Look at this sentence which uses the phrase "hit ___ at an angle":

The light from the sun hits different parts of the Earth at different angles.

When a phrase like this is used as a clause, you still have to keep the preposition. For example, if you're talking about what happens when the Earth orbits the sun, you can choose between the following:

The angle which sunlight hits the Earth at changes.

The angle at which sunlight hits the Earth changes.

The first version is a lot more casual. The second version sounds more intelligent. It's also considered more grammatically correct, so please use it when you're writing or answering a grammar question on a test.

This same structure works with other prepositions:

• for which
• by which
• on which