When I'm sitting at the middle or the back of my classroom ... At first i can see the board clearly..... but, as soon as I look down to write in my book and again look up at the board , my vision goes blurry for a while and i can't see clearly what's written on the board. Then it gets clear on its own and i can see...Again i  look down to write and then up to look at the board, my vision gets blurry for a while and then clear and this cycle repeats ... Why is this happening?