# Homework Help: Need Help With Lens

1. Apr 11, 2005

### suf7

Need Help With Lens!!

An object is placed 10cm in front of a convex lens of focal length 20cm. Where is the image formed and what is its magnification??..Your solution should include an accurately drawn ray diagram???

Pleeez help me with this question???...im so stuck??

Thanks.

2. Apr 11, 2005

### IntuitioN

Use the lens formula:

1/f = 1/u + 1/v

where f = focus, u = object distance from lens, and v = image distance from lens

So:

1/20 = 1/10 + 1/v

Solve for v
v = -20cm

So the image is located at 20cm

Last edited: Apr 11, 2005
3. Apr 11, 2005

### ZapperZ

Staff Emeritus
You should have at least[/b] be able to know where to start! The lens equation would have been a good one. If you didn't even know this, then you should review the material again and know what that equation is for, and how to use it.

If you do that, then you would have been able to derive what IntuitioN did, AND, be able to decipher what the "v = -20 cm" answer means (i.e. what does the NEGATIVE sign signifies). So look at the lens equation, and understand how each symbol are defined.

Zz.

4. Apr 11, 2005

### suf7

oh right, thank you...so when i work out the magnification would it be 20/10 which is 2???

5. Apr 11, 2005

### Ba

And for your ray diagram draw the lens with a compass. Then put in the object and draw the rays.

6. Apr 11, 2005

magnification formula is $$m = - \frac{d_{i}}{d_{o}}$$