# Find the focal length of a concave lens using a convex lens?

• jias
In summary, to find the focal length of a concave lens using a convex lens, one can use the lens formula -1/f = 1/a - 1/b where a is the distance from the center of the concave lens to the object and b is the distance from the center of the concave lens to the image. By considering the reverse path of light, the focal length can be determined.

#### jias

Can anyone explain to me the procedure that one would follow to you find the focal length of a concave lens using a convex lens? Thanks.

Yeah there is a way

Assume the convex lens creates an image of S at the point S'. Now if you put a concave lens in front of the convex one then the image of S will move further and will be at point S''. Now consider a reverse path of light: S'' is now the object and you'll see it's image at S' which is created by the concave lens. Let the distance from the "center" of the concave lens to S'' be a and the distance from S' to the "center" of the concave lens be b. Then using the lens formula:

$$-\frac{1}{f}=\frac{1}{a}-\frac{1}{b}$$ notice there is a minus near b and f. It's because the image is imaginary

Thus

$$f=\frac{ab}{b-a}$$

I hope I didn't make any mistakes

erm, armis … you're doing it again!

If you'd changed "Then using the lens formula:" to "Then use the lens formula! ", and stopped there, then that would have been a good hint.

(The OP can always come back and ask for more help, if necessary, of course )

Oh... I thought I was supposed to do that only in the homework section, sorry. But it does make a lot more sense your way. I think I finally got the idea

## 1. What is the process for finding the focal length of a concave lens using a convex lens?

The process for finding the focal length of a concave lens using a convex lens involves placing the concave lens between the object and the convex lens, adjusting the distance between the two lenses until a clear and inverted image is formed on a screen, and then measuring the distance between the lenses.

## 2. What equipment is needed to find the focal length of a concave lens using a convex lens?

To find the focal length of a concave lens using a convex lens, you will need a convex lens, a concave lens, an object to act as the source of light, a screen, and a meter stick or ruler to measure distances.

## 3. How does the focal length of the convex lens affect the measurement of the focal length of the concave lens?

The focal length of the convex lens affects the measurement of the focal length of the concave lens because it determines the distance between the two lenses. A shorter focal length for the convex lens will result in a shorter distance between the two lenses, and vice versa.

## 4. What are the potential sources of error when finding the focal length of a concave lens using a convex lens?

Potential sources of error when finding the focal length of a concave lens using a convex lens include inaccurate measurements of distances, changes in the position of the lenses while taking measurements, and imperfections in the lenses that may affect the formation of a clear and inverted image.

## 5. How does the curvature of the concave lens affect its focal length?

The curvature of the concave lens directly affects its focal length. A more curved concave lens will have a shorter focal length, while a less curved concave lens will have a longer focal length. This is because a more curved lens bends light more, causing it to converge at a shorter distance from the lens.