Lenses

Dixon999 SG

A point light source P placed in front of a thin converging lens L forms an image at the point Q as shown in the figure. If the light source P was moved to P', at which points A, B, C or D will the image be likely to form?

qdtimes2 SG

There's a pattern: As the light source move closer to the lens, the point at which the light rays converge move further away from the lens.

(Provided that the light source is more than a focal length away from the lens)

Dixon999 SG

the ans is C. The part which i dun understand is in the 1st place, shouldn't the orginal light source P form an image Q which is situated between f and 2f? How come the image Q is located way beyond 2f? Pls enlighten. Thanx!!!

qdtimes2 SG

yeah, normally the image Q should be between F and 2F. so, i guess this qn tests us on the concept of the pattern thing.

maybe the physics pros around here can explain lol.

eagle SG

One way to see

An object at 2f will have an image at 2f
An object a f will have an image at infinity (further than 2f)

So, as the object moves towards the right, the image also moves towards the right. Vice versa, as the object moves towards the left, the image also moves towards the left.

The reason being, when you are closer to the lens, the emerging rays are intersects at an angle that is smaller, hence it will be at a further distance (draw out to see)
And why is because the one day that goes through the centre of the lens is now 'steeper' (draw out to try as well!)