Answer:
θ = 30°
Explanation:
Firts, the angle when the beam of light passes through the block cam be calculated using Snell Law:

<u>Where</u>:
n₁: is the index of refraction of the incident medium (air) = 1
θ₁: is the incident angle = 30°
n₂: is the medium 2 (plastic) = 1.46
θ₂: is the transmission angle
Hence, θ₂ is:

Now, when the beam of light re-emerges from the opposite side, we have:
n₁: is the index of refraction of the incident medium (plastic) = 1.46
θ₁: is the incident angle = 20.03°
n₂: is the medium 2 (air) = 1
θ₂: is the transmission angle
Hence, the angle to the normal to that surface (θ₂) is:
Therefore, we have that the beam of light will come out at the same angle of when it went in, since, it goes from air and enters to a plastic medium and then enters again in this medium to go out to air again. This was proved using the Snell Law.
I hope it helps you!