Bending light on purpose
A lens is a curved piece of clear glass or plastic that bends light to form an image. Lenses are in your eyes, glasses, cameras, phones, microscopes, telescopes and projectors.
Lenses work by refraction: light slows down when it enters glass and bends. The curved shape bends rays by different amounts, so they come together or spread apart.
Two kinds of lenses
| Convex lens (converging) | Concave lens (diverging) | |
|---|---|---|
| Shape | Thicker in the middle | Thinner in the middle |
| Effect on parallel light | Brings rays together at a point | Spreads rays apart |
| Images | Real or virtual, depending on distance | Always virtual, upright and smaller |
| Uses | Magnifying glasses, cameras, projectors, the eye, glasses for farsightedness | Glasses for nearsightedness, peepholes, some telescopes |
Focal point and focal length
When parallel rays (like sunlight) pass through a convex lens, they meet at the focal point. The distance from the lens to the focal point is the focal length.
- A fatter, more curved lens has a shorter focal length and bends light more strongly.
- A flatter lens has a longer focal length.
You can find a lens’s focal length by focusing sunlight or a distant window onto paper and measuring the distance.
Warning: never focus sunlight onto anything near your eyes, skin or flammable materials. Concentrated sunlight can burn.
Images from a convex lens
What you see depends on where the object is.
| Object distance | Image | Example |
|---|---|---|
| Very far away | Small, upside down, real, at the focal point | Camera photographing a landscape |
| Farther than 2 focal lengths | Smaller, upside down, real | Camera, the eye |
| Between 1 and 2 focal lengths | Larger, upside down, real | Projector |
| Closer than 1 focal length | Larger, upright, virtual | Magnifying glass |
- A real image is formed where light actually meets. It can be shown on a screen and is upside down.
- A virtual image only appears to be there. You can see it by looking through the lens, but you can’t project it.
Magnification
Magnification compares the image size to the object size.
magnification = image height ÷ object height
A magnifying glass with 3× magnification makes things look three times bigger.
Lenses in action
Cameras
A camera lens focuses a small, upside-down real image onto a sensor. Moving the lens closer or farther focuses on near or far objects. Zoom lenses change focal length to make distant objects look bigger.
Microscopes
A compound microscope uses two convex lenses: the objective lens near the specimen makes a magnified image, and the eyepiece magnifies it again. Total magnification = objective × eyepiece. A 40× objective and 10× eyepiece give 400×.
Telescopes
A refracting telescope uses a large objective lens to collect light from distant objects and an eyepiece to magnify the image. Galileo built telescopes in 1609 and discovered four moons of Jupiter.
Glasses and contact lenses
| Vision problem | What happens | Lens used |
|---|---|---|
| Nearsightedness (myopia) | Far objects are blurry; image forms in front of the retina | Concave lens |
| Farsightedness (hyperopia) | Near objects are blurry; image forms behind the retina | Convex lens |
| Presbyopia | Eye lens stiffens with age, making reading hard | Convex reading glasses |
| Astigmatism | Uneven curve on the cornea | Specially shaped lenses |
Eyeglass lens strength is measured in diopters. Positive numbers are convex lenses; negative numbers are concave.
Your eye’s lens
The lens inside your eye is flexible. Tiny muscles change its shape: thicker for nearby objects, thinner for distant ones. This is called accommodation.
Common mistakes
- Thinking concave lenses magnify. Convex lenses do; concave lenses make things look smaller.
- Thinking real images are upright. Real images from a single lens are upside down.
- Confusing lenses and mirrors. Lenses bend light (refraction); mirrors bounce it (reflection).
Try it yourself
- Use a magnifying glass to project an image of a bright window onto paper. Is it upside down? Measure the focal length.
- Put a drop of water on plastic wrap over newspaper print. What happens to the letters?
- Look through the bottom of a clear glass at text. Is the glass acting as a convex or concave lens?
Quick recap
- Lenses bend light by refraction to form images.
- Convex lenses converge light; concave lenses diverge it.
- The focal length is the distance to the focal point.
- Convex lenses make real or virtual images depending on object distance.
- Lenses are used in cameras, microscopes, telescopes and glasses.