Grade 11 Physics Note

Photometry

Photometry

The branch of optics which deals about the quantitative study of light energy is called photometry.

Importances of photometry are;

1. It helps us to design the lighting schemes of houses of houses, factories etc.

2. The sensitivity of human eye varies with wave length. The sensitivity is maximum for wave.

 

Luminous intensity;

Luminous intensity is defined as the amount of luminous flux emitted by a source per unit solid angle Ω in the direction.  Its unit is lumen per steradian or simply candela.

 

Luminance:

Luminous objected objects are visible because they emit light. The luminance of a surface is a measure of brightness of the surface. The luminance of a surface is measured by the luminous flux reflected by unit area of the surface. Smooth and white surfaces reflect more light and hence appear brighter. The amount of light reflected by the surface depends upon the nature of the surface. 

Luminous and illuminated objected:

Luminous objected objects are visible because they emit light whereas illuminated objected objects are visible because they reflect light.

 

The factors on which the illumination of a surface depends areas;

1. Distance of the surface from source.

2. The direction along which light is incident on the surface.

3. The illumination power of light source.

 

Difference between luminous flux and luminous intensity;

luminous flux

luminous intensity

1. The amount of light energy emitted by a source per second is called luminous flux. A source of light emits radiation of wide wavelength range.

 

2. It is denoted by Q.

 

3. Its unit is lumen.

 

4. One lumen is the luminous flux emitted per unit solid angle by a uniform point source of one candle power.

 

1. Luminous intensity is defined as the amount of luminous flux emitted by a source per unit solid angle Ω in the direction. 

 

2. It is denoted by L.

 

3. Its unit is lumen per steradian or simply candela.

 

4. A source is said to have one candela of luminous intensity in a given direction if it emits monochromatic radiations of frequency 5.40 *1014HZ and has a radiant intensity of $\frac{1}{{683}}{\rm{watt}}\backslash {\rm{sr}}$ in that direction.

 

 

Inverse square law as applied in photometry:

Consider a point source of light S which is emitting light in all direction uniformly. Let the light energy emitted per sec by the source is Q.

Draw two spheres of radius r1 and r2 as shown in the figure. Then intensity of illumination for area $4{\rm{\pi r}}_1^2$$4{\rm{\pi r}}_2^2$ is;

Source of light, S at the centre

 

                                   ${{\rm{I}}_1} = \frac{{\rm{Q}}}{{4{\rm{\pi r}}_1^2}} \ldots  \ldots ..1$

                                   ${{\rm{I}}_2} = \frac{{\rm{Q}}}{{4{\rm{\pi r}}_2^2}} \ldots  \ldots ..2$

Dividing 1 and 2

$\frac{{{{\rm{I}}_1}}}{{{{\rm{I}}_2}}} = \frac{{\rm{Q}}}{{4{\rm{\pi r}}_1^2}}{\rm{*}}\frac{{4{\rm{\pi r}}_2^2}}{{\rm{Q}}}$

Or,${\rm{\: \: }}\frac{{{{\rm{I}}_1}}}{{{{\rm{I}}_2}}} = \frac{{{\rm{r}}_2^2}}{{{\rm{r}}_1^2}}$

In general, ${\rm{I}} \propto \frac{1}{{{{\rm{r}}^2}}}$

Thus the intensity of illumination of a point is inversely proportional to the square of the distance from the source.

 

Bunsen grease spot photometer to compare the luminous intensities of two light sources:

http://members.ozemail.com.au/~macinnis/scifun/bunsenp.gif

The Bunsen grease-spot photometer 


This photometer was once used to compare the intensities of two light sources. It is just a piece of brown paper with a greasy spot on it. When the light on the far side is brighter than the near-side light, the grease spot looks lighter than the surrounding paper. When the light on the far side is less intense, the spot appears dark, but if the two light sources are balanced, the spot disappears altogether. The photometer is dead easy to make: use ordinary brown paper and butter or margarine, or even the paper which comes around your fish and chips.


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