题目内容：

It seems to be strange to you there is a blind spot (盲点)on the eyes. Here is an interesting experiment (实验) that can make something disappear(消失) when one eye is open.

Make a card which is as big as a postcard, and write two English letters L and R on it. Write L on the left and R on the right. First, hold the card about 80 cm(厘米) away and you see both the letters. Then close your right eye and look at the letter R only with your left eye. And now, as you move the card slowly towards you, you’ll find the letter L disappearing. But if you move the card nearer to your face, the letter will be seen again. Now do the same experiment with your left eye closed, you’ll find the letter R disappearing.

Why does the letter disappear? It is because there is a blind spot on the eye. When the image (影像) of the letter falls on the blind spot, it won’t be seen. That is why either of the letters disappears.

1. Where should we write the two letters?

2.Which letter should be written on the left?

3.Which eye do you close when R disappear?

4.What will happen if the image (影像) of the letter falls on the blind spot?

5..Do all people know that there is a blind spot (盲点)on their eyes?