Since the refracting surfaces are parallel to each other the emergent rays are parallel.The angle of incidence on the first surface { from air to glass} be i and the angle of refraction at the first surface be r.Since the surfaces are parallel the ray inside the glass falls at the other surface at angle r and therefore the ray emerges at angle i that is the angle of incidence.Therefore the incident ray is parallel to the emergent ray.

Explanation:

When a light ray travels from one medium to another, it undergoes a change in its direction. This phenomenon is known as refraction. Refraction occurs due to the change in speed of light as it travels through different mediums. The emergent ray is the ray that exits the second medium after refraction.

Law of Refraction:
The behavior of light during refraction is governed by the law of refraction, also known as Snell's law. According to this law, the ratio of the sine of the angle of incidence (θi) to the sine of the angle of refraction (θr) is constant for a given pair of media. Mathematically, this can be represented as:

n1 * sin(θi) = n2 * sin(θr)

Where:
n1 and n2 are the refractive indices of the first and second mediums, respectively.

Parallel Emergent Ray:
When the light ray travels from one medium to another, it bends or changes its direction. The direction of the emergent ray depends on the relative refractive indices of the two mediums involved. If the refractive indices are the same, the emergent ray will continue to travel in the same direction as the incident ray.

In the case where the refractive indices of the two mediums are different, the emergent ray will change its direction. However, the emergent ray will still be parallel to the incident ray if the angle of incidence is adjusted accordingly.

Reasons:
1. Snell's Law: According to Snell's law, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a given pair of media. When the angle of incidence is adjusted in such a way that the emergent ray becomes parallel to the incident ray, this ratio remains constant.

2. Conservation of Energy: Light travels in the path that minimizes the time taken to travel from one point to another. When the emergent ray is parallel to the incident ray, it indicates the path that minimizes the time taken for light to travel between the two mediums.

3. Symmetry: The behavior of light is often symmetric. If the incident ray enters the second medium at a certain angle, the emergent ray will also exit the medium at the same angle but on the opposite side. This symmetry results in the emergent ray being parallel to the incident ray.

4. Law of Reflection: The emergent ray can be considered as the refracted ray after passing through the interface between the two mediums. The angle of incidence and angle of refraction obey the law of reflection, where the angle of incidence is equal to the angle of reflection when light reflects off a surface. This further supports the parallel nature of the emergent ray.

In summary, the emergent ray is parallel to the incident ray when light passes through different mediums due to the behavior of light governed by Snell's law, conservation of energy, symmetry, and the law of reflection.