Question

A parallel beam of monochromatic light of wavelength 663 nm is incident on a totally reflecting plane mirror.The angle of incidence is 60

Answer 1

We can solve this by Newton's second law, which states that force is equal to rate of change of momentum.

If you look at the figure attached, the horizontal component of the initial and final momentum (of the photons) are the same. The only part changing is the vertical component.

Hence, Change in momentum = pcos(60) - {-pcos(60)} = 2pcos(60)

From the de Broglie relation, p=h/λ

so,

force exerted = Change in momentum/time taken

=[2×p×cos(60)]×(no. of photons striking per second)

=[2×(6.626×10⁻³⁴/663×10⁻⁹) × 1/2] × (10¹⁹)

=10⁻⁸N