Jessica Robinson, of St. Mary's School, asks :-

Why can you see dots when sitting close to a TV screen?

Steve Shrimpton, a physicist at Massey University, responded.

The TV screen shows a colour picture by using many small groups of three dots, that give out red, green and blue light. When part of the picture is green, like grass, the green dot in all the groups in that part are made bright. For blue sky, the blue dots are made bright, and for a red rose, the red dots are made bright.

But suppose part of the picture needs to be some other colour, like yellow. There aren't any yellow dots but the TV set can trick the eye into thinking it sees yellow. Our eyes have red, green and blue receptors, cells that are sensitive to red, green and blue light. When yellow light enters our eyes, the red and green receptors both send signals to the brain, and the brain says that's yellow. The TV set can make the red and green receptors send the same signals to the brain by lighting up both the red and green dots. The brain can't tell the difference between this mixture and real yellow light; in fact the TV set makes you think you are seeing all the other colours with just the right mixture of light from the red, green and blue dots.

The dots are made very small so that you cannot see them individually unless you get quite close to the TV screen. At a normal viewing distnace you see patches of colour, not dots.