Tag: physics

Quizzes (598) Questions (13441)

Questions Related to physics

Multiple choice luminous intensity measurements physics

A battery operated torch is adjusted to give a parallel beam of light. It produces illuminance of 60 lux on a wall 2m away. The illuminance produced 3 m away is 

  1. $60$ lux
  2. $\displaystyle\ \frac{80}{3}$ lux
  3. $40$ lux

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

Since $\dfrac {dF}{dA}$ is a constant, where $F$ is the illuminance and $A$ is the area of cross section. Hence the illuminance produced 3 m away does not change and it is 60 lux.

Multiple choice luminous intensity measurements physics

Two coherent source must have the same :

  1. amplitude

  2. phase difference

  3. frequency

  4. both B and C
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Two coherent source have the same :

1) Frequency
2) Phase difference.

The correct option is D.

Multiple choice luminous intensity measurements physics

An electric lamp and a candle produce equal illuminance on a screen when placed $80 cm$ and $20 cm$ from the screen respectively. The lamp is now covered with a thin paper which transmit $49\%$ of the luminous flux. By what distance the lamp should be moved to balance the intensities at the screen again?

  1. $24 cm$
  2. $12 cm$
  3. $18 cm$
  4. $456 cm$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Case(i)
$\displaystyle\ \frac{I _{1}}{I _{2}}$ = $\displaystyle\ \left (\frac{80}{20} \right)^{2}$

Case(II)
$\displaystyle\ \frac{.49I _{1}}{I _{2}}$ =$\displaystyle\ \left(\frac{d}{20} \right)^{2}$

$d$ = $20\sqrt{.49(16)}$ = $20(2.8)$
 = $56$cm
Lamp be moved by $80$ - $56$ = $24$ cm

Multiple choice luminous intensity measurements physics

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

  1. increases continuously

  2. decreases continuously

  3. first increases then decreases

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

The luminosity increases with increase in wavelenth, reaches a peak at around 550 nm then decreases.


Answer. C) first increases and then decreases. 

Multiple choice luminous intensity measurements physics

The parameter that determines the brightness of a light source sensed by an eye is 

  1. energy of light entering the eye per second

  2. wave length of the light

  3. total radiant flux entering the eye

  4. total luminous flux entering the eye
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The parameter that determines the brightness of a light source sensed by an eye is total luminous flux entering the eye.
$B = F$ where $B$ is the brightness and $F$ is the total flux entering the eye.

Multiple choice luminous intensity measurements physics

Light from a point source falls on a screen. If the separation between the source and the screen is increased by 1% the illuminance will decrease 

  1. $0.5$ %
  2. $1$ %
  3. $2$ %
  4. $4$ %
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

$E$  $\propto\displaystyle\ \frac{1}{r^{2}}$

$\displaystyle \frac{\triangle E}{E}$ $=\displaystyle\ \frac{2\triangle r}{r}$ 

$\Rightarrow$ $2$ ($1$%) $= 2$%

Multiple choice luminous intensity measurements physics

$1$ % of light of a source with luminous intensity $50 $candela is incident on a circular surface of radius $10 cm$. The average illuminance of the surface is 

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

Illuminance = $\dfrac{Luminous \ intensity  \times 4\pi } {area}$

                    =$ \dfrac {50 \times 4 \pi}{100\times \pi\times0.1\times0.1} = 200 lux$

Answer B) $200 \ lux$

Multiple choice luminous intensity measurements physics

The illumination produced by A is balanced by B on the screen when B is 60 cm apart from the screen. A smoked glass plate is placed in front of A and to balance the illumination B is to move 15cm further away. Find the transmission coefficient of the smoked glass

  1. $0.36$
  2. $0.64$
  3. $0.49$
  4. $0.51$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$\alpha$ = $\displaystyle\ \frac{I _{2}}{I _{2}}$ = $\displaystyle\ \left(\frac{60}{75} \right)^{2}$ = $\displaystyle\ \left( \frac{4}{5} \right)^{2}$ = $\displaystyle\ \frac{16}{25}$ = $0.64$

Multiple choice luminous intensity measurements physics

Two light sources of $8 Cd$ and $12 Cd$ are placed on the same side of the photometer screen at a distance of $40 cm$ from it. Where should a $80 Cd$ source be placed to balance the illuminance?

  1. $40 cm$
  2. $60 cm$
  3. $20 cm$
  4. $80 cm$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Intensity of illumination = $\dfrac {L} {d^{2}}$


Therefore
$I _1+I _2=I$

$\dfrac {8} {0.4^{2}} + \dfrac{12} {0.4^{2}} = \dfrac{80}{d^{2}}$

=>$\dfrac {20} {0.16} = \dfrac{80}{d^{2}}$

=> d = 0.8 m =80 cm

Answer. D) 80 cm

Multiple choice luminous intensity measurements physics

The luminous intensity of a light source is $300 Cd$. The illuminance of a surface lying at a distance of $10$ $m$ from it will be if light falls normally on it 

  1. $30$ lux
  2. $3$ lux
  3. $0.3$ lux
  4. $0.03$ lux
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Illuminance ($E$)  is  measured  in  lux.  The  lux  is  an  SI  unit  used  when  characterizing illumination conditions of a surface:
$E = \dfrac {I}{R^2}    lux$
where $I$ is the luminous intensity and $R$ is the distance.
Given $I = 300 cd$ and $R = 10 m$
$\implies E = \dfrac {300}{10^2} = 3   lux$