Đặt mắt ở một đầu thước, đầu kia của thước hướng về một nguồn sáng, nhìn dọc theo thước. Điều chỉnh hướng của thước sao cho điểm đầu của cạnh thước ở phía mắt che khuất điểm đầu kia của cạnh thước. Nếu tất cả các điểm trên cạnh thước cũng đều bị che khuất thì cạnh thước thẳng. Lí do là vì tia sáng phát ra từ nguồn đi theo một đường thẳng bị đầu thước gần nguồn chặn lại nên không đến được các điểm khác cũng nằm trên đường thẳng ấy trên cạnh thước để đến mắt

Nếu em không nhìn thấy người thứ hai ở phía trước em có nghĩa là em đã đứng thẳng hàng. Đội trưởng đứng trước người thứ nhất sẽ không thấy được những người còn lại trong hàng.

cảm ơn

ta dùng 1 cái đèn pin, ta bật đèn thẳng với đầu thước , xem các phần của thước có được ánh sáng chiếu đều không, nếu đều là thẳng và ngược lại.

Để kiểm tra xem cạnh thước có thẳng có thẳng hàng hay không thì mình làm cách sau:

Đặt mắt ở đầu thước, đầu còn lại của thước hướng về một nguồn sáng rồi nhìn dọc theo thước. Nếu đầu cạnh thước ở phía mắt che khuất điểm còn lại của thước thì cạnh thước thẳng.

Đó là vì ánh sáng truyền theo đường thẳng trong không khí đến mắt nên những điểm thẳng hàng sẽ che khuất nhau vì ánh sáng từ những điểm đó đến mắt trùng nhau.

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