Photometry Physics - Photometry

A room is illuminated by an extended source. The illuminance at a particular portion of a wall can be increased by:

1. Moving the source.
2. Rotating the source.
3. Bringing some mirrors in proper positions.
4. Changing the colour of the source.

A light source emits monochromatic light of wavelength 555nm. The source consumes 100W of electric power and emits 35W of radiant flux. Calculate the overall luminous efficiency.

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 2m away. The illuminance produced when it falls on a wall 4m away is close to:

1. 40 lux
2. 20 lux
3. 10 lux
4. 5 lux

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?

The luminous flux of a monochromatic source of 1W is 450 lumen/ watt. Find the relative luminosity at the wavelength emitted.

A source emits 31.4W of radiant flux distributed uniformly in all directions. The luminous efficiency is 60 lumen/ watt. What is the luminous intensity of the source?

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 luminous flux of a 1W sodium vapour lamp is more than that of a 10kW source of ultraviolet radiation. Comment.

The relative luminosity of wavelength 600nm is 0.6. Find the radiant flux of 600nm needed to produce the same brightness sensation as produced by 120W of radiant flux at 555nm.

A point source emitting light uniformly in all directions is placed 60cm 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 80cm away from the first point.