Advertisements
Advertisements
प्रश्न
A point source emitting 628 lumen of luminous flux uniformly in all directions is placed at the origin. Calculate the illuminance on a small area placed at (1.0 m, 0, 0) in such a way that the normal to the area makes an angle of 37° with the X-axis.
Advertisements
उत्तर
Given,
Luminous flux = 628 lumen
Angle made by the normal with the x axis (θ) = 37°
Distance of point, r = 1 m
Since the radiant flux is distributed uniformly in all directions, the solid angle will be 4π.
∴ Luminous intensity, l = `"Luminous flux"/"Solid angle"`
`=628/(4pi)=50 "candela"`
I lluminance (E) is given by,
`E=l cos (theta/r^2)`
On substituting the respective values we get,
`E=50xxcos37^o/1^2`
`50xx"(4/5)"/1=40 "lux"`
So, the illuminance on the area is 40 lux.
APPEARS IN
संबंधित प्रश्न
The luminous flux of a 1 W sodium vapour lamp is more than that of a 10 kW source of ultraviolet radiation. Comment.
The sun is less bright at morning and evening as compared to at noon although its distance from the observer is almost the same Why?
Why is the luminous efficiency small for a filament bulb as compared to a mercury vapour lamp?
Three light sources A, B and C emit equal amount of radiant energy per unit time. The wavelengths emitted by the three source are 450 nm, 555 nm and 700 nm respectively. The brightness sensed by an eye for the sources are XA, XB and XC respectively. Then, ________ .
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 ____________ .
A battery-operated torch is adjusted to send an almost parallel beam of light. It produces an illuminancle 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 _________ .
The intensity produced by a long cylindrical light source at a small distance r from the source is proportional to _________ .
A photographic plate is placed directly in front of a small diffused source in the shape of a circular disc. It takes 12s to get a good exposure. If the source is rotated by 60° about one of its diameter, the time needed to get the same exposure will be ___________ .
Figure shows a glowing mercury tube. The intensities at point A, B and C are related as __________ .
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 a white 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.
Mark out the correct options.
(a) Luminous flux and radiant flux have same dimensions.
(b) Luminous flux and luminous intensity have same dimensions.
(c) Radiant flux and power have same dimensions.
(d) Relative luminosity is a dimensionless quantity.
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 31.4 W of radiant flux distributed uniformly in all directions. The luminous efficiency is 60 lumen watt−1. What is the luminous intensity of the source?
The illuminance of a small 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.
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?
Photo diodes are used to detect ______.
Light travels through a glass plate of thickness t and having a refractive index μ. If c is the velocity of light in vacuum, the time taken by the light to travel this thickness of glass is ______.
