23 + Interview Questions in PHOTOMETRY IN GENERAL AWARENESS Page 1 InterviewSolution

1.	The illuminance of a small -2 area changes from 900 lumen m^-2 to 400 lumen m^-2 when it is shifted along its normal by 10 cm. Assuming that it is illuminated by a point source placed on the normal, find the distance between the source and the area in the original position.
Answer» Correct Answer - B::C Let, l=Luminous intensity of source E_A=900 lumne/m^2` `E_B=400 lumen/m^2` Now,` E_a=(costheta)/x^2` and `E_B=(lcostheta)/((x+10)^2)` so, `l=(E_Ax^2)/(costheta)=(E_A(x+10)^2)/(costheta)` `rarr 900x^2=400(x+10)^2` `rarr x/(x+10)=2/3` `rarr x=2x+20` `or x=20 cm` So the distance between the source and the origiN/Al position is 20 cm.

Discussion

2.	A point source of light moves in a straight line parallel to a plane table. Consider a small portion of the table directly below the line of movement of the source. The illuminance at this portion varies with its distance r from the source as(a) I∝ 1/r(b) I ∝ 1/r2(c) I ∝ 1/r3(d) I ∝ 1/r4
Answer» The correct answer is (c) I∝1/r³

2.

A point source of light moves in a straight line parallel to a plane table. Consider a small portion of the table directly below the line of movement of the source. The illuminance at this portion varies with its distance r from the source as(a) I∝ 1/r(b) I ∝ 1/r2(c) I ∝ 1/r3(d) I ∝ 1/r4

Answer»

The correct answer is

(c) I∝1/r³

Discussion

3.	Light from a point source falls on a small area placed perpendicular to the incident light. If the area is rotated about the incident light by an angle of 60°, by what fraction will the illuminance change ?
Answer» The illuminance will not change.

Discussion

4.	A battery-operated torch is adjusted to send an almost parallel beam of light. It produces an illuminance of 40 lux when the light falls on a wall 2 m away. The illuminance produced when it falls on a wall 4 m away is close to(a) 40 lux(b) 20 lux(c) 10 lux(d) 5 lux.
Answer» The correct answer is (a) 40 lux

Discussion

5.	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 (nearly) by(a) 0.5%(b) 1%(c) 2%(d) 4%.
Answer» The correct answer is (c) 2%

Discussion

6.	A source emits light of wavelengths 555 nm and 600nm. The radiant flux of the 555 nm part is 40 W and of the 600 nm part is 30 W. The relative luminosity at 600 nm is 0.6. Find (a) the total radiant flux, (b) the total luminous flux, (c) the luminous efficiency.
Answer» The radiant flux of 555nm part is 40W and of the 600nm part is 30W (a) Total radiant flux = 40W + 30W = 70W (b) Luminous flux = (L.Fllux)_555nm + (L.Flux)_600nm = 1 × 40× 685 + 0.6 × 30 × 685 = 39730 lumen (c) Luminous efficiency =Total lumin ous flux / Total radiant flux =39730 / 70 = 567.6 lumen/W

Discussion

7.	A circular area of radius 1.0 cm is placed at a distance of 2.0 m from a poit source.Teh source emits light uniformluy in al directions. The line joining the sorce to the dcentre of the area is normal to the area. It ils found that `2.0xx1^-3` lumen of luminous flux is incident on the area. Calculate the total luminous flux emitted by the soure and teh luminous intensity of the source along the axis of the area.
Answer» Thesoid angle subtended by the area on the point source is `/_omega=(pi(1.0cm)^2)/((2.0m)^2)=pi/4xx10^-4sr`. Thus `2.0xx10^-3` lumen of flux is emitted in `pi/4xx10^-4` sr. The total solid angle at the source is 4pi. As the source radiates uniformly in all directions, the total luminous flux is `F=(4pi)/(pi/4xx10^-4)xx2.0xx10^-3lumen` `=320 lumen. the luminous intensity =/_F//_omega` `=(2.0xx10^-3lumen)/(pi/4xx10^-4sr)=25cd`

Discussion

8.	Figure shows a small diffused plane source S placed over a horizontal table-top at a distance of 2.4 m with its plane parallel to the table-top. The illuminance at the point A directly below the source is 25 lux. Find the illuminance at a point B of the table at a distance of 1.8 m from A.
Answer» Correct Answer - A

Discussion

9.	Using figure (22.1), find the relative luminosity of wavelength (a) 480 nm, (b) 520 nm (c ) 580 nm and (d) 600 nm.
Answer» Correct Answer - A::B::C::D It can be found out from the graph by the student.

