Tag: luminous intensity

Questions Related to luminous intensity

In the interference pattern, energy is

luminous intensity measurements physics

  1. created at the position of maxima

  2. destroyed at the positions of minima

  3. conserved but is redistributed

  4. not conserved

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Correct Option: C
Explanation:

conserved but is redistributed

A lamp placed 60 cm from a screen produces the same illumination as a standard 100 W lamp placed 90 cm away on the other side of the screen. The luminous intensity of the first lamp is 

luminous intensity measurements physics

  1. $49.44W$

  2. $44.44W$

  3. $54.44W$

  4. $34.44W$

Show answer
Correct Option: B
Explanation:

For same illumination $\dfrac{Power} { r^{2}}$ will be same


=> $\dfrac {100} { 0.9^{2}} = \dfrac {P} {0.6^{2}}$
=> P = $44.44 W$

Light has a wave nature, because-

luminous intensity measurements physics

  1. the light travel in a straight line

  2. Light exhibits phenomenon of reflection and refraction

  3. Light exhibits phenomenon interference

  4. Light exhibits phenomenon of photo electric effect.

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Correct Option: C

Light with energy flux 36$\mathrm { w } / \mathrm { cm } ^ { 2 }$ is incident on a well polished metal slate of side 2$\mathrm { cm }$ . Theforce experienced by it is

luminous intensity measurements physics

  1. 0.96$\mu N$

  2. 0.24$\mu N$

  3. 0.12$\mu N$

  4. 0.36$\mu N$

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Correct Option: A

If relative luminosity is $0.65$ then the luminous flux of a $10 W$ source is

luminous intensity measurements physics

  1. $12130.6$

  2. $4125.5$

  3. $4352.5$

  4. $4452.5$

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Correct Option: D
Explanation:

Luminous flux of a 10W source=Relative luminosity$\times$ luminous flux of 555nm source of 10W

=$0.65\times (685\times 10)=4452.5$

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 

luminous intensity measurements physics

  1. $60$ lux

  2. $\displaystyle\ \frac{80}{3}$ lux

  3. $40$ lux

  4. none of these

Show answer
Correct Option: A
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.

Two coherent source must have the same :

luminous intensity measurements physics

  1. amplitude

  2. phase difference

  3. frequency

  4. both B and C

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Correct Option: D
Explanation:

Two coherent source have the same :

1) Frequency
2) Phase difference.

The correct option is D.

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?

luminous intensity measurements physics

  1. $24 cm$

  2. $12 cm$

  3. $18 cm$

  4. $456 cm$

Show answer
Correct Option: A
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

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

luminous intensity measurements physics

  1. increases continuously

  2. decreases continuously

  3. first increases then decreases

  4. first decreases then increases

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Correct Option: C
Explanation:

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


Answer. C) first increases and then decreases. 

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

luminous intensity measurements physics

  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

Show answer
Correct Option: D
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.