Lenses and Visible Light (Higher Tier)

This lesson covers lenses and visible light — including convex and concave lenses, ray diagrams, and the formation of images — as required by the AQA GCSE Combined Science Trilogy (8464) Higher Tier content, Physics Paper 2, section 6.2. This topic is Higher Tier only — it will not appear on Foundation Tier papers.

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Types of Lenses

There are two types of lens:

Lens type	Shape	Also called	Effect on light
Convex (converging)	Thicker in the middle, thinner at the edges	Converging lens	Brings parallel rays together to a focus
Concave (diverging)	Thinner in the middle, thicker at the edges	Diverging lens	Spreads parallel rays apart (they appear to come from a focus behind the lens)

graph LR
    subgraph "Convex Lens (Converging)"
    direction LR
    A["Parallel rays →"] --> B["Convex lens"]
    B --> C["→ Focus (F)"]
    end

graph LR
    subgraph "Concave Lens (Diverging)"
    direction LR
    D["Parallel rays →"] --> E["Concave lens"]
    E --> F["Rays diverge ← appear to come from F behind lens"]
    end

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Key Terms

Term	Definition
Principal axis	The straight line passing through the centre of the lens, perpendicular to it
Principal focus (F)	The point where parallel rays converge (convex) or appear to diverge from (concave) after passing through the lens
Focal length (f)	The distance from the centre of the lens to the principal focus
Real image	An image formed where light rays actually converge — can be projected onto a screen
Virtual image	An image formed where light rays appear to come from — cannot be projected; seen by looking through the lens

Exam Tip (AQA 8464, Higher): A real image can be caught on a screen (rays actually meet). A virtual image cannot — you see it by looking through the lens. Convex lenses can produce both; concave lenses always produce virtual images.

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Drawing Ray Diagrams for a Convex Lens

To draw a ray diagram for a convex lens, use these three standard rays (you only need two to locate the image):

1. Ray parallel to the principal axis → passes through the principal focus (F) on the other side.
2. Ray through the centre of the lens → continues straight through without changing direction.
3. Ray through the principal focus (F) → emerges parallel to the principal axis on the other side.

Where two (or more) of these rays meet on the far side of the lens, that is where the image is formed.

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Images Formed by a Convex Lens

The nature and size of the image depend on the position of the object relative to the focal length:

Object position	Image position	Image type	Image size	Image orientation	Example use
Beyond 2F	Between F and 2F (other side)	Real	Diminished (smaller)	Inverted	Camera
At 2F	At 2F (other side)	Real	Same size	Inverted	Copying documents
Between F and 2F	Beyond 2F (other side)	Real	Magnified (larger)	Inverted	Projector
At F	At infinity	No clear image	—	—	Spotlight / searchlight
Between F and lens	Same side as object	Virtual	Magnified	Upright	Magnifying glass

graph TD
    A["Object beyond 2F"] -->|"Real, diminished, inverted"| B["Image between F and 2F"]
    C["Object between F and 2F"] -->|"Real, magnified, inverted"| D["Image beyond 2F"]
    E["Object between F and lens"] -->|"Virtual, magnified, upright"| F["Image same side"]

    style A fill:#2980b9,color:#fff
    style B fill:#3498db,color:#fff
    style C fill:#27ae60,color:#fff
    style D fill:#2ecc71,color:#fff
    style E fill:#e74c3c,color:#fff
    style F fill:#c0392b,color:#fff

Exam Tip: When the object is inside the focal length (between F and the lens), the image is virtual, upright and magnified — this is how a magnifying glass works. Learn this case carefully as it is commonly tested.

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Images Formed by a Concave Lens

A concave (diverging) lens always produces an image that is:

• Virtual (cannot be projected onto a screen)
• Upright (the same way up as the object)
• Diminished (smaller than the object)

This is true regardless of where the object is placed.

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Magnification

The magnification of a lens (or mirror) tells you how many times larger or smaller the image is compared to the object.

$\text{magnification} = \frac{\text{image height}}{\text{object height}}$magnification=object heightimage height​

• Magnification > 1 → the image is larger than the object (magnified).
• Magnification = 1 → the image is the same size as the object.
• Magnification < 1 → the image is smaller than the object (diminished).
• Magnification has no unit — it is a ratio.

Worked Example

An object is 3 cm tall and the image formed by a lens is 9 cm tall. Calculate the magnification.

$\text{magnification} = \frac{9}{3} = 3$magnification=39​=3

The image is 3 times larger than the object.

Worked Example 2

A lens produces an image that is 2 cm tall. The magnification is 0.5. Calculate the height of the object.

$\text{object height} = \frac{\text{image height}}{\text{magnification}} = \frac{2}{0.5} = 4 \text{ cm}$object height=magnificationimage height​=0.52​=4 cm

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The Human Eye and Lenses

The human eye contains a convex (converging) lens that focuses light onto the retina at the back of the eye.

Part	Function
Cornea	Refracts (bends) light as it enters the eye — does most of the focusing
Iris	Controls the amount of light entering the eye by changing the size of the pupil
Lens	Fine-focuses light onto the retina by changing shape (accommodation)
Retina	Contains light-sensitive cells (rods and cones) that convert light into electrical signals
Optic nerve	Carries electrical signals from the retina to the brain

Eye Defects

Defect	Cause	Corrective lens
Short-sightedness (myopia)	Eyeball too long or lens too powerful — image forms in front of retina	Concave (diverging) lens
Long-sightedness (hypermetropia)	Eyeball too short or lens too weak — image forms behind retina	Convex (converging) lens

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Specular and Diffuse Reflection

When visible light hits a surface: