# Find Focal Length of Biconcave Lens with Galilean Telescope

• davevresk
In summary, the experiment involves setting up a "galilean telescope" with a convex lens with focal length of 20 cm and a biconcave lens of unknown focal length, placed 15 cm away from the convex lens. The objective lens is used to focus parallel light onto a screen, with the distance between the screen and the lens being slightly longer than the focal length. The goal is to use ray diagrams and the thin lens law to determine the focal length of the biconcave lens, but the lack of information on the radius of the lenses and refractive index makes it challenging. However, it is possible to determine the focal length by considering the angle at which the light is bent by the ocular lens.
davevresk
1. For a Lab I have set up a "galilean telescope" where the objective lens a convex lens with focal length about 20 cm (F) and a biconcave lens of unknown focal length. I placed the biconcave lens at about 15 cm away from the convex lens as the ocular (so closer than the focal point) and then focused parallel light onto a screen. The distance between the screen and the objective lens was measured to be slightly longer than the focal length of the objective lens (about 22 cm). The question is to find the focal length of the biconcave lens using ray diagrams and the thin lens law.

## Homework Equations

Thin lens law: 1/f=(n-1)(1/r1 - 1/r2) where n is the refractive index of the lens.

## The Attempt at a Solution

I have drawn a ray diagram but it doesn't seem to match what is in the textbook. The book only shows diagrams where light enters the biconcave lens and come out parallel, but that wouldn't focus the light right? So it must not come out parallel in this case. So in my diagram the incident (parallel) light is bent inwards by the objective lens to a focal point that is beyond the ocular lens, so before it is actually focused, it goes through the ocular (biconcave) lens at which point it is bent to an angle that doesn't converge quite as fast so it focuses at point beyond the focal point of the objective light. I don't really know how this helps me because the light entering the ocular is not parallel and so it doesn't converge on the lens focal point (or does it). Also we are not given the radius of the lenses or the index of refraction in the lab manual, so I don't really see how thin lens law really helps at all.

nm, i think i figured it out

## 1. What is a biconcave lens?

A biconcave lens is a type of lens that is thinner at the center than at the edges, causing light rays passing through it to diverge. It is concave on both sides, meaning it curves inward.

## 2. What is a Galilean telescope?

A Galilean telescope is a type of telescope that uses a combination of a convex lens and a concave lens to magnify distant objects. It was invented by Galileo Galilei in the 17th century.

## 3. How do you find the focal length of a biconcave lens with a Galilean telescope?

The focal length of a biconcave lens can be calculated by using the formula: f = R/2(n-1), where R is the radius of curvature of the lens and n is the refractive index of the material the lens is made of. This focal length can then be used to determine the magnification of the Galilean telescope.

## 4. What factors affect the focal length of a biconcave lens?

The focal length of a biconcave lens can be affected by several factors, including the radius of curvature, the refractive index of the material, and the thickness of the lens. The distance between the two lenses in a Galilean telescope can also affect the overall focal length.

## 5. Why is it important to know the focal length of a biconcave lens in a Galilean telescope?

Knowing the focal length of a biconcave lens in a Galilean telescope is important because it determines the magnification of the telescope. This information is crucial for accurately observing and studying distant objects and phenomena, such as stars and planets.

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