Tag: measurements

Questions Related to measurements

 The luminous flux emitted by the sun will be

luminous intensity measurements physics

  1. $4.4\times10^{25}$ lm

  2. $4.43\times10^{26}$ lm

  3. $4.43\times10^{27}$ lm

  4. $4.43\times10^{28}$ lm

Show answer
Correct Option: D
Explanation:

$ luminous \ flux = luminous \ intensity \times solid \ angle$

$ I = 3.53\times 10^{27}$
$ Solid angle  = 4 \pi$

So Luminous flux = $3.53\times10^{27}\times4 \pi$
= $ 4.43\times10^{28} lm$

Answer. D) $ 4.43\times10^{28} lm$

A screen recieves $3$ watt of radiant flux of wavelength $6000$ $\mathring{A}$. One lumen is equivalent to $1.5\times10^{-3}$ watt of monochromatic light of wavelength $5500$ $\mathring{A}$ is $1.00$, then the luminous flux of the source is ___

luminous intensity measurements physics

  1. $4\times10^{3}$ $lm$

  2. $3\times10^{3}$ $lm$

  3. $2\times10^{3}$ $lm$

  4. $1.37\times10^{3}$ $lm$

Show answer
Correct Option: D
Explanation:

radiant flux in lumens = $\dfrac{3}{1.5 \times 10^{-3}} = 2000  lm$

relative luminosity at 6000 angstorm is 0.685   
relative luminosity at 5500 angstorm is 1

luminous efficiency  = $\dfrac{0.685}{1}$  

therefore luminous flux = luminous efficiency $\times$ radiant flux = 0.685 $\times$2000 = $1.37 \times 10^3lm$

The luminous intensity of a $100$ W unidirectional bulb is $100$ candela. The total luminous flux emitted from the bulb will be

luminous intensity measurements physics

  1. $100\pi$

  2. $200\pi$

  3. $300\pi$

  4. $400\pi$

Show answer
Correct Option: D
Explanation:

We have total luminous flux,
$L = 4 \pi I = 4 \pi \times 100 = 400 \pi$

A point source of $100$candela is held $5$$m$ above a sheet of blotting paper which reflects $75$ % of light incident upon it. The illuminance of blotting paper is 

luminous intensity measurements physics

  1. $4$ phot

  2. $4$ lux

  3. $3$ phot

  4. $3$ lux

Show answer
Correct Option: B
Explanation:
Illuminance = $\dfrac{L}{d^{2}}$
$\dfrac{100}{5^{2}}$ = 4 lux

Answer. B) 4 lux

Inverse square law for illuminance is valid for 

luminous intensity measurements physics

  1. isotropic point source

  2. cylindrical source

  3. search light

  4. all types of sources

Show answer
Correct Option: D
Explanation:

Inverse square law for illuminance is valid and can be applied for all types of sources.

In the above problem, the luminance of blotting paper is 

luminous intensity measurements physics

  1. $3$ phot

  2. $3$ lux

  3. $4$ phot

  4. $4$ lux

Show answer
Correct Option: B
Explanation:

The source of luminance of the blotting paper is the light reflected by it. Since 75 % of light is reflected,


luminance = $\dfrac{100}{5^{2}}\times \dfrac{75}{100}$ = 3 lux

Answer. B) 3 lux

The luminous efficiency of a lamp is $8.8$ lumen/watt and its luminous intensity is $700\ Cd$. The power of the lamp will be 

luminous intensity measurements physics

  1. $10^{1}$ $W$

  2. $10^{2}$ $W$

  3. $10^{3}$ $W$

  4. $10^{4}$ $W$

Show answer
Correct Option: C
Explanation:
$Luminous \ flux = Luminous \ intensity * solid \ angle $

Total solid angle = 4 $\pi$

So, Luminous flux = 700 x 4 $\pi$

Luminous flux = Luminous efficiency x power
=> 700 x 4 $\pi$ = 8.8 x power
=> Power of lamp = 1000 W

Answer. C) 10$^3$ W

The luminous efficiency of a lamp is $5$ lm $W^{-1}$ and its luminous intensity is $30$ candela. The power of the lamp will be 

luminous intensity measurements physics

  1. $6\pi$ $W$

  2. $12\pi$ $W$

  3. $24\pi$ $W$

  4. $48\pi$ $W$

Show answer
Correct Option: C
Explanation:

$Luminous \ flux = luminous \ intensity * 4 \pi$

$\longrightarrow$ F = $30 * 4 \pi   lm$

Luminous efficiency = $ 5  lm / W$

Power = $ F / Power$
            = $ 30 * 4 \pi / 5 = 24 \pi$

Answer. C) $24 \pi  W$

The light from an electric bulb is normally incident on a small surface. If the surface is tilted by $60^{0}$ from this position, then the illuminace of the surface will become

luminous intensity measurements physics

  1. half

  2. one fourth

  3. double

  4. four times

Show answer
Correct Option: A
Explanation:

Illuminance is proportional to  Cos of angle at which light strikes the surface

If surface is tilted by $ 60^{\circ}$, then illuminace become Cos $60^{\circ}$ of original value
Cos $60^{\circ}$ = 0.5, so Illuminace becomes half
Answer. A) half

The illuminance on screen distance $3$ m from a $100$ $W$ lamp is $25$ lm/$m^{2}$. Presuming normal incidence, the luminous intensity of the bulb will be 

luminous intensity measurements physics

  1. $100$ $Cd$

  2. $25$ $Cd$

  3. $225$ $Cd$

  4. none of these

Show answer
Correct Option: C
Explanation:

We have illuminance
$I = \dfrac {\phi}{R^2}$
$\implies \phi = I  R^2 = 25 \times 3^2 = 225 \ Cd$