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Multiple choice luminous intensity measurements physics

As the wavelength is increased from violet to red, the luminosity

  1. continuously increases

  2. continuously decreases

  3. increases, then decreases

  4. decreases ,then increases
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

With increase in wavelength of visible light, the sensitivity to eye increases, reaches a peak at 555 nm and then decreases. So luminosity increases then decreases.


Answer. C) Increases, then decreases

Multiple choice luminous intensity measurements physics

The brightness of a source based upon sensation of eye is determine by:

  1. radiant flux entering the eye

  2. luminous flux entering the eye

  3. wavelength of light

  4. none of the above
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Only light of visible range excites the retina of eyes. The range Light, which can human eyes sense is called Luminus of light.
Hence, (b) is correct.

Multiple choice luminous intensity measurements physics

The lumen efficiency, if an electric bulb emit $68.5\dfrac{lumen}{watt}$ is:

  1. $2.5\%$
  2. $5\%$
  3. $10\%$
  4. $20\%$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Luminous efficiency is the ratio of luminous flux to power. A standard 100% efficient source (monochromatic green light at 555 nm) produces 685 lumens/watt. Efficiency = (68.5 / 685) * 100% = 10%.

Multiple choice luminous intensity measurements physics

A surface is receiving light normally from a source, which is at a distance of $8\ m$ from it. If the source is moved closer towards the surface, so that the distance between them becomes $4\ m$, then the angle through which the surface may be turned so that illuminance remain as it was: 

  1. $\theta=\cos^{-1}(1/3)$
  2. $\theta=\cos^{-1}(1/4)$
  3. $\theta=\cos^{-1}(1/8)$
  4. $\theta=\cos^{-1}(1/6)$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Illuminance E = (I / d^2) * cos(theta). Initially, E = I / 8^2 = I / 64. Finally, E = (I / 4^2) * cos(theta) = (I / 16) * cos(theta). Equating them: I / 64 = I / 16 * cos(theta) => cos(theta) = 16 / 64 = 1/4. Thus, theta = cos^-1(1/4).

Multiple choice luminous intensity measurements physics

A point source generates $10\ J$ of light energy in $2\ s$. The luminous flux of source is:

  1. $5$ lumen
  2. $10$ lumen
  3. $50$ lumen

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

Luminous flux $=\dfrac{light\ energy}{time}=\dfrac{10}{2}=5$ lumen

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 $\displaystyle \frac{1}{1.2 - x} = \frac{2}{x}$
$3x = 2.4, x = 0.8 m$