When it comes to optics, the most basic equation to use is the Snell's Law:
n₁ sin θ₁ = n₂ sin θ₂
where θ₁ is the angle of incidence, θ₂ is the angle of refraction, and n are the index of refractions depending on the type of medium.
This equation is used when light strikes from one medium to another and you want to determine the angle of refraction. For example, as light travels from air to water, the light bends as it strike the water. The observable change you can see is that things tend to look bigger when viewed from afar.
In this case, the first medium is air which has an index of refraction of 1. So, n₁ = 1. Then, n₂ is for the glass which is equal to 25°. Substituting the values,
(1) sin 25° = (1.5) sin θ₂
θ₂ = 16.36°
