Lenses (Higher tier)

Two types of lens

• Convex (converging) lens — bulges out; bends parallel rays towards a single focal point beyond the lens.
• Concave (diverging) lens — curves inwards; spreads parallel rays apart so they appear to come from a focal point in front of the lens.

The focal length (f) is the distance from the centre of the lens to the focal point.

Ray diagram rules (convex lens)

To locate an image, draw two of these three rays from the top of the object:

1. A ray parallel to the principal axis → after the lens it passes through the focal point on the far side.
2. A ray through the centre of the lens → continues straight, undeviated.
3. A ray through the focal point on the same side → emerges parallel to the principal axis.

The image forms where any two rays cross.

Image classifications

• Real — rays actually meet on the far side; can be projected onto a screen (camera, projector).
• Virtual — rays only appear to meet (when extended back); cannot be projected (magnifying glass).
• Inverted — upside-down (real images for an object beyond f).
• Upright — same orientation (virtual images, for an object inside f, or any concave-lens image).

A concave lens always gives a virtual, upright, smaller image.

Magnification

magnification = image height ÷ object height

A magnification of 2 means the image is twice as tall as the object. Magnification has no units (it’s a ratio).

Edexcel exam tip

Always label your ray diagrams: object, image, F (focal point), 2F, principal axis, and arrows on every ray. A common dropped mark is missing the arrows or failing to extend rays back as dashed lines when locating a virtual image.