Tag: luminous intensity

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Questions Related to luminous intensity

Multiple choice luminous intensity measurements physics

The luminous efficiency of the bulb in lumen/watt, if luminous intensity of a $100$ watt unidirectional bulb is $100$ candela, is

  1. $12$
  2. $12.56$
  3. $13$
  4. $15$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$F=4\pi l$
$F=(4\times 3.14)\times 100=1256$ lumen
$\therefore$ Luminous efficiency $=\dfrac{luminous\ flux}{electric\ power}=\dfrac{1256}{100}=12.56$

Multiple choice luminous intensity measurements physics

An electric bulb is suspended at a vertical height $2\ m$ from the centre of a square table of side $2\ m$. If the luminous intensity of bulb is $60\ cd$ (candela), then the illumination at one corner of the table is:

  1. $8.16\ cd/m^{2}$
  2. $6.24\ cd/m^{2}$
  3. $9.25\ cd/m^{2}$
  4. $8.72\ cd/m^{2}$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Distance from bulb to corner r = sqrt(2^2 + 1^2 + 1^2) = sqrt(6). Cos(theta) = height / r = 2 / sqrt(6). Illuminance E = (I / r^2) * cos(theta) = (60 / 6) * (2 / sqrt(6)) = 10 * 2 / 2.449 = 20 / 2.449 = 8.16 cd/m^2.

Multiple choice luminous intensity measurements physics

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

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

Luminous flux = luminous intensity * solid angle = $ 10* 4\pi = 400\pi$ lumen


Answer. D) $400\pi$ lumen

Multiple choice luminous intensity measurements physics

The intensity produced by a long cylindrical light source at a small distance $r$ from the source is proportional to

  1. $\displaystyle \dfrac{1}{r^2}$
  2. $\displaystyle \dfrac{1}{r^3}$
  3. $\displaystyle \dfrac{1}{r}$

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

At a distance $r$ from a line source of power$P$ and length $l$, the intensity will be


$ I=\dfrac { P }{ S } =\dfrac { P }{ 2\pi rl } \Rightarrow I\propto \dfrac { 1 }{ r } $

Multiple choice luminous intensity measurements physics

Two light sources with equal luminous intensity are lying at a distance of 1.2 m from each other. Where should a screen be placed between them such that illuminance on one of its faces is four times that on another face?

  1. 0.2 m

  2. 0.4 m

  3. 0.8 m

  4. 1.6 m
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

$E _2 = 4 E _1$. If x is distance from 1st source,
then, $\displaystyle \frac{I}{(1.2 - x)^2} = \frac{4 I}{x^2} $ or $\displaystyle \frac{1}{1.2 - x} = \frac{2}{x}$
$3x = 2.4, x = 0.8 m$

Multiple choice luminous intensity measurements physics

Light from a source is analysed by an analyser. When the analyser is rotated, the intensity of the emergent light.

  1. Does not vary

  2. Remains uniformly dark

  3. Varies between maximum and zero

  4. Varies between maximum and minimum
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Let the intensity of the unpolarised light be $I _o$.
An analyser is used to polarized the light in one direction.
Intensity of light after passing through the analyser  $I' = \dfrac{I _o}{2}$
Intensity of light after passing through analyser remains constant $I _o/2$ even if the analyser is rotated by some angle.

Multiple choice luminous intensity measurements physics

 If light falls on the surface at an angle of $60^{0}$, then illuminance will be____

  1. $12$ lux
  2. $6$ lux
  3. $3$ lux
  4. $1.5$ lux
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Illuminance ($E$)  is  measured  in  lux.  The  lux  is  an  SI  unit 

used  when  characterizing illumination conditions of a surface:
$E = \frac {I}{R^2}  cos \alpha     lux$
where $I$ is the luminous intensity, $R$ is the distance and $\alpha$ is the surface angle.
Given $I = 300 cd$, $R = 10 m$, $\alpha = 60^o$
$\implies E = \frac {300}{10^2}    cos  60^o= 1.5   lux$

Multiple choice luminous intensity measurements physics

A lamp is hanging along the axis of a circular table of radius r. At what height should the lamp be placed above the table, so that the illuminance at the edge of the table is $\displaystyle \frac{1}{8}$ of that at its centre?

  1. r/2
  2. r/$\sqrt{2}$

  3. r/3
  4. r/$\sqrt{3}$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

$E _2 = \displaystyle \frac{1}{8} E _1$ or $\displaystyle \frac{1}{(r^2 + h^2)} \times \frac{h}{\sqrt{r^2 + h^2}} = \frac{1}{8} \frac{1}{h^2}$
(by lambert's cosine law)
or, $(r^2 + h^2)^{3/2} = (2h)^3 $ or $(r^2 + h^2)^{1/2} = 2h$
or $r^2 + h^2 = 4h^2$
$h = r / \sqrt{3}$