Lenses (AQA GCSE Physics): Revision Notes
Lenses
Lenses are pieces of glass or plastic that bend light to create images. They work by refracting (bending) light rays as they pass through. Understanding how lenses work helps us explain how our eyes see and how cameras take pictures.
Lenses are found everywhere in our daily lives - from the lenses in our eyes and glasses, to those in cameras, microscopes, and telescopes. The same principles apply to all of them!
What are lenses?
A lens creates pictures by bending light rays. When light travels from one material to another (like from air into glass), it changes direction slightly. This bending is called refraction.
There are two main types of lenses that bend light in different ways.
The amount of bending depends on the angle at which light hits the lens surface and the difference in density between the two materials.
Types of lenses
Convex lenses
- Shape: Thicker in the middle, thinner at the edges
- What they do: Bend light rays towards each other
- Effect: Bring light rays together at one point
- Example: Magnifying glasses use convex lenses
Concave lenses
- Shape: Thinner in the middle, thicker at the edges
- What they do: Bend light rays away from each other
- Effect: Spread light rays apart
- Example: Some spectacles for short-sighted people use concave lenses
Memory tip: Think of convex lenses as "caving in" towards the middle (thicker in centre), while concave lenses "cave out" from the middle (thinner in centre).
Focal length and principal focus
When parallel rays of light hit a convex lens, they all meet at one special point. This point is called the principal focus.
The focal length is the distance from the centre of the lens to the principal focus. This is an important measurement because:
- Thicker lenses have shorter focal lengths
- Thinner lenses have longer focal lengths
- The focal length affects how much the lens magnifies objects
Key relationship: The thicker the lens, the more it bends light, so the shorter the focal length. This is why strong magnifying glasses have very thick, curved lenses.
Real and virtual images
Lenses can create two different types of images:
Real images
- What they are: Images that can be projected onto a screen
- How they form: Light rays actually meet and focus at a point
- Which lenses make them: Convex lenses can make real images
- Example: The image on a cinema screen is a real image
Virtual images
- What they are: Images that you can see but cannot project onto a screen
- How they form: Light rays appear to come from a point but don't actually meet there
- Which lenses make them:
- Concave lenses always make virtual images
- Convex lenses make virtual images when objects are very close to the lens
- Example: Your reflexion in a mirror is a virtual image
Critical difference: If you can project the image onto a piece of paper or screen, it's real. If you can only see it by looking through or at the lens, but can't project it, it's virtual.
Key facts about lens behaviour
- Convex lenses can create both real and virtual images depending on where you place the object
- Concave lenses only ever create virtual images
- The thickness of a lens affects how much it bends light
- Parallel light rays always pass through the principal focus of a convex lens
The behaviour of lenses follows predictable rules, which is why we can design precise optical instruments like cameras, telescopes, and microscopes.
Key Points to Remember:
- Convex lenses are thick in the middle and bring light rays together
- Concave lenses are thin in the middle and spread light rays apart
- Focal length is the distance from the lens centre to where parallel rays meet
- Real images can be shown on screens, virtual images cannot
- Concave lenses always make virtual images