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A point source of 3000 lumen is located at the centre of a cube of side length 2m. The flux through one side is ____.
500 lumen
600 lumen
750 lumen
1500 lumen
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 60o, by what fraction will the illuminance change?
It is doubled
It is halved
It will not change
It become one-fourth
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 ____.
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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 ____.
50W
72W
200W
A lamp is hanging at a height of 4m above a table. The lamp is lowered by 1m. The percentage increase in illuminance is ____.
40 %
64%
78%
92%