Lenses and Visible Light (Converging and Diverging) — AQA GCSE Physics (Separate)
Lenses refract light to form images. There are two types: converging and diverging.
Types of lens
- A convex lens is a converging lens — it brings parallel rays of light together to a focus.
- A concave lens is a diverging lens — it spreads parallel rays apart.
Key terms
- Principal focus (focal point) — where parallel rays converge (convex) or appear to diverge from (concave).
- Focal length — the distance from the centre of the lens to the principal focus. A more powerful lens has a shorter focal length.
Images
- A real image is formed where light rays actually meet, and can be projected onto a screen (always inverted for a convex lens with the object beyond the focus).
- A virtual image is formed where rays only appear to come from; it cannot be projected onto a screen (always upright).
A convex lens used as a magnifying glass (object closer than the focal length) produces an enlarged, upright, virtual image. A concave lens always produces a smaller, upright, virtual image.
Magnification
$$\text{magnification} = \frac{\text{image height}}{\text{object height}}$$ Magnification has no units. A value greater than 1 means the image is enlarged.
Exam tips
- Convex = converging; concave = diverging.
- Define principal focus and focal length.
- Real images can be projected (inverted); virtual images cannot (upright).
- Learn magnification = image height ÷ object height.