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Slide the screen until the candle is sharp. Why is its picture upside down?
Candle, Lens and Screen lab · Last updated: 16 October 2026
A candle burns on one side of a lens and a screen stands on the other. You slide the screen until the flame appears sharp. The image is clear but upside down. The lab shows exactly why.
What you'll discover
- A convex lens bends light rays together to form a real image on a screen.
- The image only looks sharp at one distance, which depends on where the candle is.
- Rays from the top of the candle cross over and land at the bottom, flipping the image.
The science in plain words
A convex lens is thicker in the middle than at the edges. Because light bends as it enters and leaves glass, rays passing through the lens are bent towards each other. Parallel rays from far away meet at a single point called the focus, and the distance from the lens to that point is the focal length.
Light from each point of the candle flame spreads out in all directions. The lens collects some of that light and bends it so that it meets again at one point on the other side. Do that for every point of the flame and you get an image you can catch on a screen, called a real image.
Rays from the top of the flame travel downwards through the lens and land low on the screen; rays from the bottom travel upwards and land high. They cross over, so the image is upside down and swapped left to right.
Move the candle closer to the lens and the sharp image forms further away and larger. Your eye works the same way: its lens forms an upside down image on the retina, and your brain turns it the right way up. Cameras and projectors use the same principle.
Where it fits
Common questions
Why is the image from a convex lens upside down?
Rays from the top of the object pass through the lens and cross over to the bottom of the image, and rays from the bottom go to the top. The crossing flips the image.
What is the focal length of a lens?
It is the distance from the lens to the point where parallel rays of light meet after passing through it. A fatter, more curved lens has a shorter focal length.
Does our eye see things upside down?
The lens in your eye does form an upside down image on the retina. Your brain interprets the signals so that you see the world the right way up.
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- Bending Light: Shine a ray into a glass block. How much does it bend?
- Prism Rainbow: Split white light into a rainbow with a glass prism.
- Glowing Glass Rod: Shine a laser into a bent glass rod. Does the light follow the bend?
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Try it yourself
The Candle, Lens and Screen lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.