Discussion

10.	In the grease spot photometer, a lamp with dirty chimney is found to balance an electric lamp of constant illuminating power at a distance of `0.1` m from the spot. When the chimney is cleaned, the electric lamp has to be shifted by 2 cm to obtain a balance. Calculate the precentage of light absorbed by the dirty chimney
Answer» Correct Answer - 0.36

Discussion

11.	Three light sources A, B and C emit equal amount of radiant energy per unit time. The wavelengths emitted by the three sources are 450 nm, 555 nm and 700 nm respectively. The brightness sensed by an eye for the sources are XA, X5 and XC, respectively. Then,(a) XA>XB, XC> XB(b) XA > XB, XB> XC(C) XB> XA,XB > XC(d) XB > XA, XC, > XB.
Answer» (C) X_B> X_A,X_B> X_C

Discussion

12.	A photographic plate placed at a distance of 5 cm from a weak point source is exposed for 3 s. If the plate is kept at a distance of 10 cm from the source, the time needed for the same exposure is(a) 3 s(b) 12 s(c) 24 s(d) 48 s.
Answer» The correct answer is (b) 12 s

Discussion

13.	A photographic plate records sufficiently intense lines when it is exposed for 12s to a source of 10W. How long should it be exposed to a 12W source radiating the light of same colour to get equally intense lines ?
Answer» To get equally intense lines on the photographic plate, the radiant flux (energy) should be same. So, 10W × 12sec = 12W × t => t =10Wx12 sec/12W= 10 sec.

Discussion

14.	A photographic print is found satisfactory when the exposure was for 16 s at a distance of 20 m from a 25-watts lamp. At what distance must the same paper be held from a 50-watts lamp in order that an exposure of 25 a may give the same result?
Answer» Correct Answer - `35.4 cm

Discussion

15.	Mark the correct options.(a) The luminous efficiency of a monochromatic source is always greater than that of a white light source of same power.(b) The luminous efficiency of a monochromatic source of wavelength 555 nm is always greater than that of a white light source of same power.(c) The illuminating power of a monochromatic source of wavelength 555 nm is always greater than that of awhite light source of same power.(d) The illuminating power of a monochromatic source is always greater than that of a white light source of same power.
Answer» (b) The luminous efficiency of a monochromatic source of wavelength 555 nm is always greater than that of a white light source of same power. (c) The illuminating power of a monochromatic source of wavelength 555 nm is always greater than that of a white light source of same power.

Discussion

16.	An electric lamp of 100 cd consumes `0.5` watt of electrical energy per candela. Calculate the efficiency of this lamp if the mechaniacal equivalent of ligt, is `0.00161` watt per lumen.
Answer» Correct Answer - 0.04

Discussion

17.	The brightness producing capacity of a source(a) does not depend on its power(b) does not depend on the wavelength emitted(c) depends on its power(d) depends on the wavelength emitted.
Answer» (c) depends on its power (d) depends on the wavelength emitted.

Discussion

18.	A point light source illuminates a screen the, maximum illuminance being `I_(0)` How will the illuminance of this point change if a large plane mirror or 100% reflectance is placed behind the source at half the distance between the source and the screen?
Answer» Correct Answer - `5I_(0)//4`

Discussion

19.	A secreen is illuminated by a lamp which is one metre away. A sheet of glass is placed between the screeen and the lamp which has to be moved 10 cm nearer to maintain the same intensity of illumination as before. What fraction of light is absorbed by the sheet of glass?
Answer» Correct Answer - 0.19

Discussion

20.	A room is illuminated by an extended source. The illuminance at a particular portion of a wall can be increased by(a) moving the source(b) rotating the source(c) bringing some mirrors in proper positions(d) changing the colour of the source.
Answer» (a) moving the source (b) rotating the source (c) bringing some mirrors in proper positions (d) changing the colour of the source.

