Wood, iron, coal, and granite.
Can we see through a glass window?
Yes; because the glass is transparent, and the light passes through it.
Can we see through a looking-glass?
No; because the back of the looking-glass is covered with quicksilver, which prevents the rays of light from passing through it.
What becomes of the light falling on the looking-glass?
It is thrown back from the glass.
What is this called?
It is called reflection.
What objects are good reflectors of light?