Finding the focal length

The sun is so far away that its rays arrive parallel. A convex lens brings them to its focus — so a small bright spot marks where the focus is.

Activity 5.5

You will need: a convex lens, a white sheet of paper

In bright sunlight, hold a convex lens facing the sun with a white sheet of paper below it.
Adjust the distance between the paper and the lens until a small bright spot forms.

Never look at the sun

Never look at the sun through a lens, and keep the bright spot off skin and anything that can burn — it is concentrated sunlight.

Activity 5.6

You will need: a convex lens, a white screen

Open the window of a room and stand inside, holding a convex lens facing the window.
Hold a white screen behind the lens and move the lens until an image of the view through the window forms on it.
When the image is very clear, small and inverted, like a photograph, measure the distance between the screen and the lens.

The observation

That distance is the rough focal length of the lens. The image forms where light from outside, refracted through the lens, actually comes together on the screen, so it is a real image.