light - telescope

By combining two convex lenses, a strong lens with a small focal length and a weak lens with a long focal length, a simple telescope can be created.
The front lens refracts the light and forms a real image. The real image formed by the front lens is inside the focal length of the second lens. Since the first image is inside the focal length of the second lens a virtual, magnified image is created.

The telescope
Click to see an animation. Click to hide the animation

The magnifying power of the simple telescope above can be calculated by the expression below, if the focal lengths of both lenses are known.

The larger the diameter of the lenses the more light that is allowed to pass through and hence a brighter image is created.

Exercises

The telescope
Telescopes can manipulate and magnify light images from distant galaxies. Large telescopes are housed in special buildings, known as observatories, and in isolated places to minimise light interference from nearby cities.
Telescopes do not only collect light. Some, such as the Parkes radio telescope shown on the right, collect radio waves from distant galaxies. Large parabolic dishes focus the incoming waves into a central point where it is collected and analysed.
Other telescopes, such as the Hubble telescope pictured on the right, orbit around the Earth away from the interference of the atmosphere. Hubble can take pictures with incredible clarity and high resolution. Click to see one such image. Hubble has on board many other devices that capture and focus radiation such as, infrared and X-ray. The combination of these different techniques gives a complete picture of our stellar neighbours. Click to see an example of an image created by combining, infrared, UV radiation, infrared and visible light.
By combining two convex lenses, a strong lens with a small focal length and a weak lens with a long focal length, a simple telescope can be created. The front lens refracts the light and forms a real image. The real image formed by the front lens is inside the focal length of the second lens. Since the first image is inside the focal length of the second lens a virtual, magnified image is created. Click to see an animation. Click to hide the animation The magnifying power of the simple telescope above can be calculated by the expression below, if the focal lengths of both lenses are known. The larger the diameter of the lenses the more light that is allowed to pass through and hence a brighter image is created. Exercises
Activity Making a telescope