2 very difficult light ray diagram questions -- help please

• bomberkid
In summary, the lines drawn in the homework statement represent the position of what, in terms of images, based on the assumption that the light ray comes from the mirror image of the torch.f

Homework Statement

Hi please help me with these 2 ray diagrams... the questions is as stated in the attachment..i have no idea where to start drawing and whatever i have drawn is wrong for both qns.. thanks a lot experts!

The Attempt at a Solution

Attachments

• Screen Shot 2015-01-05 at 12.28.28 PM.png
70.9 KB · Views: 1,474
• Screen Shot 2015-01-05 at 12.24.20 PM.png
44.1 KB · Views: 1,343
• Screen Shot 2015-01-05 at 3.52.43 PM.png
48 KB · Views: 818
In the first one, I assume the dashed line and the three sloping solid lines above it are all parts of your construction.
Erase the lowest of the three solid lines (the one with the arrow pointing up and left).
The remaining two solid lines you show brought to convergence to the left of the mirror. This looks correct. But what do you think that point of convergence represents? What is it the position of, in terms of images?

thanks so much for your input haruspex..but pardon me, i still can't grasp how should i draw it..should i be drawing the image of the light in the mirror? if my light ray coming from the torch is reflected in the mirror, should i be drawing an angle of incident and reflection at a normal to the mirror? really confused

thanks so much for your input haruspex..but pardon me, i still can't grasp how should i draw it..should i be drawing the image of the light in the mirror? if my light ray coming from the torch is reflected in the mirror, should i be drawing an angle of incident and reflection at a normal to the mirror? really confused
First, please confirm or correct my guess as to which of the lines in the picture you had added. If confirming, please explain the basis on which you drew these lines.
It looks to me as though you started with an image of the torch in the mirror, and treated that as the source of light (in several directions) passing through the lens. If so, that all looks good.

hihi, sorry for the confusion, this is my test paper, all the lines drawn are wrong and just based on my guesswork, except for the normal. this is the original question, as attached. i have erased all the lines that were drawn wrongly, except for the normal at the plane mirror. that's the only thing i have gotten right so far. hope this is clearer now!

Attachments

• Screen Shot 2015-01-05 at 3.52.43 PM.png
48 KB · Views: 1,244
hihi, sorry for the confusion, this is my test paper, all the lines drawn are wrong and just based on my guesswork, except for the normal. this is the original question, as attached. i have erased all the lines that were drawn wrongly, except for the normal at the plane mirror. that's the only thing i have gotten right so far. hope this is clearer now!
Ok, that makes it clearer, but as I wrote in post #2 some of your lines do make sense and are useful. I'm asking you to explain why you drew those lines. This will help me lead you to the answer.

Well I drew the lines thinking that the light ray got to pass through one of the focal length, and another through the center of the lens. But that's totally wrong, the examiner totally marked it wrong.. So I don't think its correct. Maybe u can show me.what is the right drawing?

Well I drew the lines thinking that the light ray got to pass through one of the focal length, and another through the center of the lens. But that's totally wrong, the examiner totally marked it wrong.. So I don't think its correct. Maybe u can show me.what is the right drawing?
The given light ray from the torch won't do those things, but forget that light ray for the moment. Think about the torch as merely an object for which there will be images. You appear to have identified where its image will be in the mirror, and your lines from there through the lens midpoint and focal point lead you to another image, right? So where they meet indicates the position of what, exactly?

Hi...the line passing through the center will be straight like you did. But the other lines do not go straight. They get deviated through..

Could it be the examiner drew the left red cross because you didn't let that lower ray diffract ?
Add the right red cross because the arrows on those lines point in the wrong direction ?
Provided your three rays come from the mirror image of the torch, I don't see anything wrong with the top two.
And now all you have to do is ask yourself where the third one ends up.
Check with a picture of a simpler situation (image fromation thin lens)

 sorry, 'Add the right red cross'was supposed to be 'And the right red cross'

Last edited:
Add the right red cross because the arrows on those lines point in the wrong direction ?
Or maybe because having arrows on them made the examiner think they were supposed to be part of the path of the light ray, not merely construction lines.

thanks for your help everyone... i believe i have gotten the correct answer from reading all your posts attached is my new drawing, isit correct?

Attachments

• Screen Shot 2015-01-06 at 10.45.35 PM.png
31.9 KB · Views: 658
The lower ray can't pass through F: rays that come in from the right parallel to the principal axis pass through F.

Then: what is I ?

I liked your first picture in post #1 better than this one ! At least it gave the impression all three rays were originating from the mirror image of the torch.

hmm ok.. where should the lower ray pass through then?

Check with a picture of a simpler situation (image formation thin lens): once you have an intersection point of two rays coming from a signle point on the source, all rays from that same point go through that intersection point in the image plane.

yes..i checked with a simpler diagram of a thin lens image formation.. but with the image in the mirror i am not sure where am i even sposed to begin drawing. everything that i have drawn is kinda trial and error..

now i am at a loss ..

You can construct the mirror image of the torch, right ?
A ray from there through the right F comes out parallel
and a ray from there through the center goes stright on
The point where these two intersect is where your ray of light coming straight out of the image of the torch goes throuh too after diffraction by the lens.

Hi BvU, I sort of get it now, do u mean like this? Thanks for ur help, hopefully I am a little right this time, but The rays don't intersect though..

Attachments

• IMG_20150106_235517.jpg
31.2 KB · Views: 597
I think that to the right of the lens it's ok now. The middle ray is also fine. The other two have to be refracted by the lens:
the top one that is going through the right F point comes out parallel to the axis and does intersect the middle one.
the lower one (dashing on the righthand side is a bit confusing) has to go through that intersection point.

(And that intersection point better be to the left of the left F point !)

yup sorry..the dashing is actually the normal from the light to the mirror.. Is the drawing correct now? :)

Attachments

• IMG_20150107_001146.jpg
34.3 KB · Views: 504
The dashing to the left of the mirror is, yes. To the right of the mirror it is the mirror image of the ray of light from the torch, I hope.
Read on in post #20 line 3 and 4

Is the drawing correct now?
We seem to be back to your first picture, almost, and as I posted then, it is mostly correct. The one wrong line is the lowest line from the mirror image. It's fine from there, through the mirror, as far as the lens, but there's no way it is going to pass through the lens in a straight line.
And what I asked right at the start I ask again now: to the left of the lens, the top two lines intersect (just above the left hand F point). What does that point represent in terms of images?

Use your known rays (In through centre out through centre, in through F out parallel to axis, in parallele to axis out through F) to locate the corresponding point in the image of the mirror then draw the reflected ray from the mirror (angle of incidence equals angle of reflection) and you know that this ray must also pass through that point on the image.

... and your sketch is incorrect.