# Ray diagrams (lenses) problem. I need full explanations.

Ray diagrams (lenses) urgent problem. I need full explanations.

1. The question

An object of height 2cm is placed 15 cm from a thin converging lens of focal length 10cm.
Complete the paths of two rays on the figure to show their passage through the lens to the image formed.

2. The attempt

http://www.flickr.com/photos/56105561@N05/5427538657/

If I have drawn wrongly, please kindly pinpoint the mistakes and suggest your answers. Thank you so much and have a nice day!

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gneill
Mentor

The rays given in the figure are not the principle ray nor the central ray, and neither passes through the focus on the object side. It doesn't look like these rays will converge at an image.

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Thank you gneill, but I want to clarify my doubts:

the thing is that one light ray travels downwards and the other upwards from the top extremity of the object to meet the convex lens. The website you suggested doesn't give a ray diagram that has a light ray travelling upwards from the object. Can you help me?

gneill
Mentor

Thank you gneill, but I want to clarify my doubts:

the thing is that one light ray travels downwards and the other upwards from the top extremity of the object to meet the convex lens. The website you suggested doesn't give a ray diagram that has a light ray travelling upwards from the object.
That's right! They don't show your rays because, for a thin converging lens, they won't converge on the image; they are divergent rays that do not contribute to the formation of the image.

The rays that will define the extent of the image are:

1. The ray along the lens axis (which is not refracted as its angle of incidence with all surfaces of the lens is 90°);

2. The principle ray which travels horizontally from the head of the object to the lens and is refracted so that when it emerges it passes through the focus on the opposite side of the lens; and

3. The 'straight through' ray that goes from the top of the object straight through the center of the lens and out the other side -- the lens refractions cancel for this ray, so it doesn't deviate from a straight line.

Rays 2 and 3 converge at the head of the image (the image is inverted). Ray 1 defined the foot of the image.