Question

In a certain design of helium-neon laser, the chamber contaning these gases has a perfect mirror at one end. as usual, but only a window at the other. Beyond the window is a region of free air space and then the second mirror, which is partially reflecting. allowing the beam to exit. The resonant cavity between the mirrors thus has a region free of the helium-neon gas - the "lasing material" - in which you can insert something. If you insert a sheet of clear plastic at any orientation in this region between the mitrors, the laser beam disappears. If the same sheet is placed in the beam ourside the partially reflecting mirror, the beam passes through it, regardless of the orientation. Why?

Short answer

The laser beam disappears when a plastic sheet is inserted within the laser cavity (between the mirrors) because the sheet alters the condition for total internal reflection within the resonator, disrupting the formation of the laser beam. However, placing the sheet outside the laser cavity does not affect the already formed laser beam, thus it can pass through the plastic sheet regardless of the sheet's orientation.

Step-by-step solution

Understand Laser Operation

In simple terms, a laser is composed of three components: a laser medium (in this case helium-neon gases), an energy source, and two mirrors which form an optical resonator. The laser medium is energized by an external source. Once energized, the atoms in the medium emit light particles or photons. These photons oscillate inside the resonator, traveling back and forth between the mirrors, which multiplies the number of photons. One of the mirrors is partially reflective, allowing a part of these photons to exit, which forms the actual laser light.

Understand the Effect of Material on Laser Beam

The path of the laser beam is crucial for the operation. The medium (air or plastic) between the mirrors can influence the beam's path. If the plastic sheet is placed inside the resonator, it may change the conditions of total internal reflection within the resonator, causing the beam to be interrupted. The orientation of the sheet does not matter because the laser beam needs a clear, unobstructed path in order to function.

Comprehend the Difference between Inside and Outside Placement

When the plastic sheet is placed outside the partially reflecting mirror, it won't affect the already produced laser beam. That's because the laser beam has already been formed and can pass through the clear plastic sheet without being disrupted. It's the difference in internal and external placement that causes the laser beam to disappear in one case and pass uninhibited in the other.

Key concepts

Optical Resonator

The optical resonator, also known as the laser cavity, is a fundamental part of any laser system, including the helium-neon laser in the exercise. It consists of two mirrors facing each other, trapping light between them. One mirror is fully reflective, bouncing all incident light back, while the other is partially reflective, allowing some light to escape as the usable laser beam.

This setup creates an environment where light can bounce back and forth, gaining energy with each pass through the laser medium. The distance between the mirrors is precisely aligned to support photon oscillations at certain wavelengths. Any obstruction within this resonator, like the insertion of a clear plastic sheet, upsets this delicate balance, effectively disturbing the resonation and, therefore, the operation of the laser.

Laser Medium

The heart of any laser is the laser medium, which in the exercise is a mixture of helium and neon gases. This medium is the substance that gets 'excited' by an external energy source, such as an electrical current. When the atoms of the helium-neon gas return to a lower energy state, they emit photons.

In the right conditions, these photons stimulate other atoms to emit more photons of the same energy and direction, leading to a chain reaction known as stimulated emission. The purity and composition of the laser medium directly impact the efficiency and wavelength of the emitted light. The exercise posits that when the medium's continuity is interrupted by the plastic sheet, the chain of stimulated emission ceases, and the laser beam vanishes.

Total Internal Reflection

Total internal reflection is a phenomenon central to guiding light within the optical resonator. It occurs when light hits the boundary of two materials at a certain critical angle, causing the light to reflect entirely back into the original material rather than passing through the boundary.

In the context of the laser operation, the mirrors of the optical resonator are positioned to utilize total internal reflection to keep photons bouncing within the laser medium, effectively amplifying them. If the medium's uniformity is disrupted, such as by the insertion of a different material like plastic, it may alter the critical angle or the conditions necessary for total internal reflection to occur, explaining why the laser beam disappears when the plastic sheet is placed inside the resonator.

Laser Beam Path

The path of the laser beam is meticulously controlled within the confines of the optical resonator. The beam's trajectory is typically linear and must remain unobstructed to maintain a coherent and powerful output. Any physical interruption, such as a foreign object, can scatter or absorb the photons, destroying the beam's integrity.

When introducing a foreign object like a plastic sheet into the resonator, as mentioned in the exercise, it alters the beam's path regardless of the sheet's orientation. The beam path is crucially important because it is the light's journey through the resonator and out of the partially reflective mirror that eventually produces the coherent light that is the laser beam.

Photon Oscillation

Photon oscillation refers to the movement of photons back and forth within the optical resonator. These oscillations are a result of the reflections happening at the mirrors on each end of the laser cavity. As the photons travel, they repeatedly pass through the laser medium, garnering energy and inducing more emission of photons through the process of stimulated emission.

This amplification through repeated oscillation is key to achieving a powerful and uniform laser output. Photon oscillation explains the cumulative effect that leads to the coherent beam of light unique to lasers. In the exercise scenario, the plastic sheet disrupts these oscillations by altering the internal conditions of the cavity, thereby stopping the amplification process and eliminating the laser beam.