Discussion

21.	A small lamp having the from of a unifromly sphere of radius R=6 cm is suspended at a height h=3m above the floor. The luminance of the lampis equal to `L=2xx10^(4) cd//m^(2)`. Find the illuminance of the floor below the lamp. (Hint:`P=sumLDeltaScostheta=LxxpiR^(2))`
Answer» Correct Answer - 25 lux

Discussion

22.	A cylindrical hall of diameter D and height h is illuminated by a lamp fixed at the centre of the celing. Compare the minimum illumination of the froor.
Answer» Correct Answer - D/2h

Discussion

23.	At what height should a lamp be hung above the centre of a round table of radius R to otain the maximum illumination at its edges?
Answer» Correct Answer - `h=r//sqrt(2)`

Discussion

Explore topic-wise InterviewSolutions in .

The illuminance of a small -2 area changes from 900 lumen m^-2 to 400 lumen m^-2 when it is shifted along its normal by 10 cm. Assuming that it is illuminated by a point source placed on the normal, find the distance between the source and the area in the original position.

A point source of light moves in a straight line parallel to a plane table. Consider a small portion of the table directly below the line of movement of the source. The illuminance at this portion varies with its distance r from the source as(a) I∝ 1/r(b) I ∝ 1/r2(c) I ∝ 1/r3(d) I ∝ 1/r4

Light from a point source falls on a small area placed perpendicular to the incident light. If the area is rotated about the incident light by an angle of 60°, by what fraction will the illuminance change ?

A battery-operated torch is adjusted to send an almost parallel beam of light. It produces an illuminance of 40 lux when the light falls on a wall 2 m away. The illuminance produced when it falls on a wall 4 m away is close to(a) 40 lux(b) 20 lux(c) 10 lux(d) 5 lux.

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 (nearly) by(a) 0.5%(b) 1%(c) 2%(d) 4%.

A source emits light of wavelengths 555 nm and 600nm. The radiant flux of the 555 nm part is 40 W and of the 600 nm part is 30 W. The relative luminosity at 600 nm is 0.6. Find (a) the total radiant flux, (b) the total luminous flux, (c) the luminous efficiency.

Figure shows a small diffused plane source S placed over a horizontal table-top at a distance of 2.4 m with its plane parallel to the table-top. The illuminance at the point A directly below the source is 25 lux. Find the illuminance at a point B of the table at a distance of 1.8 m from A.

Using figure (22.1), find the relative luminosity of wavelength (a) 480 nm, (b) 520 nm (c ) 580 nm and (d) 600 nm.

A photographic plate placed at a distance of 5 cm from a weak point source is exposed for 3 s. If the plate is kept at a distance of 10 cm from the source, the time needed for the same exposure is(a) 3 s(b) 12 s(c) 24 s(d) 48 s.

A photographic plate records sufficiently intense lines when it is exposed for 12s to a source of 10W. How long should it be exposed to a 12W source radiating the light of same colour to get equally intense lines ?

A photographic print is found satisfactory when the exposure was for 16 s at a distance of 20 m from a 25-watts lamp. At what distance must the same paper be held from a 50-watts lamp in order that an exposure of 25 a may give the same result?

An electric lamp of 100 cd consumes `0.5` watt of electrical energy per candela. Calculate the efficiency of this lamp if the mechaniacal equivalent of ligt, is `0.00161` watt per lumen.

The brightness producing capacity of a source(a) does not depend on its power(b) does not depend on the wavelength emitted(c) depends on its power(d) depends on the wavelength emitted.

A point light source illuminates a screen the, maximum illuminance being `I_(0)` How will the illuminance of this point change if a large plane mirror or 100% reflectance is placed behind the source at half the distance between the source and the screen?

A secreen is illuminated by a lamp which is one metre away. A sheet of glass is placed between the screeen and the lamp which has to be moved 10 cm nearer to maintain the same intensity of illumination as before. What fraction of light is absorbed by the sheet of glass?

A room is illuminated by an extended source. The illuminance at a particular portion of a wall can be increased by(a) moving the source(b) rotating the source(c) bringing some mirrors in proper positions(d) changing the colour of the source.

A small lamp having the from of a unifromly sphere of radius R=6 cm is suspended at a height h=3m above the floor. The luminance of the lampis equal to `L=2xx10^(4) cd//m^(2)`. Find the illuminance of the floor below the lamp. (Hint:`P=sumLDeltaScostheta=LxxpiR^(2))`

A cylindrical hall of diameter D and height h is illuminated by a lamp fixed at the centre of the celing. Compare the minimum illumination of the froor.

At what height should a lamp be hung above the centre of a round table of radius R to otain the maximum illumination at its edges?