Physics 2000 JAMB Past Questions - Question 31: What optical instrument can best be constructed with converging lenses of focal lengths 50cm and 5cm?

Correct Answer

A

Explanation

To construct an optical instrument with converging lenses of focal lengths 50 cm and 5 cm, you can create a simple compound microscope. A compound microscope is an instrument used to magnify small objects or specimens. It typically consists of two lenses: an objective lens (the lens closer to the object being observed) and an eyepiece lens (the lens closer to your eye).To determine the optical instrument that can be constructed using converging lenses of focal lengths 50cm and 5cm, we need to consider the properties and applications of converging lenses.

Converging lenses are thicker in the middle and thinner at the edges. They are commonly used in various optical instruments, such as cameras, telescopes, and microscopes. The focal length of a lens determines its ability to converge or diverge light.

In this case, we have two converging lenses with focal lengths of 50cm and 5cm. One lens has a longer focal length, while the other has a shorter focal length. By combining these lenses, we can create a compound lens system.

One optical instrument that can be constructed using these converging lenses is a compound microscope. Compound microscopes use multiple lenses to magnify small objects. The objective lens, which has a longer focal length, collects light from the specimen and forms a real, inverted image. The eyepiece lens, which has a shorter focal length, magnifies the image formed by the objective lens and allows the viewer to see the magnified image.

Therefore, with converging lenses of focal lengths 50cm and 5cm, a compound microscope can be constructed.

Here's how you can construct a compound microscope using these lenses:

1. Objective Lens: Use the converging lens with a longer focal length (50 cm) as the objective lens. This lens is responsible for collecting light from the specimen and forming a real, inverted, and highly magnified image.

2. Eyepiece Lens: Use the converging lens with a shorter focal length (5 cm) as the eyepiece lens. This lens is positioned near your eye and is responsible for further magnifying the image formed by the objective lens. The eyepiece creates a virtual image that you can see and observe through the microscope.

Here's a basic principle of how a compound microscope works:

- The objective lens forms a real inverted image of the object being observed. This image is located at a distance equal to its focal length from the objective lens.

- The eyepiece lens takes this real image and magnifies it, creating a virtual, upright image that you can see when you look through the eyepiece.

The combination of these two lenses allows you to achieve a high level of magnification, making it possible to see fine details of small objects.

To focus the microscope, you can adjust the distance between the two lenses by moving the eyepiece lens closer or farther from the objective lens until the image appears sharp and clear.

Keep in mind that for a practical implementation, you will also need a tube or body to hold and align the lenses, a stage to place your specimen, and adjustments for fine focus and coarse focus. Additionally, you may need an illumination source to properly light the object you're observing.

This basic compound microscope configuration is a common design used in many microscopes, and you can further refine and optimize it for specific applications and requirements.