# refract(_:n:eta:) Returns the refraction direction of an incident vector, a unit normal vector, and an index of refraction eta. ``` func refract(_ x: SIMD4, n: SIMD4, eta: Double) -> SIMD4 ``` --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)