Lenses and visible light
Convex and concave lenses (Separate Physics only)
A lens forms an image by refracting light.
- A convex lens is thicker in the middle. It makes parallel rays of light meet at a point called the . It can make a real or a virtual image. In ray diagrams it is drawn as a line with arrowheads pointing outwards at each end.
- A concave lens is thinner in the middle. It makes parallel rays spread out, as if they came from a principal focus behind the lens. It always makes a virtual image. It is drawn as a line with arrowheads pointing inwards.
The distance from the lens to the principal focus is the .
Key terms
Real and virtual images (Separate Physics only)
- A forms where light rays actually meet. It can be shown on a screen. A camera and a projector make real images.
- A forms where rays only seem to come from. It can't be shown on a screen. A magnifying glass makes a virtual image: hold an object closer to a convex lens than its focal length, and you see it upright and bigger.
A convex lens can form a real image. An object at twice the focal length (2F) makes an inverted image the same size at 2F on the other side:
You must recall this equation:
Example: a 2 cm object makes an image 6 cm tall.
- magnification = 6 ÷ 2
- magnification = 3
Exam tip
Magnification has no units. Measure both heights in the same unit, such as mm or cm.
Key terms
Colour (Separate Physics only)
Each colour of visible light has its own narrow band of (and frequencies). Red has the longest wavelength and violet the shortest.
The colour of an object depends on which wavelengths it reflects most strongly. The rest are absorbed.
- A red shirt reflects red light and absorbs other colours.
- A white object reflects all wavelengths equally.
- A black object absorbs all wavelengths.
A lets some wavelengths through (transmits them) and absorbs the rest. A red filter only lets red light through. So a green leaf looks black through a red filter: the leaf reflects green, but the filter absorbs it.
Objects that let light through are (clear) or (blurry).
Specular and diffuse reflection (Separate Physics only)
- light reflects off a smooth surface, such as a mirror, in a single direction. Parallel rays stay parallel, so you see a clear image.
- light reflects off a rough surface, such as paper, and is scattered in many directions. You don't see an image.
On a rough surface, each ray still obeys angle of incidence = angle of reflection. The normals just point in different directions.