TypeScript module visibility

hello.ts

module hello {
  export interface IHelloWorld {
    greet(name:string):string;
  }
  
  class HelloWorld implements IHelloWorld {
    greet(name:string):string {
      return 'Hello ' + name;
    }
  }
}

hello_test.ts

module hello {
  var helloWorld = new HelloWorld();
  
  console.log(helloWorld.greet('John Snow'));
}