When a short pulse of monochromatic light strikes a small mirror suspended by a thread, several interesting phenomena occur. Let's break down the situation to understand why the mirror would oscillate and why the wavelength of the reflected rays remains unchanged.
Understanding the Interaction of Light and the Mirror
First, when light hits the mirror, it exerts a pressure on the surface due to the momentum carried by the photons. This pressure is known as radiation pressure. The amount of pressure depends on the intensity of the light and the properties of the mirror.
Why the Mirror Oscillates
When the pulse of light strikes the mirror, it imparts momentum to the mirror. Since the mirror is suspended by a thread, it is free to move. The light pulse creates a force that pushes the mirror in the direction of the incoming light. Once the pulse passes, the mirror will not only move in the direction of the light but will also experience a restoring force due to the tension in the thread. This combination of forces causes the mirror to oscillate.
- Initial Movement: The mirror moves in the direction of the light pulse.
- Restoring Force: The thread pulls the mirror back to its equilibrium position.
- Oscillation: This back-and-forth motion continues as the mirror swings due to the interplay of forces.
Wavelength of Reflected Rays
Now, regarding the wavelength of the reflected rays, it’s important to note that the wavelength of light does not change upon reflection from a mirror. The properties of the medium (in this case, air and the mirror) remain the same before and after reflection. Therefore, the wavelength of the reflected rays is equal to that of the incident rays.
- Monochromatic Light: Since the light is monochromatic, it has a single wavelength.
- Reflection Properties: Reflection does not alter the wavelength; it only changes the direction of the light.
Conclusion on the Options
Given this understanding, let's evaluate the options:
- (a) The mirror will start to oscillate - This is correct because of the momentum transfer from the light pulse.
- (b) Wavelength of the reflected rays will be greater than that of incident rays - This is incorrect; the wavelengths are the same.
- (c) Wavelength of the reflected rays may be less than that of incident rays - This is also incorrect for the same reason as (b).
- (d) None of these - This is incorrect because (a) is true.
Thus, the correct answer is that the mirror will oscillate, but the wavelength of the reflected rays remains unchanged. This understanding of light-matter interaction is fundamental in optics and helps explain various phenomena in physics.
