Questions on Optics experiment

[Mohit Pandey]

Hello to all!

Here are the questions.

1.A student obtains a blurred image of an object on a screen by using a concave mirror. In order to obtain a sharp image on the screen, he will have to shift the mirror-

a)towards the screen

b)away from the screen

c)either towards or away from the screen depending upon the position of the object.

d)to a position very far away from the screen.

 

I know when the position of the object is beyond centre of curvature©, then the image is diminished.

It may be that the sharp is being formed at the focus which is nearer to mirror which means that the distance between them is larger than the focal length. Thus, A is correct. Am I correct?

 

2. A student obtaied a sharp image of a the grill of a window using a convex lens . For getting better results , the teacher suggested focussing of a distant tree instead of the grills . In which direction should the lens be moved for this purpose?

A) away from the screen

B) very far away from the screen

C)Behind the screen

D) towards the screen

 

In first case, grill is at the infinite position and image is formed at focus. Now, we need to place the tree at the infinite position for the lens. I think for it we need to move the lens away from the screen(option A). But we need to move the screen also in the direction of lens as the focal lenth is sam, don't we?

 

Please explain.;)

[modest]

There is a virtual java optics bench here:

Optics Bench

that will help you. It has a concave mirror and convex lens.

 

I would suggest using an aperture to simulate a screen (probably an open aperture for question 1). Place the object and mirror/lens and simulate the question.

 

You can move the mirror or lens back and forth and track the image point (notated with a numeral one) seeing when it meets the screen. The object would then be in focus.

 

I would suggest placing 2 objects for question 2. A nearer object simulating the window grill and a further object simulating the tree.

 

Good luck

 

-modest

[Mohit Pandey]

after encountering problem with java, I have started using the great website. But I am having following problems-

1. Where is the screen?

2. How can I know when the image is sharp and at focus?

[modest]

after encountering problem with java, I have started using the great website. But I am having following problems-

1. Where is the screen?

2. How can I know when the image is sharp and at focus?

 

Your setup will probably look something like this:

 

question 1

• The mirror has been scaled to 1.3 to help.
  
• The object is the thick white line pointing up (the object is at the top)
  
• The image point is at the bottom of the thin white line - it is denoted with a 1.
  
• The screen is represented by the aperatuer which is the yellow line going up and down. It is open so light will pass through it.
  
• The three lines that start at the object, move right, reflect off the mirror, and move off screen left are the light.
  

 

When the image point (thin white line) is vertically aligned with the screen (yellow, vertical, broken line) the image will be in focus. The current setup you see above has the image out of focus. The idea is to move the mirror left and right and watch what happens to the imagte point. You should then be able to answer the question.

 

question 2

 

Differences here:

• There are 2 objects (thick white lines). They represent the tree (farthest left) and the window grill (farthest right)
  
• The lens is scaled to .75 which will help a lot.
  
• There are 2 image points (be sure and keep straight which object they belong to)
  
• The screen (yellow vertical line) is now on the right of the optic.
  

 

When an object's image point aligns vertically with the screen then that object will be in focus. Move the lens left and right and see when each object is in focus. In this current setup neither object is in focus.

 

If you set up the applet correctly and understand what each thing represents then it should be very insightful. This is as much as I can say without answering the questions.

 

-modest

[Mohit Pandey]

Thanks modest for your kind, prompt and great help.;)

I earnestly request you to check my inferences if they are correct or not.

 

I didn't know that aperture is the screen itself. That's why I was little bit confused .

now I think I have found out the answer.

My biggest mistake was to think that the formed images given in questions can be formed only when the object is at infinity and the image formed is at focus.

 

1.A student obtains a blurred image of an object on a screen by using a concave mirror.

I have now realised that the blurred image of question 1) was so formed because the screen was behind or in front of the image formed.

 

2. A student obtained a sharp image of a the grill of a window using a convex lens

It meant that the image was formed at screen but not necessarily is at focus.

 

for first question , answer is

C) either towards or away from the screen depending upon the position of the object.

 

for second question , answer is

 

D) towards the screen

 

Thanks again.:)

[modest]

My biggest mistake was to think that the formed images given in questions can be formed only when the object is at infinity and the image formed is at focus.

 

I was hoping you would realize that, and you did - very nice.

 

 

for first question , answer is

C) either towards or away from the screen depending upon the position of the object.

 

for second question , answer is

 

D) towards the screen

 

Thanks again.:)

 

Perfect. Looks like you understand it very well now.

 

-modest

[Mohit Pandey]

I was hoping you would realize that, and you did - very nice.................

Perfect. Looks like you understand it very well now.

 

-modest

Dear Modest

I would like you to tell that this mistake would not have arisen if I had done that experiment myself or done it using Optics Bench. Sadly, the school in which I study emphasises on rote leaning, not learning by doing.

However, I hope with the help of technology such as Java enabled Optics Bench can help me a lot in visualising experiments as if I was doing it. Therefore, I request you to give me such links whenever you can as it would help me a lot. If you had told me simply by writing, then perhaps I would have taken more time( though I did in present case also:))

 

Thanks Again.:hihi: