Lower Secondary Science

[mum_sugoku]

Koky462:

My DS has a physics question. It is on the topic of light.


Converging lens A & B have the same focal length, but B is only half the diameter of A. Both lenses are used to form the images of distant objects on a screen. Which of the statement is correct?

A. For both lenses, the distances from the lens to the screen is the same.
B. For both lenses, the images are of the same brightness.
C. For both lenses, the images are the same size as the object.
D. Lens B gives a smaller image than A.

The correct answer is C, but my DS still do not understand. May anyone enlighten us?

Think the correct answer is wrong. Should be A instead.

[mum_sugoku]

Dr.Daniel:

Yes, C is correct. It is the curvature of a lens that determines the magnification. Just take a magnifying glass and cover half of it. When you look through, you still see the same magnification. But not as much light makes it through. So the image from a full lens will be brighter than an image from half of the lens. This might not be very apparent at normal lighting conditions. But the magnification depends on how much the light bends – and that is due to the curvature of the lens, not how big the lens is.


With telescopes you can have two telescopes that have the same curvature to the lens, or the same magnification. But one with a big lens collects more light and you can generally see a sharper image. I was out looking at the lunar eclipse with the Singapore astronomers last month. One of the best views was a guy who had a huge set of binoculars. The magnification wasn’t that much but the image was very clear because he was collecting so much light due to the size of the lenses.

At O-Level there are lots of problems where you move an object closer to and further from a lens and the image focal point changes and so does the magnification. Doing those kind of problems (and some experiments with lenses) help students get a feel for it.

Agree that it is the curvature of a lens that determines the magnification.. However, did you notice that the question already specified that both A & B have the same focal length?

My understanding is that for image to have exactly the same size as the object, the object should be positioned at exactly 2x the focal length (2F) away from the (centre of) lens.. However, for this particular question, we are looking at images formed by distant objects (>>2F away), and based on what I know, image formed by distant object should be smaller (ie diminished) - instead of of the same size as object - and with an image distance (location where you can get a sharp image) at exactly 1F away from the lens..

Which is why I believe that C is incorrect and it should be A instead lor.

[Dr.033430Daniel]

Oh yes - good point. You are right - A looks like the right choice.


It says the images are the same size as the object, not necessarily true. Its true that the images are the same size as each other, but not the same size as the object.

Thanks

[mum_sugoku]

Dr.Daniel:

Oh yes - good point. You are right - A looks like the right choice.


It says the images are the same size as the object, not necessarily true. Its true that the images are the same size as each other, but not the same size as the object.

Thanks

:oops: actually, I had made exactly the same mistake as you - failed to notice that \"same focal length\" thingy and had also thought \"C\" was the right answers :slapshead: . (Alas nowadays, some questions can be so tricky that we'll have to read and re-read before we could correctly figure out what they are asking ya )

[Koky462]

Thanks everyone for contributing their replies!


My doubts are cleared for now!

[Koky462]

My DS still has another question on lens.


When light which is perpendicular to the principal axis passes through a converging lens, what happens to its path?

I would appreciate if anyone can enlighten me again.

[KS-Lim]

Hi everyone


Pls advise / recommend any good guide book for Sec 1 Science.

Thanks

[mum_sugoku]

Koky462:

My DS still has another question on lens.


When light which is perpendicular to the principal axis passes through a converging lens, what happens to its path?

I would appreciate if anyone can enlighten me again.

It will undergo refraction, ie light ray will bend towards the normal..

[Koky462]

My DS has another question on electrochemistry.


Why must the electrolyte be Copper(II)Sulphate solution for the purification of copper? Can’t it be other molten or aqueous ionic compounds?

Thank you.

[mum_sugoku]

Koky462:

My DS has another question on electrochemistry.


Why must the electrolyte be Copper(II)Sulphate solution for the purification of copper? Can't it be other molten or aqueous ionic compounds?

Thank you.

Becos it's the Cu2+ ions from CuSO4 solution that get deposited on the cathode. (Meanwhile Cu at anode \"melts\" into the solution so concentration of solution remains constant)