Three Red Beams Crossing After a Lens

In the picture

1. 1The lamp housing that sends out the three beams
2. 2One of the parallel beams before it reaches the glass
3. 3The thick glass lens that bulges on both sides
4. 4The single point where the three beams cross
5. 5The white card catching the spread-out beams

Examples

• A magnifying glass focusing sunlight into a tiny hot spot on paper
• The lens in a camera or phone bringing distant light to a sharp point on the sensor

Quiz: Three Red Beams Crossing After a Lens (Age 15)

1. 1

   Several rays of light travelling parallel to each other pass through a lens that bulges outwards on both sides. What do the rays do after they leave the lens?

   • They carry on parallel, just travelling a little more slowly
   • They bend towards each other and cross at a single point
   • They start spreading apart the moment they leave the glass
2. 2

   What makes the rays change direction as they go through the lens?

   • The curved surfaces of the glass bend the light as it enters and leaves
   • The rays pull on one another once they are close together
   • The lens adds energy to the light and pushes it inwards
3. 3

   A screen is held well beyond the point where the rays cross. What appears on the screen?

   • A spread-out patch of light, because the rays separate again after crossing
   • One tiny bright dot, because the rays stay together once they meet
   • Nothing at all, because the light stops at the crossing point

The answers, each with its explanation, are in the app. This lesson has left the free week, so its quiz is part of the subscription.