CBSE-XI-Physics
17: Light Waves
- #9The wavefronts of a light wave travelling in vacuum are given by x + y + z = c. The angle made by the direction of propagation of light with the X-axis is
(a) 0°
(b) 45°
(c) 90°
(d)
cos-1 1/3digAnsr: dAns : (d) `` {\,\mathrm{\,cos\,}}^{-1}\left(1/\sqrt{3}\right)``
On writing the given equation in the plane equation form lx + my + nz = p,
where l2 + m2 + n2 = 1 and p>0, we get:
`` \frac{1}{\sqrt{3}}x+\frac{1}{\sqrt{3}}y+\frac{1}{\sqrt{3}}z=\frac{c}{\sqrt{3}}``
If `` \theta `` is the angle between the normal and +X axis, then
`` \,\mathrm{\,cos\,}\theta =\frac{1}{\sqrt{3}}``
`` \Rightarrow \theta ={\,\mathrm{\,cos\,}}^{-1}\left(\frac{1}{\sqrt{3}}\right)``
Page No 379: