All Leaving Cert Physics posts
• #### Finding the focal length of a convex lens. ChristianDaly5

Hi, could anyone help me with this... When doing it in tests i see to drop a few marks and don't really see what I'm missing. Any tips on what to say for this sort of question?

1. #### granners

If the object is located outside of the focus, the image is real, formed on the opposite side of the lens, it is inverted and diminished. If the object is located on the focus or inside the focal length it is virtual, the image is located on the same side as the object, it is upright and either an image at infintiy or magnified

2. #### granners

Just realised it was the experiment: set up your object (cross threads), a ray box, a convex lens and a screen. Put the cross threads into the front slit of the ray box. Turn on the ray box and get the lens, move the lens backwards or forwards until you get a clear, sharp image. Measure the distance between the cross threads and the lens to get the object distance "u" and measure the distance between the lens and the screen to get the image distance "v". Repeat this 3-4 more times, but move the screen further away. You could also add about getting the approximate focal length too and how you did the math to get the focal length

3. #### ChristianDaly5

Thanks a million, I'll be sure to add this!

4. #### what.the.matter.is.the

Make sure you try to avoid error of parallax, to make sure you measure the distances right, make sure that image produced on the screen is very sharp, the sharpest possible. I don't know what else you could be missing that could make you lose marks, sorry.