# Homework Help: Why image is blurry or focused with convex lenses?

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1. Aug 24, 2016

### TheCapacitor

1. The problem statement, all variables and given/known data

I have a few fundamental questions in optics about the focus and blurry images.

Each text book says that according to this picture the object image is the same.

1) If we move the screen to point further then the image will be blurry. What causes it to be blurry?

2) Why do we need the rays to meet in the same point for focus? What does that mean when few rays from the top are coming to the same point? What does that do? Anyway, we see the reflection of them. Does the number affect the intensity?

3) What does blurry mean at lowest level possible? Is it rays from different parts of the object coming to the same point? Is it the different number of rays coming to a certain point from different points on the object?

4) As far as I see the objects top will now be in 3 different places. We should see the reflection of the top 3 times then? Of course there are infinite number of rays. But i"m trying to get a constructive vision on what's going on there.

Thanks

#### Attached Files:

• ###### optics_que.png
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2. Aug 24, 2016

### BvU

Hi C,

FIrst a comment on your picture: we usually start with expressions for thin lenses. The position of the lens in a picture is then the center plane, not the exit surface, when the lens is drawn with thickness (for clarity). The rays that in reality are diffracted when entering the lens and when exiting, are supposed to diffract at the midplane only. That way the two f points (which you did draw equidistant from the midplane) make more sense too. And the 2f points should really be at twice the f distance.

I'll lump 1 and 3: blurry, out of focus: that's when rays from a single point of the object don't end up in a single point but in a finite area of the image plane.

2. Fewer rays from a part of the object means less brightness in the image plane. In a photo that would be an underexposed area.

4. no, not in 3 different places: all the in between rays end up in between. So in an area (as opposed to: in a point). But basically that's repeating the 1/3 answer.
You can sketch the approximate bounds of the area by drawing the uppermmost and lowermost rays.

3. Aug 24, 2016

### TheCapacitor

Can you give me more info about "underexposed area"? if we have the top object spread upon an area, I still don't get why it is blurry.

4. Aug 24, 2016

### BvU

I thought I explained about in focus/out of focus.
Perhaps I misunderstood 'few rays'. Once you found the point in the image plane where two rays from the same point on the object meet, all rays from that point on the object that reach the lens go through that focal point. A sharp image must be a one to one correspondence between points from the object and points in the image.

All this for ideal lenses, of course.

5. Aug 24, 2016

### TheCapacitor

Firstly, thank you for the answers.
I understand this part. Maybe I better ask: What does really make me think it is blurry? You say that the more lines meet the more focus achieved. If less lines are meeting then it is blurry, and some are of the object is spread around the screen (for example lets take a screen). Let's look at my picture:
Let's assume that each light ray that is getting at a point on the screen. And it's reflecting to my eye, so I see my top 3 times according to the picture. Of course, there is unlimited number of rays so I see my top infinite number of times in some area where the image is.
That is happening with each part of my body. Small area on the body is seen unlimited number of times on a small area (instead of a smaller focused area) on the screen which makes it blurry. I'm sure what i've said now is incorrect or not precise. Then what is the reason behind it the blurry image? We can call it "bad reflection" from the screen. What's the cause of that "bad reflection"? Is this mixing of rays from different parts of the object into the same area? I'm trying to imagine what's is that bad reflection upon the screen.

6. Aug 24, 2016

### BvU

I hope I did not write that. What are you referring to ?
Yes. If you see parts of the nose mixed with the eyes, the portrait is blurry. Only if separate points of the object end up on separate points of the image (and nowhere else) can you have a sharp picture.

7. Aug 25, 2016

### TheCapacitor

"Only if separate points of the object end up on separate points of the image (and nowhere else) can you have a sharp picture."
That is exactly what I was looking for! Thank you!

8. Aug 25, 2016

### BvU

Test it by constructing the image of another object point in the picture of post #2.

A further test you can do: where do the in-between the pink rays in that picture end up ? The idea is that all those end up in the same image point -- provided they make it through the lens aperture.