Look at a point, draw the line

The idea

The line of sight is the line drawn from the eye of an observer to the point in the object being viewed.

The student and the Minar

A student stands near the Qutub Minar and looks up at its top. Then she looks at a bird. Then at a flower pot on the ground.

Each time, one straight line joins her to the thing she is looking at.

Which straight line joins her to the top of the Minar?

• From her eye to the top of the Minar
• From her feet to the top of the Minar
• From the top of the Minar down to the ground

Aim the line

Drag the ring. The student looks at it, and the orange line shows where she is looking. Make her look at the top of the Minar, the bird and the flower pot.

Eye to the point viewed

The line of sight is the line drawn from the eye of an observer to the point in the object viewed by the observer.

It starts at the eye and ends at the point you are looking at. Look at another point and you get another line of sight. It can slope up (the bird, the top of the Minar), run straight ahead, or slope down (the flower pot).

Notes

The line of sight is the line drawn from the eye of an observer to the point in the object viewed by the observer.

Check yourself

The student at A looks at the top C of the Minar. Which line is her line of sight?

A girl on a balcony looks at a flower pot on a stair. Where does her line of sight start and end?

A student stands still. She looks at the bird, then at the flower pot. Is her line of sight the same both times?

She looks at a point straight ahead, exactly at the level of her eyes. Is the line from her eye to that point a line of sight?

• AB, the dashed line. AB joins her eye to B, a point on the Minar at the height of her eye. It would be her line of sight if she looked at B. She is looking at C.
• CD, the Minar. CD is the Minar itself. It does not start at her eye.
• AC — correct. Yes! AC is drawn from her eye A to the point C that she is looking at.
• From her feet to the pot. The line of sight starts at the eye, not at the feet.
• From her eye to the pot — correct. Yes! It joins her eye to the point she is looking at.
• From the pot straight down to the ground. That line does not touch her eye, so it is not her line of sight.
• Yes. She is in the same place, so the line is the same.. The line ends at the point she looks at. A new point gives a new line.
• Yes. A line of sight is always horizontal.. A line of sight can slope up, run level or slope down. You saw it in the toy.
• No. Each point gives a different line from her eye. — correct. Yes! The line of sight joins her eye to the point she is looking at, so it changes with the point.
• Yes. It is still the line from her eye to the point she is looking at. — correct. Yes! It is a line of sight that happens to be horizontal.
• No. A line of sight must slope up or down.. A line of sight has to join the eye to the point viewed. It may also be level.