Department of Pre-University Education, KarnatakaPUC Karnataka Science Class 12

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. - Physics

Advertisement Remove all ads
Advertisement Remove all ads
Advertisement Remove all ads
Fill in the Blanks

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 ___________ .


  • \[1 \propto \frac{1}{r}\]

  • \[1 \propto \frac{1}{r^2}\]

  • \[1 \propto \frac{1}{r^3}\]

  • \[1 \propto \frac{1}{r^4}\]

Advertisement Remove all ads


\[1 \propto \frac{1}{r^3}\]


Let the distance between the parallel straight lines be L.

Angle with normal = ​θ

We know,

\[I = \frac{I_o \cos\theta}{r^2}\]

From the above figure, we  get

\[I = \frac{I_o \cos\left( {90}^0 - \alpha \right)}{r^2}\]

\[ \Rightarrow I = \frac{I_o \sin\alpha}{r^2}\]

\[ \Rightarrow I = \frac{I_o}{r^2}\left( \frac{L}{r} \right)\]

\[ \Rightarrow I = \frac{I_o L}{r^3}\]

L =  constant for parallel moving source

So, \[I_o L   =   k ............\left(\text{constant}\right)\]

\[ \Rightarrow I = \frac{k}{r^3}\]

\[ \Rightarrow I\alpha\frac{1}{r^3}\]

Concept: Light Process and Photometry
  Is there an error in this question or solution?


HC Verma Class 11, Class 12 Concepts of Physics Vol. 1
Chapter 22 Photometry
MCQ | Q 10 | Page 454

View all notifications

      Forgot password?
View in app×