# Electromagnetic radiation pressure

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1. Jan 12, 2016

### Small bugs

I am confused with the fact of radiation pressure on Wiki and my homework solution. They are both arguing that the pressure produced by a ray of light to be reflected with incident angle θ and intensity I is:
$$\frac{2I\cos^2(\theta)}{c}$$
My thinking
We know that:
$$Ft=\Delta p$$
We see the situation where the ray is reflected:

The change of momentum is
$$2\cos\theta\frac{h}{\lambda}$$
And also with other equalities then we get (also I got)
\frac{2I\cos(\theta)}{c}
Rather that cos2... Why?
The second question is about polarisation. Does polarisation direction mean the direction from the peak and perpendicular to the axis so it is just the direction of the E-or H-field?? What is unpolarized light? But as long as the sinusoid wave is spreading and oscillating, then it must have a direction of oscillation, so how can we say it is UNpolarized?

2. Jan 12, 2016

### Staff: Mentor

In addition to the smaller momentum change, your intensity gets reduced by another cosine factor.
The E-field.
The direction can change incoherently and randomly.