Images from a Magnifying Glass

06 May 2007

Part of the show Naked Scientists Question and Answer Show

LensImage.jpg

Credit:

Dave Ansell

Ingredients

	A magnifying glass
	A nice bright window
	A white wall (or piece of paper) opposite the window

Instructions

Put the magnifying glass between the wall and the window - about 10-30 cm from the wall.

Move it in and out looking at the wall, do you see anything interesting?

You may have 'known people' who set fire to ants as children with a magnifying glass, so it is best not to do this experiment if the sun is visible through the window because the point where the sun is focused can get very hot. We have it on good authority that focusing the sun on emulsion paint is not good for the finish.

Result

If you get the magnifying glass the right distance from the wall you should see an upside down image of what is outside projected on the wall.

Image from a Magnifying glass

You will find that if you move the magnifying glass too close or too far away from the wall the image will go all fuzzy.

Explanation

The magnify glass is a lens. This is a specially shaped piece of glass that has an interesting property. All the light coming from one spot on one side of the lens is bent so that it is concentrated (focused) through a single point on the other side.

Light going through a lens

This means that if you put a piece of white paper at this point you will get a bright spot there. If there are lots of lights on one side each will be focused into a different point starting to build up an image.

Focussing

So if you point the lens at a bright scene out of a window every point in the scene will give off light which will be focused to a point on the paper. So you get an image on the paper. The image will look upside down because the light goes in a straight line through the lens - like the
pinhole camera.

Why does the image go fuzzy at the wrong distance?

If the screen is put at the wrong distance instead of each point in the scene making a point on the wall, it makes a big splurge, so each point in the scene will overlap with its neighbours and the image will look fuzzy.

Out of focus

What has this got to do with a camera?

A camera works on exactly the same principle, there is a lens at the front and where the screen would be there is a piece of film which will change chemically when light falls on it. When you change the focus setting it changes the distance between the lens and the film.

A digital camera works on the same principle but instead of a piece of film it has a CCD, an electronic sensor which is read by a small computer in the camera.


Looking in the back of a conventional camera you can see the lens.	If you put a piece of greeseproof paper in the back of the camera you can see what is being projected onto the film.