Final answer:
The first federal law that showed racial prejudice and failed to provide equal treatment to African American people was the Civil Rights Act of 1875. It was seldom enforced and invalidated by the Supreme Court in 1883.
Step-by-step explanation:
The first federal law that is being referred to in the question is the Civil Rights Act of 1875. This law aimed to prohibit racial segregation, but it was seldom enforced and faced many obstacles. Despite its passage, the law was declared unconstitutional by the Supreme Court in 1883.
Although the law was intended to prevent racial prejudice towards African American people, it ultimately failed to provide significant protection and equal treatment due to lack of enforcement and subsequent segregation laws known as Jim Crow laws.