Answer (Photometry) HC Verma Part 1

 Q#1

The brightness producing capacity of a source
(a) does not depend on its power
(b) does not depend on the wavelength emitted
(c) depends on its power
(d) depends on the wavelength emitted.

Answer: (c), (d).
The brightness produced by radiation depends on the wavelength of the radiation besides depending on the total radiant flux. The total radiant flux is the power of the source. More the power of the source more will be the brightness. Also more the contribution of the wavelengths around 555 nm will make the source more bright. Hence the brightness of the source depends on the power of the source and the wavelengths emitted.    

Q#2
A room is illuminated by an extended source. The illuminance at a particular portion of a wall can be increased by
(a) moving the source
(b) rotating the source
(c) bringing some mirrors in proper positions
(d) changing the color of the source.

Answer: (a), (b), (c), (d).
The illuminance at a point is inversely proportional to the square of the distance from the source. Hence moving the source nearer to the portion of the wall will increase the illuminance. Hence the option (a).

Since the source is an extended source, it will subtend different angles at the wall portion when rotated. The maximum angle subtended at the wall portion will be when the line joining the source and the wall portion is perpendicular to both. Hence the illuminance can be increased by rotating the source in this position. Hence the option (b).

If some mirrors are used to gather the lights going in other directions to the wall portion, the intensity of the light at that point will be increased. This will make the illuminance to increase. Hence the option (c).

Since the wavelengths of the light around 555 nm produce maximum brightness, and the wavelengths are related to the color of the light, so by changing the color of the source, the illuminance can be increased. Hence the option (d).      

Q#3
Mark the correct options.
(a) The luminous efficiency of a monochromatic source is always greater than that of a white light source of the same power.
(b) The luminous efficiency of a monochromatic source of wavelength 555 nm is always greater than that of a white light source of the same power.
(c) The illuminating power of a monochromatic source of wavelength 555 nm is always greater than that of a white light source of the same power.
(d) The illuminating power of a monochromatic source is always greater than that of a white light source of the same power.

Answer: (b), (c).
The white light is a mixture of different colors of light having different wavelengths. But of all the wavelengths the light having the wavelength around 555 nm have a maximum luminous flux. The luminous efficiency is the total luminous flux per unit radiant flux. Hence only in the case of the monochromatic light having a wavelength around 555 nm, the luminous efficiency will be more than the white light, not always. So the option (b).

The illuminating power or the luminous intensity of a source is the total luminous flux per unit solid angle. As stated above the total luminous flux of the monochromatic light having a wavelength of 555 nm will be more than the white light of the same power. So it is a special case, not a general case. Hence the option (c).

Q#4
Mark the correct options.
(a) Luminous flux and radiant flux have the same dimensions.
(b) Luminous flux and luminous intensity have the same dimensions.  
(c) Radiant flux and power have the same dimensions.
(d) Relative luminosity is a dimensionless quantity.

Answer: (b), (c), (d).
The unit of radiant flux is watt but the unit of luminous flux is lumen which is cd*sr (Candela*Steradian). Hence both do not have the same dimensions. (a) is not true.
The luminous intensity is the total luminous flux per unit solid angle. Since the solid angle is dimensionless, Hence the luminous flux and the luminous intensity have the same dimensions. The option (b) is true.

The radiant flux is the energy of radiation emitted per unit time which is same as the power of the radiant source. Hence the radiant flux and the power have the same dimensions. The option (c) is true.

The relative luminosity of a wavelength is a fraction comparing luminous flux of this source to the luminous flux of a 555 nm source of the same power. Since the units of the numerator and the denominator are same in this fraction hence the relative luminosity is dimensionless quantity. The option (d) is true.