Advertisements
Advertisements
Question
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 Adirectly 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.
Solution
Let x be the distance moved.
Let the illuminance of the candle and the lamp at a distance 1 m be IC and ILrespectively. According to question,
`E=I_L/(0.8)^2=I_c/(0.2)^2`
`rArr I_L/I_c=16`
Again after the wrapping the paper around
`49/100I_L/(0.8+x)^2=I_c/(0.2)^2`
Either,
x = -0.24
Or,
x = -1.36 this value is not acceptable because it is more than 0.8
so, x = 0.24m (-ve sign denotes distance is to be decreased)
x = 24cm
APPEARS IN
RELATED QUESTIONS
What is the luminous flux of a source emitting radio waves?
The luminous flux of a 1 W sodium vapour lamp is more than that of a 10 kW source of ultraviolet radiation. Comment.
A bulb is hanging over a table. At which portion of the table is the illuminance maximum? If a plane mirror is placed above the bulb facing the table, will the illuminance on the table increase?
Why is the luminous efficiency small for a filament bulb as compared to a mercury vapour lamp?
The yellow colour has a greater luminous efficiency as compared to the other colours. Can we increase the illuminating power of a white light source by putting a yellow plastic paper around this source?
The one parameter that determines the brightness of a light source sensed by an eye is ____________ .
An electric bulb is hanging over a table at a height of 1 m above it. The illuminance on the table directly below the bulb is 40 lux. The illuminance at a point on the table 1 m away from the first point will be about ___________ .
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 ____________ .
The intensity produced by a long cylindrical light source at a small distance r from the source is proportional to _________ .
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 ___________ .
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 source emits 45 joules of energy in 15 s. What is the radiant flux of the source?
The relative luminosity of wavelength 600 nm is 0.6. Find the radiant flux of 600 nm needed to produce the same brightness sensation as produced by 120 W of radiant flux at 555 nm.
The luminous flux of a monochromatic source of 1 W is 450 lumen watt−1. Find the relative luminosity at the wavelength emitted.
A source emits light of wavelengths 555 nm and 600 nm. 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.
A light source emits monochromatic light of 555 nwavelengthm. The source consumes 100 W of electric power and emits 35 W of radiant flux. Calculate the overall luminous efficiency.
A point source emitting light uniformly in all directions is placed 60 cm above a table-top. The illuminance at a point on the table-top, directly below the source, is 15 lux. Find the illuminance at a point on the table-top 80 cm away from the first point.
