# Does the focal length of a concave lens change in a different medium?

1. Jul 22, 2012

### Clyner791

My question is as follows:

Does the focal length of a concave lens change when put into a medium such as water?
Initially the focal length of the lens is 20 cm in air. Would it get larger or small in water?

2. Jul 22, 2012

### tiny-tim

Hi Clyner791!

Tell us what you think, and then we'll comment!

3. Jul 22, 2012

### Clyner791

I think the focal length will get larger. My reason for thinking this is because light rays diverge less in a more viscous medium? (I.e. they refract less?).

4. Jul 22, 2012

### tiny-tim

you'll need to explain it a lot better than that …

where is the image? where do the rays go?

what difference does it make whether it's concave or convex?​

5. Jul 22, 2012

### Clyner791

In air: an object is placed 50 cm from the lens (concave) with a focal length 20cm. This produces an image 33.33 cm from the lens (again in air)

So, what change (if any) will occur to the focal length of the lens