Question

What is the critical angle for vegetable oil and water and plexiglass

Answer 1

Snell's law describes the relationship between the angles of incidence and refraction.
n1*sin(theta1) = n2*sin(theta2)
When light travels from a medium with a higher refractive index to one with a lower refractive index, a phenomenon known as total internal reflection occurs. The largest possible angle of incidence which still results in a refracted ray is called the critical angle; in this case the refracted ray travels along the boundary between the two media.
theta(crit) = arcsin(n2/n1) where n1>n2.
Assuming light enters the following media and refracts to the air (n=1):
For acrylic: n=1.590, theta(crit)=arcsin(1/1.490)=42.4 degrees
For water: n=1.333, theta(crit)=arcsin(1/1.333)=48.6 degrees
For vegetable oil: n=?