View Full Version : Woot! High School Physics!

Boondock Saint
02-06-2005, 01:02
Hey, finals are coming up, just got a quick question for you all. It concerns refraction of light through concave and convex lenses (I quite literally slept through the whole chapter). What I need to know is how to do a few problems, what formulas, and the steps involved.... any help would be greatly appreciated. Also, I just realized that I now have a credit card... however, I will not make any donations to Portent until I know for sure its going to be back, and on a secure server...

Ok, here goes.

A converging lens has a focal length of 20 cm. If it is placed 50 cm from an object, at what distance from the lens will the image be?

If an object is 10 cm from a converging lens of 5 cm focal length, how far from the lens will the image be formed?

An object 3.0 cm tall is placed 20 cm from a converging lens. A real image is formed 10 cm from he lens. What is (a) the focal length of the lens, (b) the size of the image?

When an object 5 cm tall is place 12 cm from a converging lens, an image is produced on the same side of the lens as the oject bu 61 cm away from the lens. What is (a) the focal length of the lens, (b) the size of the image?

An optical bench pointer 1 cm tall is placed at the 55 cm mark on a meter stick. When a diverging lens is placed at the 0 mark, an image is formed at the 5 cm mark. What is (a) the focal length of the lens, (b) the size of the image?

Thanks in advance (and no, this isn't homework, its from the book)

Boondock Saint
02-06-2005, 02:54
Come on.... anyone?

02-06-2005, 07:36
the rule you are looking for is 1/S + 1/S' = 1/f

f= focal point
S = object distance
S' = image distance

for example, in your first Question

1/50 + 1/S' = 1/20

this 1/S' = 3/100

so S' is 100/3 cm from back of lense...

also M = -S'/S (magnification)

ill go away and have ago at the rest for you...

Brother Munro
02-06-2005, 12:49
Athamas is indeed correct.

Wow, ray optics, haven't done that in a while, I don't think you ever really have to do it again (at least not remember any formulae).

BTW Boondock Saint, Portent is now on its own server, as you can see from the announcement at the top right of the page. :-)

Boondock Saint
02-06-2005, 15:00
Great thanks, I shall then indeed go to the new server and become a guilder (if thats still the term).

EDIT: My bad, I assumed this was still the temporary, anyway... donation sent ;)

02-06-2005, 17:06
Hmm... I did he whole light ray thingy once... It isnt that difficult after you practiced it a bit...

*reaches over for formula book*

Lets see...

Main law of Snellius, for how the light travels through a clear object:

sini / sin r =n

In which i is the angle of the ingoing light and the rest of the things I forgot...

Mind you, they find this part mucho more important than the babble on lenses.... (in Holland that is...