Tag: luminous intensity

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Questions Related to luminous intensity

Multiple choice luminous intensity measurements physics

 The luminous flux emitted by the sun will be

  1. $4.4\times10^{25}$ lm
  2. $4.43\times10^{26}$ lm
  3. $4.43\times10^{27}$ lm
  4. $4.43\times10^{28}$ lm
Reveal answer Fill a bubble to check yourself
D Correct answer
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$

Multiple choice luminous intensity measurements physics

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 ___

  1. $4\times10^{3}$ $lm$
  2. $3\times10^{3}$ $lm$
  3. $2\times10^{3}$ $lm$
  4. $1.37\times10^{3}$ $lm$
Reveal answer Fill a bubble to check yourself
D Correct answer
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 = $1.37 \times 10^3lm$

Multiple choice luminous intensity measurements physics

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

  1. $100\pi$
  2. $200\pi$
  3. $300\pi$
  4. $400\pi$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

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

Multiple choice luminous intensity measurements physics

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 

  1. $4$ phot
  2. $4$ lux
  3. $3$ phot
  4. $3$ lux
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
Illuminance = $\dfrac{L}{d^{2}}$
$\dfrac{100}{5^{2}}$ = 4 lux

Answer. B) 4 lux
Multiple choice luminous intensity measurements physics

Inverse square law for illuminance is valid for 

  1. isotropic point source

  2. cylindrical source

  3. search light

  4. all types of sources
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

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

Multiple choice luminous intensity measurements physics

In the above problem, the luminance of blotting paper is 

  1. $3$ phot
  2. $3$ lux
  3. $4$ phot
  4. $4$ lux
Reveal answer Fill a bubble to check yourself
B Correct answer
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

Multiple choice luminous intensity measurements physics

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 

  1. $10^{1}$ $W$
  2. $10^{2}$ $W$
  3. $10^{3}$ $W$
  4. $10^{4}$ $W$
Reveal answer Fill a bubble to check yourself
C Correct answer
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
Multiple choice luminous intensity measurements physics

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 

  1. $6\pi$ $W$
  2. $12\pi$ $W$
  3. $24\pi$ $W$
  4. $48\pi$ $W$
Reveal answer Fill a bubble to check yourself
C Correct answer
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) $24 \pi  W$

Multiple choice luminous intensity measurements physics

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

  1. half

  2. one fourth

  3. double

  4. four times
Reveal answer Fill a bubble to check yourself
A Correct answer
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

Multiple choice luminous intensity measurements physics

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 

  1. $100$ $Cd$
  2. $25$ $Cd$
  3. $225$ $Cd$

  4. none of these
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

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