Use of 'auquel' / à qui instead of 'que'

Question

In the lesson ‘Que = Whom, which, that (relative pronouns)’ an example sentence is given as:
‘La femme que je dessine’

An example sentence from this lesson is: ‘Le chat, auquel tu as fait peur’ OR ‘Le chat, à qui tu as fait peur’ 
I’m struggling to understand why is ‘Le chat, que tu as fait peur’ is not used?

Thanks

Answer

He Stewart,
It is "J'ai peur à quelque chose." Hence you need an indirect object like "à qui" or "à laquelle/auquel". "Que" is the direct object pronoun.
-- Chris (not a native speaker).

Answer

It's because the expression is "faire peur à qqn/qq chose" or in English, "to make/give fear TO someone/something". If you don't know the phrase, you won't get the right relative pronoun (though with time and experience, you'll be able to figure it out from the rest of the sentence, if they give it to you).
And then it's easy: "Tu as fait peur au chat." It's the "à" that triggers "à qui" or "auquel".
(An aside. To be afraid of something is "avoir peur DE qq chose." I've never heard of "avoir peur à quelque chose" but I'm not a native speaker, so anything is possible.)