Given:
A ray diagram
To find:
Identify the incident ray.
Explanation:
An incident ray is a ray that falls on the surface that separates two media.
In the given ray diagram, it can be seen that light ray 1 is falling on the surface which separates the two media. Thus, a light ray 1 is an incident ray.
Ray 3 is called a refracted ray and a line represented by number 2 is called a normal which is perpendicular to the surface.
Conclusion:
Ray represented by number 1 is the incident light ray. Thus, the correct option is option a.
