Tag: physics

Questions Related to physics

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

luminous intensity measurements physics

  1. continuously increases

  2. continuously decreases

  3. increases, then decreases

  4. decreases ,then increases

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Correct Option: C
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

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

luminous intensity measurements physics

  1. radiant flux entering the eye

  2. luminous flux entering the eye

  3. wavelength of light

  4. none of the above

Show answer
Correct Option: B
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.

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

luminous intensity measurements physics

  1. $2.5\%$

  2. $5\%$

  3. $10\%$

  4. $20\%$

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Correct Option: C

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: 

luminous intensity measurements physics

  1. $\theta=\cos^{-1}(1/3)$

  2. $\theta=\cos^{-1}(1/4)$

  3. $\theta=\cos^{-1}(1/8)$

  4. $\theta=\cos^{-1}(1/6)$

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Correct Option: B

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

luminous intensity measurements physics

  1. $5$ lumen

  2. $10$ lumen

  3. $50$ lumen

  4. none of these

Show answer
Correct Option: A
Explanation:

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

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

luminous intensity measurements physics

  1. $12$

  2. $12.56$

  3. $13$

  4. $15$

Show answer
Correct Option: B
Explanation:

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

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:

luminous intensity measurements physics

  1. $8.16\ cd/m^{2}$

  2. $6.24\ cd/m^{2}$

  3. $9.25\ cd/m^{2}$

  4. $8.72\ cd/m^{2}$

Show answer
Correct Option: A

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

luminous intensity measurements physics

  1. $100 \pi$ lumen

  2. $200 \pi$ lumen

  3. $300 \pi$ lumen

  4. $400 \pi$ lumen

Show answer
Correct Option: D
Explanation:

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


Answer. D) $400\pi$ lumen

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

luminous intensity measurements physics

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

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

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

  4. None of these

Show answer
Correct Option: C
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 } $

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?

luminous intensity measurements physics

  1. 0.2 m

  2. 0.4 m

  3. 0.8 m

  4. 1.6 m

Show answer
Correct Option: C
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 $\displaystyle \frac{1}{1.2 - x} = \frac{2}{x}$
$3x = 2.4, x = 0.8 m$